Note: Descriptions are shown in the official language in which they were submitted.
1~3~33
This invention relates to controllers for industrial processes
and particularly to the type of controller that provides so-called proportion-
- al speed floating control. Such controllers are generally classified in the -
United States Patent Office in CLASS 318 ELECTRICITY, MOTIVE, POWER SYSTEMS,
POSITIONAL SERVO SYSTEM, SUB-CLASS 599, Pulse ~idth Modulated Power Input to
Motor (e.g. "duty cycle systems"). ~he invention is particularly concerned
with an electronic controller that is operative in response to signals re- ;
presentative of a process condition to produce a bi-directional low le~el
direct current (d-c) output that may be utilized to regulate the direction W
and speed of operation of an electric motor valve actuator for a final control -~
element~ without valve position feedback, to maintain an industrial process at
a desired value. The controller is particularly suitable for controlling
industrial processes having short time constants.
,:
Proportional speed floating control is a type of control action
in which the direction and rate of adjustment of the final control element,
such as a valve, damper, etc., is proportional to the direction and extent of
deviation, from a desired set point, of the process variable that is being
controlled. The final control element is said to "float" since the position
of adjustment can be anywhere within the operating range when the deviation
is zero.
Proportional speed floating controllers accept low level direct
current voltage or current input signals from primary sensors responsive to
process variables to provide, for example, rate of flow control, by means of
motor driven valves, of water, sewage, sludge, slurry, and other process fluids.
Such controllers compare the signal from the flow or other process variable
transmitter with a set point signal representing a desired flow rate, which set
point signal may be either locally or remotely generated. When a difference
appears between the actual and the desired process variable value, the con-
troller provides output signals that control the supply of energizing current
, .................................................................... .
- 1~39833 ~
to a reversible electrical motor for rotation at a speed and in a direction
to operate a control valve as required to restore the process variable to the
desired value. The basic control mode is proportional velocity. That is to
- say, the output signals of the controller are speed signals that are propor- `~
tional to deviation. These speed signals are integrated by the electrical
motor as the latter drives to the correct valve position.
Controllers of this type may be employed to directly control the
operation of a light-duty motorized valve having, for example, a rating of one
ampere or less. In modern industrial process control systems, however~ heavy-
duty motorized valves having much higher power requirements are common. Inorder to enable such controllers to control the operation of such heavy-duty
.,~- ;::.~ ::
motorized valves, a power relay is employed therewith to control the required
heavy current and or high voltage to the motorized valve. Power relays utili-
zing semi-conductor switches of the type normally referred to as an SGR, triac
or thyristor have been employed because of their heavy current and high volt- ~ -
age handling capabilities. Such power relays are generally mounted adjacent ; ~;~
the motorized valve at a location remote from the controller, with the control- -
ler output signals being transmitted to the power relay over a plurality of
relatively high v~ltage transmiss~onn circuits.
The control of such remotely located power relays by electronic ~`
controllers has had certain drawbacks that have added substantially to the
-- . . .
cost of achieving acceptable performance and reliability. These drawbacks
have resulted from the fact that measuring low-level voltages afid currents in
circuits that are not referenced to the same potential can be rather diffi-
culto A primary reason for such difficulty is that both the controller and
power relay must be grounded, and when remotely located from each other one is
grounded at one location and the other is grounded at another location which
may be at a different potential than the ground at the first location. Another
factor that has contributed to increased costs and decreased performance and
2 -
11~3~833
reliability is the difficulty in transmitting without phase shift and signal
degeneration, that is, at a one-to-one time rate, the output signals from the
controller to the relay. This difficulty has been due to the effects of
capacitance and inductance in the transmission circuits connecting the control-
- ler to the power relay.
Among the objects of the present invention is the provision of an
improved industrial process control system embodying a two-wire low energy
level transmission line for transmitting direction and magnitude signals from ~;
a proportional speed controller to a remotely located power relay for manipu-
lation of a final control element, without position feedback, to maintain an
industrial process at a desired value.
A further object of the invention is to provide an improved electron-
ic industrial process control system wherein a proportional speed floating
controller and a remotely located power relay may be maintained at different
reference potentials without adverse effect on the operation of the system.
A f~rther object of the inYention is to provide an industrial pro-
cess control system in which a remotely located power relay is completely
isolated from the measuring and controlling circuit and the signal transmis-
sion circuitO
Still another object of the invention is to provide an industrial
process control system in which a proportional speed floating controller and
a remotely located power relay are electricàlly isolated by optical means from
each other.
Another object of the invention is to provide an improved electronic
industrial process control system in which signals from a proportional speed
floating controller to a remotely located power relay are transmitted by a
two-wire low level, low energy, transmission;line, and wherein no electromag-
netic radiation nor radio fre~uency energy that may be generated in the
controller is transmitted to the power relay or vice versaO
. .
~039~33 -`
Another object of the invention is to provide an industrial process
control system in which the transmission of the direction and magnitude sig-
nals from a proportional speed floating controller to a remotely located power
relay are transmitted at a one-to-one time rate.
Still another ob~ect of the invention is to provide an improved
industrial process control system in which stray electromagnetic signals that
may be induced on the transmission line are ineffective to produce a control
or other action on the p~er relay operation.
Still another obJect of the invention is to provide an improved
industrial process control system wherein the possibility of a transmission
line fault in producing an undesired response by the power relay is substan-
tially eliminated or minimi~ed.
A further object of the invention is to provide in such an improved
industrial process control system means to signal an open transmission line
condition.
In accordance with this invention there is provided a proportional
speed floating controller adapted in response to a process-variable signal `~
to produce bidirectional time-proportioned direct current signal pulses
comprising a deviation amplifier responsive to the departure of said process-
variable signal from a set point to produce an error signal the magnitude and
polarity of which depends upon the extent and direction of departure of said
process-variable signal from said set point, a triangular wave generator
operative to produce a triangular wave signal, a modulator said modulator
including error signal polarity detecting means for producing a first steady
signal at a first terminal when said error is of one polarity and to produce
a second steady state signal at a second terminal when said error signal is
of the opposite polarity, said modulator further including means to compare
said triangular wave signal with the absolute value of said error signal for
producing at a third terminal a signal the duration of which varies in accord-
ance with the relative magnitudes of said error and triangular wave signals,
~ _ 4 _
~1
~39~333
and an output circuit including means connected to said terminals of saidmodulator to produce bidirectional direct current pulses in said output
circuit, the direction and duration of said pulses varying in accordance
with the polarity and amplitude of said error signal.
mere has been provided in accordance with the present invention
improvements in a proportional speed floating controller and in transmission
line circuitry for transmitting control signals from said controller to a
remotely located power relay, which power relay is electrically isolated from
said controller. m e improved controller comprises a deviation amplifier to
eompare process variable and set point signals and to provide an output
signal related to the magnitude and polarity of the difference. The output
signal is applied to a derivative or rate network to provide an error signal
of the same polarity but of a magnitude representative of the deviation
between the process variable and set point signals, with a rate component
added. This error signal is algebraically compared by a modulator with a
periodic sawtooth wave form signal that is produced by a sawtooth generator.
The comparison results in repetitive time-proportioned pulses each of duration
dependent upon the magnitude of said
- 4a -
:-- - . --- - - .:
: ~ . ., , . :
~39~33
error signal, but having a minimum duration of 8 milli seconds. Such time-
proportioned pulses are converted by a unique polarity responsive controller
output switching circuit into a bi-directional time-proportioned direct cur-
rent (d-c) flow in a two-wire low voltage and current transmission circuit~
the direction of such current flow being in accordance with the polarity of
said error signal. By the use of logic circuitry one such pulse only is
allowed to be produced within each cycle of the sawtooth wave of the sawtooth
generator. The bi-directional time-proportioned d-c is conducted by the
transmission line to a remotely located power relay. At the power relay a
pair of optical isolators are provided and arranged to be selectively activa-
ted depending upon the direction of flow of the received d-c pulses. The
optical isolators serve to electrically isolate the power relay from the
transmission line and the controller. The power relay includes means res- `-
- ponsive to the actuation of the optical isolators to provide appropriate
gating signals to a triac or other motor control circuit thereby to energize `~
a reversible valve motor actuator for operation in one direction or the otherO `"
:.: -.:
The duty cycle of such energization is in accord with the time-proportioning
of the current pulses transmitted from the controller and received over the
two wire transmission line by the said optical isolators.
A better understanding of the present invention may be had from the
following detailed description when read in connection with the accompanying ; ~ -
drawings wherein:
Figure 1 is a simplified block diagram showing a process control `
system, a rate of flow control loop, including an electronic controller and
a remotely located power relay arranged to control a motorized valve that may
be a heavy-duty type and comprises the final control element of the process
control system;
Figure 2 is a simplified block diagram showing an electronic con-
troller and power relay, as in Figure 1, arranged in conjunction with a direct
` :
1)3~833
digital control (DDC) computer in a rate of flow control system;
Figure 3 is a block diagram illustrating the computer-backup con-
trol interface of the system shown in Figure 2;
Figure 4 is a schematic diagram of an input portion of the electron-
ic controller of the system illustrated in Figures 1 and 2;
Figure 5 is a schematic diagram of an output portion of the said
electronic controller of Figures 1 and 2;
- Figure 6 illustrates voltage wave forms generated by the controller
of Figure 4; and
Figure 7 which is located on the second page of drawings together
with Figure 3 is a diagram, in block and s`chematic form, showing a typical
form which the power relay and electric motor actuator of Figures 1 and 2 may
take to effect the desired control of the process control system~
In Figure 1 there is illustrated in simplified block form a propor-
tional speed floating controller. The controller includes a deviation indica-
ting control station generally designated by numeral 10 and a solid state
power relay designated by the numeral 11. Analog signal values representing
process variable ¦PV) and desired set point (SP) are applied to station 10.
A reversible electrical motor 12 is arranged to be energized by the power
relay 11 to drive a final control element or valve damper, etc. 13 which
regulates a process~ shown as the flow of fluid through a controlled flow
line 14.
The process variable signal to the control station 10 is derived
by means of a transducer 15 which is connected to control station 10 by a ~rans-
mission line 16. The set point signal (SP) may be manually determined as by
manipulation of a set point adjustment thumbwheel lOa. Movement of th~mb-
wheel lOa positions a contact along a slide wire, not shown in Figure 1. ~
Alternatively, the set point signal may be established at a remote point as `
illustrated in Figure 4O
~3g~333
Typically the process controlled may comprise a flow of water,
sewage, sludge, slurry or other liquid through line 14. Transducer 15 may
comprise a flow transmitter which senses the rate of flow of such liquid and
develops the analog process variable signal PV that is applied to the control
station 10. The control station 10, as illustrated, is provided with indica-
- tors 10b, and 10c and a scale 10d to display the value of the process varia-
ble and the set point value, respectivelyO A meter 10e is provided to display
the valve opening as effected by the operation of the reversible electrical
motor 12 when a separate transmitting circuit for the purpose is provided. -~
That is to say, the meter 10e can be made to display the percent valve opening
in response to an external slidewire, not shown, operated by the valve. The
control station 10 is also provided with a control transfer switch lever 10f -
and manual control push buttonslOg and 10ho Control transfer switch lever 10f
has three positions to which it may be adjusted. Lever 10f is moved to posi-
tion "A" for automatic control and position "M" for manual control of the
process. For computer control, lever 10f is moved to position "C". A control-
ler status light 10i is provided to indicate whether the output cf the control
station is under control of the computer or the controller. The status light
10i is behind a translucent front bezel of .the control station.
The control station 10 senses the departure or deviation of the
process variable PY from the set point SP and produces a nominal 10 ma d-c
time-proportioned current pulse in one direction in the line 17 when the
departure of the process variable PV from the set point SP and produces a
10 ma d-c time-proportioned current pulse in the opposite direction in the
line 17 when the departure of the process variable is in the opposite direc-
tion.
The d-c time-proportioned current signal appearing at the output
~f control station 10 is transmitted by the transmission line 17 to power
relay 11, wherein it is sensed by one or the other of a pair of photo-isola-
1~39833
tors, depending upon its polarity, as is explained in detail hereinafter with
reference to Figure 7. Thus, upon the appearance of a signal at the input of
power relay 11~ the power relay is activated to establish an alternating cur-
rent energi~ing cirouit for the reversible motor 12 for rotation in one direc-
tion or the other depending upon the polarity of said signal. The speed at
which the reversible motor rotates is in accordance with the time-proportion-
ing of the said signal. The rotation of the motor and consequent adjustment ~;
of valve 13 restores the process 14 to the condition at which the process
variable and set point signals are in acco~d.
The basic control mode, as noted hereinbefore, is proportional
velocity. Thus the output of the power relay 11 in addition to being a dir-
ection signal, is a speed signal that is proportional to the deviation of PV
from SP, which signal is integrated by the electrical motor 12 as the latter
drives to the correct valve position The time-proportioned pulse signals
from control station 10 determines motor speed by duty cycle while their ~
direction of flow determines the direction of motor rotation. -
Figures 2 and 3 illustrate the basic control station of Figure 1
modified to allow it to be employed as a backup controller in a DDC computer
installationO The control st~tion in Figures 2 and 3 is designated by numeral
10'. In one mode of operation of the arrangement of Figure 2, the controller
mode, the flow rate in controlled flow line 14 is controlled by comparing the
flow signal PV from the transducer 15 to set point SP and proportioning the
speed and direction of reversible motor 12 through the power relay llo The
arrangement of Figure 2, however, also provides for operation in a computer
mode wherein a computer 24 directly provides time-proportioned signals to an
output sec~ion of the control station 10', as described hereinafter by refer- ;
ence to Figure 5.
Referring to Figure 3 it is noted that when transfer switch lever
10f is adjusted to computer position a transfer switch 20 is actuated by a
_ 8 -
833
control transfer circuit 21 to provide the output drive circuit 22 of the con-
trol station 10' with bi-directional d-c signals through a circuit directly ;
from the computer 24. This circuit includes a computer adapter card 23 having
an optical isolator 24a, to provide signals at the output of control station
10', which signals represent the velocity and direction of motion required of
the valve actuator motor to maintain the process variable under control. Thus
with switch 20 in computer position, the computer directly controls the output
of station 10' and thereby motor 12 and valve 13. With the switch 20 in
controller position~ the control station 10' controls the output and the motor
12 and valve 130 The controller status light lOi, as previously noted, is
provided to indicate whether the output of the station 10' is under control
of the computer or controller. For example, light lOi may be illuminated when
the output is not under computer control.
Figures 4 and 5 collectively show in simplified form the circuitry ;
of the control station 10. The portion of the circuitry shown in Figure 4~ -
includes a deviation amplifier 30, a set point generator, a rate amplifier 32,
.",., ,:
a sawtooth waveform generator 33, a modulator 34, a direct/reverse switch 35,
and a power supply 36. The set point generator includes an input terminal 31
for receiving a remotely-set set point signalO
As shown in Figure 4 the process variable and set point signals are
applied to the input terminals of the deviation amplifier 30. The SP signal
is obtained from the adjustment ofa contact 37 along a slide wire 380 That
adjustment may be manual, as previously noted, or, may be effected automatic-
ally in response to a signal received at terminal 31 from a remote point. For -~
such remote-set operation a servo motor amplifier 39 and reversible motor 40
are provided to effect the automatic adjustment of the set point slide wire 38 --
in response to the remote set point adjustmentO Such adjustments are indicat-
ed by the position of the indicator lOc along the scale lOd.
A single pole-double throw switch 41 is provided whereby in one
- _ 9 _
.... , . , - -- . . ,, -
r . . ` - ~ ~ . ` , : . `. - . .
t ~ . . - r .
3~33
position, the signal from the set point slide wire 38 is applied to the input
of amplifier 30, and in the other position, the remote set point signal is
applied directly to the input of the amplifier 30. With the latter connection
the servo amplifier 3~ and motor 40 continue to operate to adjust the slide ~
wire, thereby tracking the remote set point adjustment. ~ -
An amplifier 42 and a meter 43 are provided in association with
deviation amplifier 30 to indicate the deviation of the process variable
signal from the desired set point. Meter 43 adjusts indicator lOb along
scale lOd to indicate the magnitude of the process variable. Amplifier 4
serves to buffer meter 43 from the output of the deviation ampll`fier 30.
Amplifier 42 also scales the deviation amplifier output~ Optional alarm loads
not shown may be associated with the meter 43.
The output of deviation amplifier 30 is also connected to the input ;~
of the rate amplifier 32 which, as shown includes a derivative network 440
The derivative network of the rate amplifier can be bypassed by a switch~
The derivative network provides, for example~ a rate time adjustment -
from 004 to 60 secondsO This time depends upon the setting of a rate time
adjustment potentiometer on slide wire 450 The signal at the output of the
rate amplifier 32 comprises the deviation signal with a derivative component
added.
The output signal from the derivative network is applied through a -
buffer amplifier 46, in an inverse manner9 to each of the input circuits of a
pair of polarity detectors which are included in modulator 34 and are indicat- -
ed at 50 and 51 The buffer amplifier 46 provides signal gain and impedance
bufferingO An offset adjust~ent 47 is part of the buffer amplifier and is
provided to permit ~ero output of rate amplifier 32 notwithstanding electron-
ic offset~
A signal derice from the tap on an adjustable potentiometer slide
wire indicated at 48 is applied to a common terminal of the input circuits of
- 10 --
. . .
1~39833
the polarity detectors 50 and 51. The signal from slidewire 48 provides a
dead band zone, sometimes referred to as a neutral zone, in which some limited
deviation between the process variable and set point signals can occur before
a command is given by controller 10 to the power relay 11 for a control action.
The outpu~ signal voltage of the rate amplifier 32 is applied also
to an amplifier 49. The amplifier 49 is a so-called absolute value amplifier
and is characterized in having a one-to-one gain and in translating both
positive and negative signal voltages at its input as a negative voltage at
its output. The output of amplifier 49 is connected by a resistor 49a to a
positive input terminal of an amplifier 54 included in a modulator comparator
indicated at 52. If desired, the output of amplifier 49 can be characterized `~
to provide a positive output regardless of the input. With such modification
appropriate circuit alterations would be required as understood by those
skilled in the art.
.
A sawtooth voltage is also applied to said positive input terminal
amplifier 54 as seen in Figure 4 from the sawtooth generator 33 through a
circuit including a gain adjustment slidewire 53 and a fixed resistor 49b. ;
The dead band signal voltage from slidewire 48 is connected to the negative
input terminal of the amplifier 54. As shown, the modulator comparator 52
also includes diodes 68 and 69, an AND gate 70~ and resistor 70~.
The sawtooth wave generator 33 includes a ramp generator 55 and a
ramp comparator 56. A reference voltage V derived from power supply 36 is
applied to the positive input terminal of generator 55. A capaci~or 57 is
connected between the negative input terminal of said generator and the output
terminal thereof; additionally, gain adjustment slidewire 53 is connected in
the output circuit thereof. A single pole-double throw switch 58 is provided
for selectively connecting the said negative input terminal of generator 55 to
the tap on an adjustable potentiometer or slidewire 59, when in a first posi~
tion~ and to the output of the absolute value amplifier 49, when in a second
:'.'''
- 11 - . '' '
j .~, ...... ,. ..... , . , -
position. 1~39833
With switch 58 in the STD ~standard) position, the negative terminal
of generator 55 is connected to the contact on the slidewire 59. With the
switch 58 in the VC~ (voltage controlled oscillator) position, the negative
terminal of amplifier 55 is connected to the output terminal of the absolute
value amplifier 49. For each switch position there is circuitry to be con-
sidered contributing to both the rising portion as well as the falling portion
of the sawtooth wave. The circuit contributing the rising portion of the saw-
tooth wave, for the STD position includes a reverse biased diode 60. This
diode isolates the 24 volt terminal of the power supply 36 and also a trans-
istor 61 from the negative terminal of amplifier 550 During the rising por-
tion of the sawtooth wave, the transistor 61 conductsO During the falling
portion of the sawtooth ~ave, the taansistor 61 is turned off, that is, it is
not conducting. The diode 60 then is effective to apply a current to the
negative terminal of amplifier 55 and capacitor 57 from the 24 volt power -
source terminal. This current flows into the capacitor 57 whereby the voltage
at the output terminal of generator 55 decreases. During the rising portion
of the sawtooth wave, the current flow is in the reverse direction, that is,
from capacitor 57 to the slidewire 59. As a result the output voltage of the
generator 55 then gradually increasesO
Included in the sawtooth generator 33 also are resistors 63 and 64
and a field effect transistor (FET) 650 The latter components are connected ;~
with the source terminal of the FET 65 connected to ground and the drain ter-
minal connected through resistors 62 and 63 to the output of generator 55,
and the gate terminal connected to the output of ramp comparator 56. The
drain junction of resistors 63 and 64 is connected to the negative input
terminal of ramp comparator 56. The positive input terminal of the latter is
connected to reference voltage V and also, through a diode 66 to the output
terminal of an AND gate 67 in the modulator 34O The output of ramp comparator
.: , . ; ,
~/~39833
56 is connected through a diode 68 to the positive input terminal of modulator
comparator amplifier 54. The output of comparator 56 is also connected to
the gate of FET 65.
With this arrangement, in the generation of each sawtooth wave, the
upward portion of each wave is provided by the slidewire 59 or the absolute
value amplifier 49 depending upon the position of switch 58. W]len the said
upward portion is provided by the amplifier 49, the time for each upward sweep
of the wave is variable in accordance with the magnitude of the error signal.
When said upward wave portion is provided by the slidewire 59, the time for
each upward sweep is determined, over the range of variation, by the position
of the tap along the slidewire. The downward portion of each saw~ooth signal
wave is determined by the diode 60 and the transistor 61. That is to say,
the time for each downward sweep is fixed at a predetermined value depending
upon the parameters of the circuit including the capacitor 57, the diode 60
and transistor 61.
A complete cycle of each sawtooth wave produced by generator 33 `~
includes an upward portion and a downward portion. The time required for ~
each such complete cycle may be adjusted manually by manipulation of the tap , -
along slidewire 59, or determined automatically in accordance with the magni-
tude of the error signal, and hence, in accordance with the variation of the -
process variable from the desired set point.
The sawtooth wave form generator 33 thus produces a periodic saw-
tooth wave form the cycle time or frequency of which is determined either by ``
the manual adjustment of the slidewire 59 or automatically in accordance with
the error signal, that is the magnitude plus rate component of the deviation
signal m~nus the dead band signal, at the output of the absolute value
amplifier 49. The amplitude of the sawtooth wave form is determined by the
setting of the gain control slidewire 53. This sawtooth wave form acts as a ?
reference signal to determine the proper output duty cycle of the controller.
To this end, the sawtooth generator outpu~ is algebraically compared by the
- 13 -
1'039833
modulator comparator 52 with the error signal. The modulator comparator 52
provides an output pulse whenever the reference sawtooth wave is smaller, in
amplitude than said error signal. The greater the error signal the longer
the duration in each cycle of such pulse from modulator comparator 52~ that
is the larger ~he duty cycle. This duty cycle is expressed in terms of the
psrcentage of time on of the pulse to the complete time of one cycle.
Normally, the sawtooth generator 33 is inoperative. The arrange-
ment is such, however, the sawtooth generator is triggered into operation `
whenever the error signal at the output of the rate amplifier 32 exceeds the
dead band, and is turned off when that error signal returns to within the
dead band. To this end, in the operation of the sawtooth generator 33, the
ramp generator 55 normally is held off by a clamp on the non-inverting or `
positive input terminal of the ramp comparator 56. This clamp is provided by
the AND gate 67, which causes the output of ramp generator 55 to ramp upward
to a saturated output level. This clamp is removed for a predetermined `-~
minimum time of 8 m sec. when an error signal greater than the dead band
appears at the output of rate amplifier 32.
The modulator 34, as seen in Figure 4, includes an AND gate 70
having one input terminal connected by a diode 68 to the positive input ter-
minal of the amplifier 54, and the other input terminal connected to the out-
put of amplifier 54. The output of the AND gate 70 is connected by a series
connected resistance 70' and a diode 69 to the positive input terminal of
the amplifier 54. The modulator 34 also includes a pair of OR gates 71 and
72 which are associated, respectively, with the output circuits of the
polarity detectors 50 and 51. Thus, one input terminal of each of the OR `
gates 71 and 72 is connected to the output of its associated polarity detector
50 and 51, respectively, and each of the other input terminals is connected -
to the output of the modulator comparator 52 and to one output terminal 73 of
the portion of the control station 10 illustrated in Figure 4. The output ~
terminal of OR gate 71 is connected to the positive input terminal of polarity -
.~
- 14 -
~6339833 ~
detector 50 by a diode 74. ~le output of OR gate 72 is connected by a diode
75 to the negative input te~)inal of polarity detector 51.
~ ith this arrangementJ the error signal at the output o~ the rate
amplifier 32 is processed by the modulator 34 to determine the controller
output duty cycle and the direction of output current~ based upon the direc-
tion of the error signal and the controlle:r gain and deadband settings.
Specifically, the polarity detectors 50 and 51 algebraically combine the d0ad
band and error signals. One polarity detector, for example, detector 50
combines the error and dead band signals and emits an active signal, indicated
by a low voltage output, when a positive error signal is sensed, resulting
from the process variable signal being greater than the setpoint signal and
in excess of the dead band setting. The other detector 51 combines the same
two signals but emits an active signal, indicated by a low voltage output, ~`
when a negative error signal is sensed, resulting when the process variable ;~
is less than the set point signal and again in excess of the dead band.
The outputs of the polarity detectors 50 and 51 are connected by
the Direct/Reverse Switch 35 to a pair of terminals 76 and 77. Switch 35
is provided to permit selective interchange of the two signal directions at
terminals 76 and 77 for valve close/open operation. In the full line position
shown, the output of the polarity detector 50 is connected to the terminal 76 ~-
and the output of detector 51 is connected to the terminal 77. The dotted ;~
line position shows the said two output circuits interchanged, wherein the
outpu* of detector 50 is connected to terminal 77 and the output of detector
51 is connected to terminal 76.
It is noted that the sawtooth wave output of the sawtooth generator ~- .;
33, as it appears at the slider 53, rides on a bias voltage which in a typical
embodiment of the invention may be 6.35 volts above ground. A similar bias
voltage is provided between the upper input terminal of the absolute value `~
amplifier 49 and ground whereby the signal voltage at the output o~ amplifier
49 lies between the value of the bias voltage and ground and approaches the
- 15 -
.. - `,', , .
~39t333
latter as the deviation signal on its input increases from zero in either
direction. The voltage V across the slidewire resistor 48 desirably also has
a similar value.
By reference to Figure 6 it will be seen that the modulator 34
generates an output signal voltage or pulse that is time-proportioned to the
deviation signal magnitude and gain control setting. The input signal to the
upper terminal of the modulator comparator 52 comprises the algebraic sum of
two signals represented respectively by curves A and B. The curve A is a
sawtooth wave of a signal taken from the tap on slidewire 53. The curve B ;~
represents the signal appearing at the output of the absolute value amplifier
49. The input to the lower input terminal of amplifier 54 is represented in
Figure 6 by the curve C. This is the dead band signal derived from the slide-
wire 48. The curve D in Figure 6 is a representation of the algebraic sum
of the curves A and B and is shown being compared with the dead band signal
represented by the curve C.
With these input signals applied to the modulator 34, the latter -
produces an output signal that is represented by the curve E in Figure 6.
Curve E, as shown~ is a square wave, and has a low value whenever the curve D ~ ~
has a negative slope or is below the value represented by the dead band -
curve C. Conversely, the curve E has a high value whenever the curve D is
above the value represented by the curve C and has a positive slope.
Upon variation in the magnitude of the deviation signal, there is
a corresponding change in the proportion of the time that the curve E has a
low value compared to the time that it has a high value. The ratio of the
time that the curve E has a low value to the total time of each cycle is
referred to as the "duty cycle." Thus, the duty cycle is variable in accor- ;
dance with the magnitude of the deviation signal from the circuit 32. It
will be apparent that the minimum time in which the curve E has a low value
is the time required for the sawtooth voltage of curve A to drop from its
maximum to its minimum value. This also is the minimum duty cycle achievable
- 16 -
~39~333
with the apparatus of Figure 4. In a typical embodiment of the invention,
tllis minimum time may well be eight m sec., as hereinbefore noted.
The frequency of the sawtooth wave A, and thereby of the square
wave ~ is determined by the setting of the cycle time adjustment slidewire
59 and may be varied over a range of 20 cycles per second to 0.002 cycles
per second. Thus, the range of variation of the period of each cycle is from
0.05 seconds to 50 seconds. `
When the controller is in the automatic mode of operation, a
controller output signal will occur whenever the outp~ts of one of the polar-
ity detectors 50 or 51 and the output of the modulator comparator 52 both are
low. In order to minimize the effect of noise on the deviation signal in
causing output drive signals when the deviation is within the dead band, logic
is incorporated within the modulator 34 to permit one and only one output
pulse to occur within one cycle of the sawtooth generator. This prevents a ;
train of pulses at noise frequency from appearing at the controller output.
This is accomplished by holding the polarity detector on whenever the output
of modulator comparator 52 is low, and clamping the modulator comparator off
following the expiration of low modulator comparator output during the
remainder of a cycle of the wave produced by the sawtooth generator. The ~ ~ ~
clamp is removed at the begi~ming of the next generator cycle. '!''i~''" ''' " '
A desirable characteristic of the controller is the provision for a ~ `
minimum output signal duration of approximately 8 milliseconds (msec.), the -~
approximate time of one-half wave of a 60 cycle alternating current supply.
The power relay to which the controller is connected has sixty cycle, zero-
crossing voltage detection circuitry which assures that the valve motor will
not be actuated unless the output drive signal is coincident with the zero
voltage crossing point of the power line. Without the minimum of 8 m sec
pulse duration, the controller settings could be such that a small error would ~;~
result in a very short (much less than 8 m sec) periodic output pulse which
might take considerable time to coincide with the power line zero voltage
~ 17 -
: , , :: :- - - . . , . ;
: ... , :. -. .. :- ~. : -. , -- . .- : -: ~
it~39~it3;~ `
point. For this reason, when the minimum output pulse width is made to be 8
m sec, the probability is great that the first controller output pulse will
trigger the power relay into action to reduce the deviation of the process
variable signal from the set point. The ramp comparator output of sawtooth
waveform generator is a two-state signal. It is in the low state for approxi-
mately 8 m sec when the sawtooth waveform is on its downward excursion and is
in the high state for the remainder of a cycle period when the sawtooth is on
its upward ramp. When the ramp comparator 56 is in the low state, its output
clamps the non-inverting terminal of the modulator amplifier 54 low, forcing
its output to the active (low)state. The appropriate polarity detector 50 or
51 is automatically clamped in the active state by the individually associated
OR gate 71 or 72 when the output of the modulator comparator 52 is low. The
minimum width of any output pulse is therefore approximately 8 m sec. Th~is
minimum width of output pulse is desirable when the alternating current supply
source is 60 cycles. When the alternating current supply source for motor 12
is a frequency other than 60 cycles, the minimum output pulse duration pre-
erably corresponds to the time of one-half wave of such source, for example, i~
10 m sec with a 50 cycle source.
There is shown in Figure 5 the computer interface and output cir-
cuit portion of the control station 10 illustrated in block form in Figures
1 and 2. This output portion includes AND gates 80, 81, 82, 83, 84 and 85
and OR gates 86 and 87. ~ ~ -
In the operation of the apparatus, the output signals from the -
polarity detectors 50 and 51 and from the modulator 34 drives the gates 80
and 83. Specifically, depending upon the polarity of the error signal at the `
output of rate amplifier 32, one or the other of the polarity detectors 50 ~ ` -
and 513 because of their inverse or reverse input connections, will be in a ~ -
low state and apply a low output voltage by way of an associated terminal 76 ~ `
or 77 to one input ~erminal of an individual associated AND gate 80 or 83.
This relationship, as previously noted, may be reversed by operation of the -
- 18 -
... . . .. . ... .. .
1~39~333
Direct/Reverse Switch 35. Such low voltage applied to a first input terminal
of an associated gate 80 or 83 will continue as long as the error is in the
same direction and is greater than the dead band value. The time-proportioned
low voltage signal then appearing at the output 73 of the modulator 34 is
applied to a second input terminal of each of said gates 80 and 83, but will
be effective to open only that one of the gates 80 or 83 that then also has a
low voltage applied to its first input terminal. This action occurs, however,
only if high voltage then also appears on a third input terminal of each of
said gates 80 and 83, as is further explained hereinafter. The time that a
gate 80 or 83 is so held in an open state is in accord with the time modula-
tions or proportioning action of the modulator 34.
As seen in ~igure 5, the third input terminal of each of the gates
80 and 83 is connected to the output of individually associated OR gates 88
and 89. One input terminal of the OR gate 88 is connected to the output of an
AND gate 90. Similarly, one input terminal to OR gate 89 is connected to the
output of an AND gate 91. The other input terminal of each of the OR gates `
88 and 89 is connected to an "AUTO MODE" terminal. One input of each of the
AND gates 90 and 91 is connected to a "DDC MODE" terminal, and the other input
of each of said AND gates is connected to a "SHED-AUTO" terminal.
One input to each of AND gates 81 and 84 is a "manual" signal, the
signal in the case of AND gate 81 being a "MANUAL CLOSE" signal and ~he signal
in the case of AND gate 84 being a "MANUAL OPEN" signal. The other input - `
terminal of each of said AND gates 81 and 84 is derived from an OR gate and an
AND gate. Thus, the second input to AND gate 81 is derived from the output - ~`
of an OR gate 92, a first input to which is a "MANUAL-MODE" signal and the
second input to which is the output of an AND gate 93. Similarly, the second -~
input signal for AND gate 84 is derived from the output of an OR gate 94. A
first input for the OR gate~94 is a "MANUAL MODE" signal and the second input
is derived from the output of an AND gate 95. Each of the AND gates 93 and
95 have two input signals, one being "DDC MODEI' and the other input being
- 1~ -
` ~ "'; - ~ ; `'.' '
1at;~9833
"SHED MANUAL".
The AND gates 82 and 85 each have four inputs three of which are ;~
respectively "DDC MODE", "DDC OVERRIDE" and ''S~iED.I' The fourth input to
AND gate 82 is "COMPUTER C~OSE," and the fourth input to AND gate 85 is
"COMPUTER OPEN."
As shown in Figure 5, the outputs of AND gates 80, 81, and 82 are
connected to an individually associated input of OR gate 86. Similarly, the
ouputs of AND gates 83, 84 and 85 are connected to an individually associated
input of OR gate 87. The arrangement is such that upon the application of a
signal to any one of the three inputs of each of the OR gates 86 and 87, the
gates will be in an open state. Opening of gate 86, for example, will produce ~`
a control effect resulting in a time-proportioned d-c pulse in one direction
in transmission line 17a, 17b and consequently in operation of the electric
motor actuator 12, as seen in Figure 1 for operation in one direction, for
example, for closing the control valve 13. Similarly, the opening of the OR
gate 87 will result in a d-c pulse in the opposite direction in line 17a, 17b
and in energization of the electric motor actuator for operation in the ~ ;
opposite direction. This direction of actuation of the electric motor ~
actuator may be reversed, as previously noted, by manipulation of the DIRECT/ - -
REVER~E SWITCH 35.
With the apparatus in the "AUTOMATIC MODE" the time that the gate
86 or 87 is so held in an open state is in accord with the time modulations or
proportioning action of the modulator 34, and in accord with the deviation of
the process variable signal from the desired set point.
The output circuits of OR gates 86 and 87 are connected respective-
~.:
ly to the input circuit of a bi-directional constant current source control
transistor pairs indicated at 99 and control transistor pairs 100, 101, and ;~
102~ 103 whereby upon deviation of the process variable signal in one direc-
tion and actuation, for example,of gate 86 to an open state, transistor 100
is rendered conductive. A current pulse is then transmitted to an output
- 20 - ` ;
.
~039~33
terminal 104 of the control sta~ion 10. This current is transmitted over line
17a to the remotely located power relay 11 through an optical isolator 121,
as illustrated in Figure 7, and then transmitted back ove~ wi~e 17b to a
terminal 105 of control station 10. The current pulse is then conducted by
transistor 103 to ground. ~hen the deviation of the process variable signal
is in the opposite direction, a current pulse is transmitted through transis-
tor 102 to the control station output term:inal 105, then over wire 17b to the
power relay 11 and through a second optical isolator 122, back over wire 17a
to the control station output terminal 104, and through the transistor 101 to
ground. This current signal in each case may have a nominal value of 10
. .:
milliamperes, with the voltage between the terminals 104 and 105 of the con~
trol station 10 floating between the source voltage and ground. The current `~
pulse signals through the transmission line 17a and 17b are time-proportioned
bidirectional d-c pulses. The transmission line, as noted, is a low-voltage,
low current line. The control station output circuitry illustrated in Figure
5 comprises a computer interface and error polarity responsive output switch.
Thus, there is combined in the outp~t switch circuitry, information from the
control transfer switch lever lOf, as seen in Figure 1, the modulator 34, as
. .. .
seen in Figure 4, and the computer interface circuitry as seen in Figures 2
and 3. This circuitry controls the bidirectional current source 99 in accor-
dance with the polarity and amplitude of the error signal and establishes the
appropriate duty cycle that should be transmitted. The operation o~ this
circuit is further explained by the following description. -
The output switch 86, for example, will transmit a signal to the
power relay 11 for that energization of motor 12 required to ef~ect a valve
closure, under the following three conditions: ~ ~
1. The control transfer switch-lever lOf is in AUTOMATIC COMPUTER ~ ~;
SHED to automatic in DDC MODES, and
a. The positive error polarity detector 51, for example, is
activated, and
~)39833
b. The output of modulator comparator 52 produces a pulse calling
for drive.
2. Mode switch in MANUAL or COMPUI`ER SHED to manual in DDC MODES3 and
a. A manual close switch 10g, for example, as seen in Figure 1, is ;
depressed. ` ~
3. Mode switch in DDC mode, and ~ :
:
a. DDC OVERRIDE not active,
b. COMPUTER SHED not active,
c. Computer transmits a valve close signal.
The output switch 87, for example, will transmit a valve open `
signal under the following three conditions. '
1. Mode switch in AUTOhlATIC or in DDC with COMPUTER SHED to automatic,
and a. The negative error detector, polarity detector 50, for example, ~ ;
- is activated, and -``
b. The output of modulator comparator 52 produces a pulse calling
for drive.
2. Mode switch in MANUAL or in DDC with COMPUTER SHED to manual, and
a. A manual open switch, for example, switch 10, Figure 1, is
depressed. `
3. Mode switch in DDC MODE, and
a. DDC OVERRIDE not active; and
b. COMPUTER SHED not active; and
c. Computer transmits a valve open signal.
The power relay 11 as shown in the block and sehematic diagram of
Figure 7, includes a pair of semi-conductor switches, for example, triacs, 108
: .
and 109. Each of the triacs includes a pair of principal electrodes, and a
control or gate elec~rode. Triacs 108 and 109 may be high-current rating n~
triacs, for example, RCA types 40927. The triacs 108 and 109 are connected
by lines 110, 111 and 112, respectively, in circuit with motor 12 and an
alternating current source of power indicated at 113. The principal elec- -
~:
- 22 - ~
` . .:~ -
~39833 `:::
trodes of triac 108 are connected between a first terminal of power source 113 -
and a first *erminal of a first winding 11~ and motor 12. The principal
electrodes of triac 109 are connected between said first terminal of source
113 and the first terminal of a second winding 115 of motor 12. A capacitor
116 is connected between said first terminals of said winding 114 and 115.
Both of the other terminals of said motor windings are connected together and
by line 112 to the second terminal of source 113. Lines 111, 112, and 113
may be heavy-duty power lines that are capable of carrying with little or no -~
loss the heavy currents that may be drawn by the motor 12 from source 113. ; ~;
With this arrangement, when triac 108, for example, is rendered
conductive by proper actuation of its gate circuit, alternating current flows
directly through motor winding 114 and in series with capacitor 116 and wind-
ing 115 to establish a rotating field in the motor 12 that produces rotation
in one direction. When triac 109 is rendered conductive, the capacitor is
placed in series with motor winding 114 and a rotating field is established ;~
that produces motor rotation in ~he opposite direction.
Triacs 108 and 109 are selectively fired by triggering pulses that
are adapted to be applied to their respective gate electrodes by an electronic
control section indicated in block form at 118. Typically, the power relay -
: ,.
including the electronic control section 11~, shown in Figure 7, is of the ;
type disclosed in detail and claimed in the copending application of Homer L. ;
Greer bearing Serial No. 216,438 and filed on even date herewith. As in the -~
power relay disclosed in said Greer application, the arrangement is such that
gating voltage pulses can be produced at gating terminals 119 and 120 of the
control section 118 only each zero cross-over of the alternating voltage across
the principal electrodes of the triacs 108 and 109. To this end, means are
provided for sensing conditions of zero voltage and current across each of the -
triacs. These means include a connec~ion from the common junction of the
triacs 108 and 109 to an input terminal 130, and individual connections from
the other terminals of the triacs to respective input terminals 131 and 132,
- -: . .
833
of the control section 118.
The control section 118 is arranged selectively in response to the
time-proportioned direct current pulses in transmission line 17a and 17b, to
produce gating pulses for the triacs 108 and 109 at gating terminals 119 and
120. Such triggering pulses are produced as a result of the selective
actuation of optical isolators 121 and 122 by the current in the transmission
line. Specifically, with d-c pulses in one direction in transmission line 17a, -~
17b, optical isolator 121, for example, responds to activate circuitry in
control section 118 to produce a gating pulse at terminal 119 to fire triac
108, thereby to energize motor 12 to rotation of one direction.
With d-c pulses in the transmission line 17a, 17b in the opposite
direction, the optical isolator 122 similarly responds whereby a gating pulse
is produced at terminal 120 to fire triac 109. Such gating pulses can occur, ~`
as noted, only at the time of a zero cross over of the alternating voltage
across the principal electrodes of the triac that is to be fired. The control
section 118 also preferably includes provisions, as illustrated and described
in said Greer application, for inhibiting operation of the gating circuit in -
response to demands from the control station 10 for reversing direction of
the rotation of the motor 12, which commands occur too rapidly for the triacs
108 and 109 properly to respond without tending to cause damage to them. Such
inhibiting provisions desirably also include means to delay the response of
the triacs 108 and 109 to the commands received from the control station 10
whereby to avoid further firing of them when their response is not in accord
with the commands. ;~
The gating pulses at the output terminals 119 and 120 of the elec-
tronic control section 118 desirably have a time duration of approximately
1/2 millisecond, a time su~ficiently long to insure that the triac is above
the minimum switching current for the triac. Accordingly, in the apparatus
shown in Figure 7, the motor 12 is supplied by source 113 with periodic or
intermittent pulses of alternating current power. The periodicity of these
- 24 -
.-. . . , , , . , ~
~()39833
pulses is determined by the time-proportioning of the low voltage-low current ;
signal in the trans~ission line 17a, 17b, and hence, by the frequency of the
sawtooth ~ave produced by the generator 33. These power pulses, moreover,
are modulated in time duration as a function of the deviation in the process
variable signal from the set point. In other words, the motor energization
or duty cycle is varied as a function of time. As the process variable
deviation increases the motor duty cycle increases, that is, alternating
current power pulses are supplied to the motor for a longer time, and vice
versa. The direction of motor rotation is determined by which one of the
triacs 108 or 109 is activated to produce the said power pulses.
The optical isolators, as seen in Figure 7, are light-operated
switches and may be of the type known commercially as the Monsanto MCT 26 ;
photo transistor opto-isolator. Each of the optical isolators 121 and 122
includes a light emitting diode (LED) indicated at 123 and 124, respectively,
and individually associated photo transistors respectively designated 125 and "
126. The light emitting diodes 123 and 124 are each connected to receive the ~`
ti~e~proportioned current signals transmitted from the control station 10 over
the transmission line 17a, 17b. The arrangement is such that with current
flow in one direction only one of the LED's 123 or 124 emits light to activate
its associated photo transistor 125 or 126. Specifically, with the output ~;
terminal 104 of the control station positive with respect to terminal 105, -~
and hence current flow over transmission line 17a in the direction toward the
power relay 11, LED 123 emits light to ac~ivate its associated photo tran-
sistor 125. With the polarity at the control station output terminals 104 and
105 reversed~ and hence, current flow over the transmission 17a and 17b in ~~
the opposite direction, LED 124 emits light to illuminate and thereby activate
the photo transistor 126. Diodes 127 and 128 individual respectively to LED's
123 and 124 are protective devices to protect the optical isolators 121 and
122 against damage that might result because of the application of reverse ~`
voltage to the LED's 123 and 124. Such reverse voltage can occur upon the
- 25 -
:
8~33
opening, for example, of limits switches 133 and 134 that, in an industrial
application, normally are provided to prevent further valve adjustment when
the latter has reached its extremes of travel. Thus, limit switch 133 has
been provided to open and hence preclude further opening valve adjustment, and
limit switch 133 has been provided to open and preclude further closing valve
adjustment.
Referring to Figure 5 there is further provided, in accordance
with the present invention, a circuit for signalling an open circuit condition
of the transmission line 17a, 17b. Such open circuit condition, as noted,
occurs when the final valve under adjustment has reached its extremes of
travel. Open circuit conditions may also be established, as further described
herein, if a faul~ should occur in the power relay or motor energi~ing circuit.
In addition, open circuit condi~ions may result because of damage to the
transmission line 17a, 17b which usually in the application of the apparatus~ ;
would pass through an industrial environment, and hence3 possibly be subject
to damage and open circuit.
Accordingly, there is provided, shown in Figure 5, a circui* that `~
will respond to an open transmission line condition to actuate an alarm lamp ~-
or other annunciator to call the attention of an operator to the open trans- ~`
2Q ~ission line condition. This circuit includes a pair of transistors 140 and ; -~
141, a pair of diodes 142 and 143 and an alarm lamp indicator 144. Diodes
142 and 143 are connected in back-to-back relationship across the control
station output terminals 104 and 105. The junction of the cathodes of the
diodes is connected to the base of transistor 140, the collector of which is
connected by resistor 145 and 146 to ~24 VDC. A reference voltage Vl is
applied to the emitter of transistor 140. The junction of the resistor 145
and 146 is connected to the base of transistor 141. The collector emitter
circuit of transistor 141 includes a connection from the 24 volt to the ;~
emitter and the collector of transistor 141 and the alarm device 144 to
ground.
- 26 -
1~3~833
With this circuit arrangement, the voltage Vl applied to the
` emitter of transistor 140 is so selected that normally transistors 140 and
141 are not conducting. Upon an open circuit condition developing in the
-~ transmission line 17a, 17b, the voltage of terminal 104 or 105 will rise de- -
pending upon the direction of the command signal, which rise in voltage will
be sensed to render transistor 140 conductive. This renders transistor 141
.
conductive which permits current flow from the voltage source through the
alarm device 144 for actuation of the latter. Alarm current will be pulsed - -
simultaneously with the pulsing current at the output terminals 104 and 105
of the control station lOo If a semiconductor device such as SCR is employed
in lieu of the transistor 141, the alarm may be made to be continuous until
:~.
the device is reset by momentary interruption of the SCR current.
As noted, if desired the alarm circuit disclosed in Figure 5 may
also be made to respond to failure conditions that may occur in the power
relay 11, the motor 12, or the triac actuating circuit therefore. Thus, if a
~ault should occur in the power relay or motor energizing circuit, such
condition could be made to cause a relay 147, as seen in Figure 7, to open a
normally closed contact 148 connected in series with the wire 17a of the
transmission line, in the power relay. Such opening of the transmission line
will result in actuation of the alarm 144 at the control station in the same
manner as previously described upon an open circuit condition occurring in
the transmission line 17a, 17b.
Thus, there has been provided in accordance with the present inven- -
tion an improved industrial process control system including a two-wire low
energy level transmission line for transmitting direction and speed signals
from a proportional speed controller to a remotely located and electrically
isolated power relay for the regulation of a final valve operator, without
position feedback, for maintaining an industrial process at value, which
system features signalling means at the controller indicating that the final
valve operator has been adjusted to one or the other of its extreme positions~ ~
-
- 27 -
, - , . -- . : ~ . .
.. .. ~ . . . ,~
.: , - . , . . -
1~39833
the occurrence of a fault in the power relay and/or the valve operator ener- .,;.
gized thereby, or an open circuit condition of the transmission line.
Subject matter disclosed but not claimed in this application is
disclosed and claimed in the copending application bearing Serial No. 216,438
filed On even date herewith by HD~er L. Greer. '~
~,
`"`
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- ,
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- 28 -
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