Canadian Patents Database / Patent 1257645 Summary

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Claims and Abstract availability

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(12) Patent: (11) CA 1257645
(21) Application Number: 578623
(52) Canadian Patent Classification (CPC):
  • 318/98
(51) International Patent Classification (IPC):
  • H02P 29/024 (2016.01)
  • B25B 21/00 (2006.01)
  • H02P 23/00 (2016.01)
  • H02P 25/14 (2006.01)
(72) Inventors :
  • BRADUS, ROBERT (United States of America)
(73) Owners :
  • BLACK & DECKER INC. (United States of America)
(71) Applicants :
(74) Agent: MACRAE & CO.
(74) Associate agent:
(45) Issued: 1989-07-18
(22) Filed Date: 1985-03-04
(30) Availability of licence: N/A
(30) Language of filing: English

(30) Application Priority Data:
Application No. Country/Territory Date
592,809 United States of America 1984-03-23

English Abstract


A microprocessor based motor controller which
provides open loop speed control at low conduction angles,
closed loop speed control at high conduction angles, and a
smooth transition between open loop and closed loop zones.
In open loop, the motor speed is selected and is permitted
to vary with applied load. In closed loop, the motor speed
is held constant, substantially irrespective of load. In
the transition zone, the motor is operated in a hybrid open
loop, closed loop fashion. Anti-kickback protection is
also provided based on a percentage change in the motor's
rotational period.

Note: Claims are shown in the official language in which they were submitted.


1. A method of detecting an impending kickback condition in a
motor driven tool comprising:
a) determining a first value indicative of the
rotational period of said motor during a first time interval;

b) determining a first limit value based upon a
predetermined percentage of said first values;

c) determining a second value indicative of the
rotational period of said motor during a second time interval; and

d) producing a predetermined response if said second
value exceeds said first value by at least said first limit value.

2. The method of Claim 1 further comprising:

adding said first limit value to said first value to

produce a first test value;
comparing said first test value with said second value;
producing said predetermined response if said second
value exceeds said first test value.


3. The method of Claim 2 further comprising determining a second
test value by determining a second limit value based upon said
predetermined percentage of said second value and adding said second limit
value to said second value.

4. The method of Claim 3 further comprising determining a third
value indicative of the rotational period of said motor during a third time

comparing said second test value with said third value;

producing said predetermined response if said third
value exceeds said second test value.

5. The method of Claim 1 further comprising comparing said first
value with a predetermined sensor limit value and disabling said
predetermined response if said first value exceeds said sensor limit value.

6. The method of Claim 1 further comprising delivering an
alternating current to said motor in a succession of half cycles of
alternating polarity; and wherein said first value is determined during a
first half cycle and said second value is determined during a later half

7. The method of Claim 6 wherein said step of producing a
predetermined response is performed at least once during each successive
half cycle.

8. The method of Claim 1 further comprising determining a
kickback sensitivity value and wherein said first value is determined in
proportion to said kickback sensitivity value.

9. The method of Claim 8 wherein said kickback sensitivity value
is determined in accordance with at least one preset conductive path.

10. The method of Claim 1 wherein said step of producing a
predetermined response includes interrupting the delivery of power to said

11. The method of Claim 10 wherein said step of producing a
predetermined response further includes waiting for an instruction from the
operator of said tool and continuing to interrupt the delivery of power to
said motor until said instruction is received.

Note: Descriptions are shown in the official language in which they were submitted.

~ ~ ~ 7 6 ~ ~

The present inventlon relates generally to power tools and
electrical motor controllers for 6uch tool~. More particularly the
invention relates to a microprocessor-ba6ed or microcomputer-based control
method for detecting an impending kickback condition in a
motor driven tool.

D~scription of the Prior Axt
In oontrolling the 6peed of an electric motor for u~e in power
tools, it i~ now generally known to u~e gated electronic pcwer
controlling device~, such a6 a SCR'~ or triacs, for periodically
transferring electrical e~ergy to the motorO Many popular power tools
employ universal notors which are readily controllable using such gated
controlling devices.
Generally speaking, gated ~peed control circuits work by
~witching the motor current on and oEf at periodic intervals in relation
to the ~ero cros~ing of the a.c. current or voltage waveform. These
periodic interval~ are caused to occur in ~ynchronism with the a.c.
waveform and are mea~ur~d in terms o~ a conduction angle, neasured as a
nu~ber of degrees. qhe conduction angle determlnes the point within the
a~c. waveform at which electrical energy is delivered to the motor. For
example, a conduction ~le o~ 1 ~ degrees per half cycle corresponds to a
condition of full conduction, in which the entire, uninterrupted
alternating current is applied to ~he mo~or. Similarly, a 90-degree
conduction angle corresponds to developing the fiupply voltage acros~ the
motor connencing ln the middle o a given half cycle and thus corresponds
-- 1 -- ~.~


to the delivery of approximately hal~ of the available energy to the
motcr. Conduction angles below 90 degrees corxe~pond to the transfer of
even lesser quantities of energy to the motor.
~ otor ~peed control circuits of the prior art have employed
gating devices to alter the conduction angle in order to deliver a
predetermuned amount of energy to the motor, and to thereby achieve a
predetermuned motor speed. With univer~al motors, which are commonly used
in power tool6, motor speed is also related to the load placed on the
notor. That is, under no load the motor delivers one given ~eed (the no
load speed) and under load, the motor speed decreases as the load
increase~. The inver~e relationship between speed (R.P.M~) and load
(torque) at various conduction angles for a given motor may be expressed
graphically as a family of curves in a speed-torque diagram.
One scheme for controlling motor 6peed 6imply selects a desired
no load speed by selecting the appropriate conduction angle. The speed
control circuit is of an open loop configuration, which mæan6 that no
speed sensing mechanism is used to provide a feedback signal for
maintaining the desired ~peed as the load is varied. Thu6 the open loop
m~tor ~peed control circuit is capable of providing a p~eselected no load
speed, but has no m~chanism for holding ~peed constant under a changing
load. In open loop, the motor ~peed will dimunish in accordance wi~h the
speed-torque relationship as a load is applied to the toolO In the hands
of a skilled operator, the open loop configuratlon Frovides a tool in
which the power demands, and potentially destructive overheating
conditions, can be Eensed ~y the decrease in motor 6peed. Bcwever, ~uch
configurations do not provide for constant speed operation.
In contrast to the open loop configuration, ~ome motor ~peed
control circuits are de~igned as a closed locp configuration. In a closed
loop configuration neans are provided for ~ensing either the rotational
speed of the motor or the current drawn by the motor to pravide a feedback

~ 2 ~7~5
signal indicative of actual ~otor 6peed. m e feedback signal is compared
with an operator selected desired ~peed to determine an error sig~al7 The
error signal i6 then used to ~peed up or slow dbwn the motor BO that a
substantially constant rotational ~peed 1~ achieved. ~hile clo~ed loop
motor speed control configurations offer the ability to operate a motor at
a relatively constant speed, to a large extent independent of the load
placed on the m~tor, they are not without problems.
~ ne significant problem with clo6ed loop motor ~peed control is
the potential for overheating the motor under heavy loads at lcw speeds.
Present day power tools use cooling fans, driven by the motor armature for
dissipating heat generated by the motor. Such cooling fans ~ecome
gradually less efficient as motor speed dimini6hes, to the point where
overheating can become a significant problemO In a closed loop
configuration, a pcwer tool can be quite readily overheated when a desired
speed corresponding to an armature speed in~ufficient to develop
efficient fan cooling (e.g. below 10,000 R~ is Eelected. Specifically,
if the pcwer tool is placed under a heavy load, the motor ~peed control
circuit will increase the conduction angle, as the load on the motor is
increased, in an effort to ~aintain a constant speed~ is causes
increasingly higher currents to flGW through the windings of the motor
with a dramatic ri&e in temperature. ~ithout adequate fan coolLng the
tool quickly overh~ats which may cau æ permanent damage to the tool's
lubricant-impregnated bearings or other comçonents. Even in the hands of
a skilled operator, it may not be readily apparent that an overheating
condition is taking place until it i8 too late. The constant low
operating speed can give a false impression that little pcwer is being
delivered to the motor, even when the power is ln fact quite high due to
the operaLion of the cloEed loop sFeed control circuit. In this state,
overheating and damage can occur quite rapidly. Thermal protection
circuits and over current protection circuit~ are kncwn for combating the

o~rerheating problem, however, in order to fully protect agains~
overheat~ng, the sensitivity of these circuits must be high and thus quite
often will falsely trigger a m~tor ~hut ~bwn when the operator is only
momentarily overloading the tool, without any danger of permanent damage
to the tool.
Another feature which is present in more ~oFhl~ticated motor
speed control circuits is an anti-kickback feature for removing pcwer from
the tool when an imminent kickback ~ituation is detected. Generally, the
kickback condition corresponds to a very rapid change in load, ~uch as
might occur when the tool grabs or 6eizes in a work piece, causing a
backward thrust of the work piece or tool. Kickback problems are m~st
significant with power tools which develop high torque. Several
anti-kickback detection schemes have been proposed. One such anti-kickback
~cheme involve~ monitoring the rate of change in motor current, while
another ~cheme involves monitoring the rate of change of motor ~peed. An
example of a ~y6tem which employs a rate of change of motor current
detection scheme may be found in U.S. Pat nt Nb. 4,249,117, to Leukhardt,
issued February 3, 19~1. An example of a rate of change of motor spe~d
detection scheme may be found in ~. S. Patent Nb. 4,267,914, to Saar,
i6sued May 19, 19~. Both of the above noted patent~ are assigned to the
assi~nee of the pre~ent invention.
While both kickback detection schemefi have Froven useful, it has
heretofore been difficult to adapt such schemes to a wide range of
operating ~peeds. In order to have ~ufficient ~ensitivity at higher
operating speeds, the kickback ~ensing circuitry of the prior art may
produce false kickback detection6 at lower operating ~peeds. Mbrecver,
it has not heretofore been po~sible to readily a~apt one kickback
detecting sch~me to a wide variety of power tool~. In thl~ r~gard, heavy
duty half-inch drill~, for example, have a high gear ratio and generate a
lot of torque. For ~uch drill~ a high kickback sensitivity is desirable.

~ever, for quarter-inch ~ills, ha~e a relatively low gear ratio and 20
not generate a lot of to~ue, rapid speed variations with c~nge in loads
are com~on and therefore the kickback sensitivity ~ould be lcw. Prior
art kickback detection schemes are not readily adaF~ble to different
sensitivity settings for use with such broad ranges of tools.

Summary of the Invention
The present invention in general provides an anti-
kickback system which reacts to the percen-tage change in
motor speed to provide sufficient sensitivity at high speeds
without being overly sensitive at low speeds. The anti-
kickback system i.s readily adaptable to different sensitivity
settings for use with a broad range of power -tools.

This application discloses control apparatus
and a method for controlling a motor operable over a range of
conduction angles. Ihe 6peed-to~ue operating characteristics of the
motor are divided or ~regated into various operat~g zones in order to
effect a combination open loop/closed loop configuration. A first
operating zone is defined, corresponding to cGnduction angles below a
predetermined first angle. A second operating zone is defined,
corresponding to conduction angles between the first conduction ~le and
a predetermined second conduction angle greater than the first anyle. A
third operating zone is defined, corresponding to conduction angles
gr~ter than the second conduction angle. One oE the above operating
zones is selected, and -the motor operated according to the
following steps.
If -the first zone is selec-ted, the rnotor is operated in an open


~ L~ 7~ 5
loop coniguration.
If the second zone is ~elected, the motor iB operated in a
hybrid configuration whereby the conduction angle i~ varied in relation to
the load to maintain a predetermined constant speed, BO long as the
required conduction angle does not exceed the ~elected conduction angle.
In other words, the motor i~ operated in a limited closed loop fashion for
selected conduction angles below the predetenmmed second angle. As loads
continue to increase, however, the motor ~peed is not held constant, but
rather is Fermitted to decrease in accordance with the characteristic
speed-torque relationship of the motor.
If the third zone is selected, the motor is operated in a closed
loop configuration. In the third zone the conduction angle selected is
interpreted as a desired operating speed, and the motor is operated at
that desired speed until the power capability of the motor i5 reached.
Selection of one of the operat mg zones is made ~y the operator
of the tool (through the use o~ a manually operable trigger or the like)
by providing an analog signal corresponding to a selected conduction
angle. In the first operating zone the ~elected conduction angle is less
than the first conduction angle and the ~otor is operated at the selected
conduction angle, which remains con~tant, while ~he ~peed of the motor is
allowed to vary in accordance with the load applied. In the second zone
the selected conduction an~le is less than the second conduction an~le and
greater than the first conduction angle, and the motor i8 operated at a
predetenmined rotational speed corresponding essentially to the no load
operating speed of the motor at the first conduction angle. In this
~econd zone, the conduction angle is automatically increased or decrea5ed
to maintain the Fredetermined speed, so long as the required conduction
angle doe~ not exceed the selected conduction angle. If the load is
increased to the point where the conduction angle reaches the selected
~onduction angle, the conduction angle i6 held at ~he selected conduction

~ S 7~;f~5
angle and motor speed is permitted to thereafter decrease with further
increases in load. In the third zone the selected conduction an~le is
-greater than the ~econd conduction angle and is interpreted as a desired
speed instruction. ~his desired fipeed is held constant while the
conduction angle is permitted to vary as required to maintain the constant
The present m~thod and apFaratus disclosed further provides for
the detection of an Impending kickback condition by determining a first
value indicative of the rotational period of the motor during a first time
interval. A first limit value is determined based up~n a percentage of
the first value. A second value, indicative o the rotational period of
the motor during a second time interval, is then determined. If the
~econd value exceeds the first value by at least the first limut value, a
predetermm ed response is produced. Mbre ~pecifically, the fir~t limit
value is added to the first value to produoe a first test value, and the
first test value i5 compared with the ~econd value. If the econd value
exceeds the first test value the p~edetenmined re~pon6e is produoe d. m e
predetermuned response typically includes removing or interrupting the
delivery of power to ~he motor, and may further include initiating a brake
routine to decrease the rotational ~peed of the m~tor. In additionf the
present invention includes a ~afety provision whereby once power is
interrupted during ~he anti-kickback routine, it remains interrupted until
an instruction from the operator 1~ receivedO ml8 inEtr~ction may ~e,
for example, a re~etting action taken by relea~ng the manually operable
trigger to its off position~
For a further understanding of the invention, a~ well as its
objects and advantages over prior art motor controllers, re$erence may be
had to the following ~pecification and to the accompanying drawings and
flcw charts.

;~ie~ crip~iOR oi~
Figure 1 i~ a schemstic cirruit diagram of the microccmputer-based
control circuit;

Figure 2 i~ a graph of ~he ~peed V5. torque curves for a mDtor,
illustrating the various operating zones;

Figure 3 is a flow chart lllustrating ~he ~teps for implementing
the ~ombinational oFen loop/closed loop method of controlling a motor;

Figure 4 i5 a flow chart diagram illu~trating a preferxed method
of obtaining an analog signal indicative of a desired operating parameter,
useful in implemRnting the invention~ and
Figure 5 is a fl~w chart diagram illu~trating the anti-kickback
detection and response producing method of the invention.

Referring to F~gure 1, a circuit diagram of an
electronic control circuit is shown. The c~ntrol circuit
compri~es microcomputer 10, which in the preferred embodiment is an
MC146 ~5FZ ~ingle chip, 8-bit microconputer unlt (MCU), containing an
on-chip oscillator, CPU, RA~ ~OMb I/O, and IIMER. Although the preferred
embodiment described herein discloEes a mucroc~mputer implementation, it
i~ to be under~tood that the teachings of the pre~ent invention nay also
be implemented utiliziny other forms of digital circuitry, such as
di~crete digital logic integrated circuits.
The microcomputer 10 receive6 p~wer through a power ~upply
circuit 12, which converts the 115 volt to 120 volt a.c. lnput signal to
t5 volt DC ~ignal~ ~n ~0 KHz. re~ona~or 14 is coupled to the oscillator
terminals (pins 4 and S) to provide a ~table clock for operating the

mucroc~mputer 10.

~ 76~5
Microcomputer 10 is provided with a fir~t group of eight
input/output lines comprising port A and a ~econd group of eight
input/output lines compri6ing port B. In addition, microcomputer 10
includes a third group of four line6 compri~ing port C~ The ~tate of ~ach
line comprising port A and port B i~ ~oftware programmable. Port C i~ a
fixed input port. In Figure 1 the lines ccmprifi mg ports P~ B and C are
identified by the alpha numeric designation PA5, PB0, PC2, znd ~o forth,
wherein the number refer6 to the binary line number (0 7) and the letter
(A, B, or C) is the port designation.
Microcomputer 10 al~o includes a reset terminal, designated
RESET, a maskable interrupt request terminal, designated IR2, as well as
the usual power Eupply connection terminal~ VDD, and ~S- The terminals
designated TIMER and NUM are tied to Vss, which is a floating ground.
Ihe invention further cnmprises a ~ignal processing circuit 20
which provides the functions of rectification, power on re~et controlD
gate current control, and ~peed signal conditioning. Signal processing
circuit 20, which is described more fully below, provides a ~peed 6ignal
to the interrupt request line I~Q of mucrocomputer 10. Signal processing
circuit 20 al~o provides a reset ~ignal to the RESET terminal of
microcomputer 10. In turn, signal processing circuit 20 receives a triac
fire signal from microcQmputer 10. In re~ponse to ~he triac fire Eignal,
circuit 20 provide~ a gating signal on lead 21 to the triac device 22
which controls the flGw of power to motor 23. A tachometer, or equivalent
motor speed EenSing device is positioned to determine the rotational speed
or rotational period of the armature of tor 23, Tachometer 24 produces
a sinusoidal signal the frequency of which i5 indicative of the rotational
speed or rotational period of the motor 23. Thi8 signal is pr w ided to
signal processing circuit 20 which conditions the ~ignal and applies it to
the interrupt request termunal IF~ for further proceEsing by microcomputer
10 aE di~cussed ~elcw.

Sigral processing circuit 20 includes a rectification circuit 62
coupled between node 63 and floating ground 64. ~ectification circuit 62
may be implemented with a diode poled to conduct ~urrent in a direction
from ground 64 to node 63, thereby placing node 63 ~ubstantially at (or at
least one diode drop belcw) floating ground potential. Signal processing
circuit 20 further includes a gate control circuit b6r preferably
comprising a current Ewitch, for supplying a current 6ignal for firing
triac 22 in response to the triac fire signal from microccmputer 10. Gate
control circuit 66 thereby isolates microcomputer 10 fr~n triac 22 while
supplying the necessary current for triggering the triac. Signal
processing circuit 20 further includes a speed signal conditioning circuit
68 such as a Schmitt trigger comparator circuit for supplying fast rise
and fall time pulses to microcomputer 10 in response to the comparatively
slow rise and fall time sinusoidal signal output of tachometer ~4. Signal
procefising circuit 20 also provides a pcwer on reset control circuit 70
which is coupled to the V~D term mal of power 8upply 12 to provide a reset
signal to microcomputer 10 upon initial power up.
Included within pcwer supply 12 is a diode 72 which is ~oupled to
t~rminal PA5 of microcomputer 10 to provide a zero cros~ing detection
signal. When line 74 of supply 12 is positive with respect to the
opposite side of the a.c. ~upply line, current flows through resistors 76
and 77 and diode 78. Nbde 63 iB thus at one diode drop below float mg
ground potential, and tenninal PA5 assumes a logical LO ~ate. When l me
75 goes positive during the next half cycle, diodes 72 and 78 block
current flow. Hence there is no voltage drop across resistor 76 and
terminal PA5 is at V~D potential to assume a logical HI ~tate. It will be
seen that terminal PA5 is thus toggled between alternating LO and ~]I
~tates in synchronism with each half cycle of the a.c. wavefonm and may
thus be u~ed to determune when each zero crossing occurs.
Ihe present invention provides a motor ~peed controlling device

-10- ,

~l2 ~7~

wnich may be utilize~ with a number of different t~pes and ~izes of tors
'n à wide range of different power t~ol application~. In order to preset
the operating characteristics of the circuit to correspond to
predetermined operating parameters or to a predetermined power tool, an
option strap arrangement, designated generally by reference numeral 26~ is
provided. Certain of the lines of port A, port B and port C may be
connected to a logical LO voltage or a logical ~I voltage to convey a
predetermined de~ired operating characteristic or characteristic6 to
microcomputer 10. For example, in Figure 1 a ~trap 32 ls shcwn connecting
PA4 to place a logical HI signal on the fourth bit of port A. It will be
appreciated, that the particular arrangem~nt of ~trap options, and the way
in which microcomputer 10 interprets the bit patterns entered by the ~trap
options will depend on the software, as ~hose Ekilled in the art will
reoognizeO In general, the strap vption selections can be effected by any
convenient neans including the use of jumper wires or Ewitches, or ~y
~electing a printed circuit board with the aFp~opriate traces being open
or closed circuited~
The invention further comprises a means for p~oduciny an analog
~ignal indicative of a desired operating characteristic of the motor,
which in Fractice is selected by ~he oFerator during operation of the
tool. Frequently, the desired operating parameter represent6 a desired
n~tor sEeed, or a desired triac firing angle, or the like, and is inputted
using a manually operable trigger. Although nany different systems may be
devi~ed for providing instructions to the control circuit in accordance
with the wishes of the operator, the presently preferred embodiment
employs rheostat 34 as a trigger position tran~ducer. Rheostat 34 is in
~eries with capacitor 36, which is in turn coupled to yround. By
appropriately ~etting the input/output llne P~l, capacitor 36 is
alternatel~ charged and discharged through rheostat 34. The charging time
i~ proportional to the resistance of rheostat 34, ~hiGh nay be varied in

~ 7~if~5

accordance with the manually operable trigger ~etting. Thus, the ~harging
and discharging time is indicative o~ the position of the trigger. By
approFriate selection of capRcitor 36, rheostat 34 ~nd ~oftware timing, as
will be discussed below, an analog ~lgnal indicative of a desired
operating parameter may be determLned in accordance with a trigger
position. Ihis analog signal may then be converted to a digital signal
for use in microcomputer 10.
While the foregoing represents one way of inputting the desired
operating parameter, or 6election o~ a desired speed for example; other
mechanisms may be employed without departing from the ~cope of the
invention. In general, ~ wide variety of digital or analog transducers
may be emplcyed, wi~h the apFropriate interface circuitry (such as A to D
converters, for example) for communicating wi~h miCroCGmputer 10~
With the foregoing circuit in mind, referenoe may nGw be had to
the flcw charts of Figures 3 through 5 and to the graph of Figure 2 or a
further understand mg of the invention and it~ operation in accordance
with the inventive method.
With reference to Figure 2, the speed V8. torque curves for the
motor at various conduction angles are shown. ~he uppenmost dlagonal line
44 represents full conduction (1 ~ degrees~. The area under the curves is
divided into three operating ranges or zQne~, namely, fir~t zone 46,
~econd zone ~8 and third zone 50. More ~pecifically, first zone ~6 is
bounded from above by diagonal line 52, which corre~ponds to a conduction
angle of apF~oxlmately ~eventy degrees. Second zone 48 is bounded between
diagor~l line 52 and diagonal line 54, which repre ænt~ a conduction angle
of approximately eighty~eight degrees. Second zone 48 i8 further bounded
by horizontal line 56 which corresponds to a constant ~peed of 10,000 RPM.
As seen in Figure 2, horizontal line 56 intercepk~ the ~peed axis at point
A and intercep~s diagonal line 54 at point Bl The third zone 50 is
bounded from above by the upFermo~t diagonal line 44 and from belc~ by

-12- ~

horizontal line 5~ which c~rrespo~ls to a motor fipeed m excess of 10,000
The area 60 which falls outside of the above de6cribed three
zones represents low speed high torque operating conditions which have
heen found to give rise to the potential for unwanted overheating
conditions, More specifically, the factor~ which control the tenperature
of the tor are the current drawn ~r the motor and the means proYided for
dissipating the heat generated by the ~otor. In most power tools, a
cooling fan is Frovided which i~ driven directly off the armat~re of the
motor. Accordingly, at lcw ~peeds and heavy loads the cr301ing effect
contributed ty the fan may not ~e Eufficient to Frevent overheating. The
area 60 in Figure 2 represents the potentially dangerous ~verheating zone
in which the cooling effect c~ntributed by the fan i insufficient to
overcome the thermal heating effects cau&ed by heavy current draw at high
Unlike prior art overlo d E~otection Echemes, which have 60ught
~erely to detect overheating conditions ~o that the tor can be ~hut dcwn
before damage occu¢s, the present invention additionally seeks to avoid
significant temperature rise by substantially preventmg the motor from
operating in the region which gives rise to the most ~ignificant
overheating problems. As will be explained more fully below, the present
invention permits the tool to be operated in any one of the above
described three zones 46, 48 and 50, while c~refully avoiding conditions
which would fall in the danger zone 6~.
The present invention utilizes the above described three
operating zones to provide a combinational open loop/closed loop
configuration. In the first zone 46 ~he m~tor i~ operated in an open loop
configuration, whereby motor speed and torque are inversely related as
illustrated ky the diagonal line speed torque curves withLn first zone 46.
Each of the diagonal line curves of first zone 46 represents an

~13- `

2 ~ 7~ 5
~ndividual, operator ~elect~d conduction angle. m u~, for exansole, if the
operator ~elects a conduction angle of le~ than approximately seventy
degrees via the position of the trigger Ewitch, the ~peed of the motor
will be determined ~olely in accordance with the load applied tbereto.
In the ~econd zone 48 the motor i5 operated in ~ comoinational
open loop/clo~ed loop configuration. In pQrticular, for operator ~elected
conduction angles between apF~oximately seventy degrees (point A) and
approximately eighty-eight degrees (point B) the control circuit is
designed to provide a nominal operating ~peed of lQ,OOO RPM, regardles~ of
the ~pecific conducticn angle between ~eventy ~nd eighty-eight degree
~elected. Moreover, a~ the motor 18 loaded above no load torque to, the
control circuit will operate initially in a clo~ed loop mLde and attempt
to maintain motor ~peed at 10,000 RPM by increasing ~he ~onduction angle
out to the operator ~elected conduction angle~ Ecwever, if the operator
Eelected conduction angle is not ~ufficient to malntain motor speed at
10,000 RPM given the loading on the motor, the speed of the motor will
thereafter be permitted to decline in open loop fashion Thu6 for
example, if an eighty-eight degree conduction angle is ~elected and an
increasing load i~ placed on the mDtor; the motor speed will initially be
held constant at 10,000 RPM as the conduction angle i~ increased from the
no load conduction angle of Eeventy degrees, follcwLng horizontal line
56, until point B is reached (corresponding to torque load tl)~ As load
increa~e~ beyond this point, the motor ~peed begins to decline, follcwing
diagonal line 54, which corre&ponds to the open loop ~pe~d vs. torque
curve for an 8a-degree conduction angle.
In the third zone 50 the operator selected conduction angle is
interpreted a~ a desired speed request. Thus, conduction angles falling
within the third operating zone each corre~pond, ln ~ one to one
relationship with a desired operating 6peed. Ihe ~peed control circuit
will endeavor to maintain this o~nstant ~peed by increasiny or decrea~ing


the conduction angle in accordance with ~he load until full conduction is
reached. Full conduction (lE3 degrees), denoted by the uppermost diagonal
line 44, represent~ ~he maxi~m pcwer which can be deliver~d by the motor.
I the motor is operating in the third zone 50 at full conduction, then
any further increase in load upon the mDtor will cause the ~otor ~peed to
drop following line 44.
The presently preferred embodiment for implementing this
combinational open loop/closed loop configuration u6es microcomputer 10
which i~ programmed to execute the algorithm~ descrI~ed below. Bcwever,
it will be understood that the particular algorithms described, while
presently preferred, do not exhaust all possible algorithms for
implementing the three zone ~peed control method or the combination21 open
loop/closed loop configuration in accordance with ~he invention.
Accordingly, changea in the follcwing algorithms may be ~ade by those
skilled in the art without departing fram the ~cope of the mvention as
defined by the appended claims~
Wi~h reference to Figure 3, the F~e~ently p~eferred algorithm for
Lmplementing the combinational open loop/closed loop ~peed mode is
described fully in the flow chart. Following the system reset, the
input/output ports are interrogated to preload the desired operating
parameters for the particular tool in which the ~nvention i6 employed.
~ext, initial low ~peed, low conduction angle and high kickback test
limits are loaded to ~tandardize the initial ~tart-up condition~ to safe
values. After the initial values are 6et, the a.c. waveform is
interrogated to determine the present half cycle, and if apFropriate, the
desired operator ~elected parameter is input by calling the analog lnput
subroutine, which will be di~cussed below in coru~ction wlth Figure 4. In
general, the analog input ~ubroutine interrogates the manually operable
trigger or other rheostat and provides a digital value r~presenting the
operator ~elected conductiorl angle. Ihe program then waits for a power

~ 2 ~;7 ~
line ~ero cros6ing to synchroni~e ~he software timing with the a.c.
wave~orn4 and, prcvided the trigger switch has act~all~ been depressed,
the actual motor Epeed is determuned or ~a6ured ~y tachometer 24. This
actual motor ~peed ~or motor rotational period) i~ load~d into a memory
cell for containing the latest actual speed data~
Next, the kickback detection algorithn~ discu6sed more fully with
reference to ~igure 5, tests whether an i~pending kickback condition
existRO If it dbes, then evasive neasure~ are taken; if it does not,
then the program determines whether the pcwer line hal~ cycle is even or
odd. In the even half cycle, oFeration branche~ to a portion of the
program which determines the desired ~peed based upon the
operator-selected conduction angle. In the odd half cycle the program
branches around the ~peed determining algorithn~ and instead executes a
countdown procedure to fire triac 22 at the appropriate time, based on the
desired conduction angle~ More specifically, the countdown sequence
includes a procedure for testing whether the triac will be fired early or
late in the cycle. In general, this is done to compen~ate or balance the
time required for making ~peed control calculations and for executing the
analog input ~ubroutine. If the triac i~ to fire e~rly in ~he half cycle,
a ccmpensation value is added to the firlng time to compensate for the
amount of time required to perform a speed control calculation. Then the
countdbwn seq~ence i~ initiated and the triac fired, followed by a call to
the analog input ~ubroutine. If the trlac i~ to fire late in the half
cycle, the analog input ~ubroutine i~ executed early, and following that
~ubroutine, the firing time value i~ oJmpensated to reflect the amount of
time spent performlng the analog Lnput ~ubroutine, le55 the amount of time
required for the ~peed control calculation. Finally the countdown
~equence i~ executed and the triac fired.
To continue with the flow chart o~ Figure 3 r as~ume that
operation i~ in the even ha:lf cycle, ~o that control has branched to the


2 5~7~
~peed control ~mput~tion algorithm b~ginning at point D. The algorithm
next tests to determine whether the opera~or ~elected conduction angle is
less than 88 ~egrees. If it is less than B8 degrees, the desired speed is
~et automatically at 10,000 RPMb In ~he alter~atiYe, if the operator
selected conduction angle is greater than 88 degrees, the ~electe~
conduction angle is converted again to a desired operator ~elected speed.
This calculation i~ based upon a 6traight line apFroximation using an
equation of the type y = ax ~ b, where "y~ denotes ~peed, ~x~ denote~ the
operator selected conduction angle, and ~a~ and "bW denote constants which
are preselected so that when "x~ equal~ 88 degr~es, "y~ equals 101000 and
when UX~ e~uals 1 ~ degree~, uyw equals the maximum ~afe operating peed
for the tool.
Once the desired speed has been determined, ~he circuit next
test~ to determine whether the desired ~peed exceeds a predetermined
maxLmum speed limit established for the tool. ~ ~ the desired ~peed
is below the maximum 6peed limit, a calculation i~ then performed to
determine the appropriate conduction angle nece~sary to achieve and
maintain ~he desired speed. If the operator selected conduction angle is
less than 88 degrees, the circuit determine6 whether the operator Eelected
conduction angle i~ greater than the full feedback conduction angle
required to maintain the desired ~peed. If the operator ~elected
conduction angle i8 greater than the full feedback conduction angle, ~he
circuit sets the de6ired conduction angle equal to the full feedback
conduction angle and a degree o closed loop control is effected. If,
hcwever, the operator selected conduction angle i~ not greater than the
full feedkack conduction angle, the desired cDnduction angle is ~et equal
to the operator ~elected conduction angle and the circuit operates in an
open loop configuration.
q~us, for example, if the operator selected conduction angle is
equal to eighty-five degrees and only seventy-five degrees conduction


~ 2~7~5
angle is required to maintain a motor ~peed o~ 10,000 RPM~ given the
pree-~nt loading of the motor, ~he control circuit will supply ~eventy-five
degrees conduction angle. Moreover, ~he control circuit will atteTpt in
this ~ituation to maintain the 10,000 RPM motor ~peed by increasing the
conduction angle as necessary to a maximlm of eighty-f~ve degrees -- the
operator Eelected conduction - before permitting the ~peed of the
motor to decline with increased loading. If~ on the other hand, the
operator selected conduction angle is greater ~han 88 degrees, the circuit
automatically assumes a complete clo6ed loop configuration and the desired
conduction angle is ~et equal to the full feedback conduction angle.
Once the desired conduc~ion angle has been set, the countdcwn
ssquence begins and the triac is fired based on the desired conduction
angle. Following the firing of the triac a new kickback limit value is
determined for u~e in the kickback detection algorithm to be discussed
Referring ncw to Figure 4, the analog input ~ubroutine reerenoed
above will now be described in further detail. The analog input
subroutine begins by loading the loop counter, which is used to establish
a predetermined time interval for interrogating the an210g position of the
trigger Ewitch, and ky clearing the ~hre~hhold counter, u6ed to store a
value indicative of the po~ition of the trigger ~witch. The circuit tests
to determine whether the power line voltage is ~n an odd half cycle or an
even half ~ycle. In the odd half cycle cap~citor 36 i~ charged through
rheostat 34 while the predetenmined tinung loop i~ executed, each time
testing to determine whether the capacitor is above a threshhold value of
the input/output port. For each pass through the loop during which
cap~citor 36 ls charged aboYe the input threshhold~ the threshhold counter
i~ incremented. m u~ the value held in the threshhold oounter at the end
of ~he odd half cycle loop is mdicative of the rate at which cap~citor 36
wa~ charged through rheostat 34 Sinoe the charging rate is determined by


~2 S~7~f~5
the analog position of rheostat 34, as Eet ty the operator through the
trigger switch, the threshold counter value or charge count is indicative
of the desired or operator-selected conduction angle.
Sim;larly, dur mg ~ach even half cycle ~aEacitor 36 i5 dis~harged
through rheostat 34 while a 6imilar tLming loop determlne~ hcw long it
takes for the capacitor to discharge belGw the input threshold voltage~
This discharge count is then averaged with the previous charge count and
the operator ~elected conduction angle is calculated in accordance with
the average value, using a straight line approxL~ation of the form y = ax
~ b, where "y" represents the operator ~elected conduction angle, nx~
represents the average count value prevlously determined, and ~a~ and rb"
represent scaling constant~.
The oFerator selected conduction angle determined accordingly is
then compared with the previously ~elected conduction angle to determ m e
whether the absolute value of the difference between the two values
exceeds a preselected ~hysteresis~ limit. If not~ the anal~g input
subroutine returns to the main program. If the absolute value is above
the hysteresis limit, the n~w operator ~elected conduction angle, thus
determined, replaces the previous operator selected oonduction angle and
control returns to the main progran~ The purpose of this procedure i~ to
prevent the tool from ujittering" in responfie to relatively small changes
in the operator selected conduction angle~ parti~ularly during full
feedback operation of the tool.
Figure 5 outlines the as~ti-kickback routine, which begins at the
reset entry point of the n~in program described above in cos~ection with
Figure 3. After prel~ading the registers and waiting for the power line
vol~age zero crossing, as described above, the circuit tests to determine
whether the trigger &witch is on. If the trigger switch 18 not on, ~he
circuit continues to cycle through the ~nitial presetting steps until the
~witch i~ turned sn by the oFerator. Once this has occurred the actual

~peed o ~he motor i6 determined by the ~peed ~ensing device such as
tachometer 24. In the presently preferred embodiment speed is actually
measured as the time interval or period be~ween impul~es fron ~he speed
sensor. ~he presently preferred embodiment utilizes a ~achometer for its
cost ~aving advantages. ~owever~ at lcw rotational 6peed~ the tachometer
produce~ an output voltage which is Lnsufficient for ~peed ~easurements.
To avoid erroneous results, the program determines whether the m~asured
speed is below the reliability lilits of the tachometer. More precisely,
the program determines whether the tine period between tachometer i~pulses
is near or above the limit of the ~ensor, If the measured period i8 near
or abcve the l~mit the program branche~ aro~nd the anti-kickback detection
point and continues a~ shown. If ~he rotational ~peed is ~ufficient for a
reliable tach~meter reading, the program tests to determune whether the
most recently determined sFeed period is greater than the anti-kickback
limit determined on a previous pass through the programO If the latest
~peed period is greater than the anti-kickback linit, a kickback condition
i~ detected and the program branches to a trap circuit, which performs an
endles~ loop, prohibiting the triac, SCR or other gating device from being
triggered. Exit from the endless loop i5 effected by releasing or turning
off the trigger ~witch, whereupon program control branches to the F~eset
point A near the begin mng of the main program.
Following the anti kickback test the program proceeds to fire the
triac or thyristor at the appropriate time, takin~ into account the time
required for determuning the cGnductlon angle. A detailed de6cription of
the ~teps involved was previously given in referenoe to Figure 3. After
firing has occurred and the desired operati~ zone selected in accordance
with the operator selected conduction angle (a~ wafi discussed in
connection with Figure 3), the program determines whether or not open 1GOP
low pcwer pha~e control has been ~electedO If open loop lcw pcwer Fhase
control exists, then the operation ifi ~or oe d to occur within the first


~2 5~
~one 46 of ~igure 2. If operation is in the first xone, a very high
anti-kickback limit ~alue is loaded mto the memory address for ~toring
the anti-kickback limit value. Ihis serves to effectively disable the
kickback feature during operation of the tool in thig low speed m~de
where low power is being supplied to the motor and cons~guently kickback
is not a problenL If the operation is not within the first zone, the
input/output port i~ interrogAted to determine the anti-kickback
sensitivity value. This value may be preset at the factory through the
selection of the approEriate strap option via option 6trap arrangement 26.
If a ano limit" kickback Eensitivity is selected, the anti-kickback limit
value is set to a very high value. If other than a ~no limitW sensitivity
is selected through the option strap arrangement, the input selection read
from the input port is converted to a numerical sensitivity value. Ihe
rotational period of the motor deter~ined ~y the tachometer 24 and ~tored
in the ~peed register is &caled ty dividing it by predetermined value. In
practice, the speed period, expressed as a binary number, i~ shifted five
digits to the right, which performs a division by 32. The 6caled ~peed
period is then mwltiplied by the sensitivity value, and the product is
added to the ~peed period valuea m is Froduct is then 6aved as the new
anti-kickback limit for testing against the next ~peed period to be
determined follcwing the next power line voltage zero cro~smg.
The ant1-kickback routine thu8 utilize~ the actual operating
~peed of the motor in determining when a kickback condition exists.
Limits are calculated, using a percentage change technique, against which
the act~al operating speed i~ compared for kickback detectionO For
example, if during a given half cycle the motor is operated at a speed
corresponding to 100 forty-microsecond countsl and the anti-kickback
factor is set at ten percent, an impending kickback condition will be
detected if, on the next half cycle, ~he actual ~peed period exceeds a
count of 110. If it~ period is le~s than 110 counts, a new limit, based

-21- ,

~ Z 5 ~ 6 ~ 5
upon the n~asur~d actual ~peed period value i6 calculsted and entered and
operation continues Unlike prior art klckback detectlon ~chemes which
atte~pt to nonitor kickback in terns o~ rate-of-change of ~otor current
(dI/dt) or rste-of-change of motor 6peed (ds/dt), the pre6ent method
detects the kickback conditlon a~ a percentage change ~n motor ~peed.
Ihus the present invention doe~ not require current shunt circuitry and
analog to digital convertor c~rcuitry needed for using the dI/dt
techni4ue. PuLthermore, the per oe ntage change technique i6 more accurate
at high ~peed6, unlike Erior art ds/dt methods, which are by their nature
les6 able to detect snall ~pee~ changes at higher operating 6peeds.
While the above de6cription con6titutes the Ereferred embod~ ent
of the present invention, it will be appreciated that the invention ls
susceptible to m~dification, variation and change without departing fro~
the proper scope or fair meaniJ~ of the acccm~anying claims.

- 22 -

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Admin Status

Title Date
Forecasted Issue Date 1989-07-18
(22) Filed 1985-03-04
(45) Issued 1989-07-18
Expired 2006-07-18

Abandonment History

There is no abandonment history.

Payment History

Fee Type Anniversary Year Due Date Amount Paid Paid Date
Filing $0.00 1988-09-27
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