CA1257645A - Method of detecting kickback condition in a motor driven tool - Google Patents

Method of detecting kickback condition in a motor driven tool

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Publication number
CA1257645A
CA1257645A CA000578623A CA578623A CA1257645A CA 1257645 A CA1257645 A CA 1257645A CA 000578623 A CA000578623 A CA 000578623A CA 578623 A CA578623 A CA 578623A CA 1257645 A CA1257645 A CA 1257645A
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Canada
Prior art keywords
value
motor
method
kickback
peed
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
CA000578623A
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French (fr)
Inventor
Robert Bradus
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Black and Decker Inc
Original Assignee
Black and Decker Inc
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Publication date
Priority to US59280984A priority Critical
Priority to US592,809 priority
Priority to CA 475641 priority patent/CA1257645C/xx
Application filed by Black and Decker Inc filed Critical Black and Decker Inc
Priority to CA000578623A priority patent/CA1257645A/en
Application granted granted Critical
Publication of CA1257645A publication Critical patent/CA1257645A/en
Application status is Expired legal-status Critical

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Abstract

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.

Description

~ ~ ~ 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 -- ~.~

~5~76~S

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 ~;~576L~5 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 speed.
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.

576~5 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- ~

76~5 horizontal line 5~ which c~rrespo~ls to a motor fipeed m excess of 10,000 RP~L
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 tor~ues.
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 ~257~5 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 ar.gle - 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 below.
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 -1~

~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.

Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
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;
and 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 interval;

comparing said second test value with said third value;
and 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 cycle.
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 motor.
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.
CA000578623A 1984-03-23 1988-09-27 Method of detecting kickback condition in a motor driven tool Expired CA1257645A (en)

Priority Applications (4)

Application Number Priority Date Filing Date Title
US59280984A true 1984-03-23 1984-03-23
US592,809 1984-03-23
CA 475641 CA1257645C (en) 1984-03-23 1985-03-04
CA000578623A CA1257645A (en) 1984-03-23 1988-09-27 Method of detecting kickback condition in a motor driven tool

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Application Number Priority Date Filing Date Title
CA000578623A CA1257645A (en) 1984-03-23 1988-09-27 Method of detecting kickback condition in a motor driven tool

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CA1257645A true CA1257645A (en) 1989-07-18

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