GB2156172A - Stepper motor torque control apparatus and method - Google Patents

Abstract

In an apparatus and method for controlling the output torque of a stepper motor composite sequential drive pulses are provided for energizing the stepper motor, with the mark-space ratio of the component pulses being modified to establish the output torque from the stepper motor to a load. <IMAGE>

Description

SPECIFICATION Stepper motor torque control apparatus and method A stepper motor is a rotary motion apparatus which provides desired incremental movement of a load in responseto sequential input control pulses and will provide the desired movement of that load coupled with the motor in responseto each control pulse until the required load torque exceeds the electromagnetic capability ofthe motor. When this pull-out condition occurs the motorwill miss desired step movements, and thetorque at which the step movements are missed is a function of the motor speed in relation to steps persecond, the basic motor design and the electric drive circuit controlling the stepper motor.

Atypical stepper motor as presently available in the open market is made and sold by Superior Electric Company of Bristol, Connecticut as their Model M112FJ12. io provide a control apparatus, which can include a programmed microprocessor, operative to sequentially energize the stepper motor stator coils with the required input control pulses to obtain a given angularspeed, including accelerating and decelerating belowthetorque limits ofthe stepper motor, is well known in the priorart.

It is known in the prior artto provide a motorfor opening and closing the passenger doors on a transit vehicle in responsetodoorcommandsignalsfroma vehicle automatictrain operation control apparatus, as described in an article entitled Recent Applications of MicroprocessorTechnologyto People MoverSystems by M. P. McDonald etal. and published in the conference proceedings for the 29th IEEE Vehicle Technology Group Conference held in Chicago, Illinoisduring March, 1979.

A control apparatus and method are provided for modifying the tarque level of a stepper motor atwhich the pull-out ofstep condition, in response to an applied control pulse, or missed rotational steps occurs. This modification can be accomplished such thatthe stepper motor slip torque can be controlled as desired.

Figurel showsasideviewofapriorarttransit vehicle including passengerdoors in a closed posi tion; Figure 2 shows a priorartdirectoralternating current motor coupled to operate a pair of biparting passenger doors ofatransitvehicle; Figure 3 shows a priorartstepping motorstatorcoil energization sequence provided for a typical four-pole stepper motor; Figures 4 through 7 illustrate the successive sequential rotational movement positions of a prior art four-pole stepping motor;; Figure 8 shows an illustrative embodiment of the present invention, including a microprocessor controller apparatus coupled with a stepper motor for moving a load and provided for establishing the stepper motortorque when moving that load; Figure 9 illustrates the stepper motortorque control operation of the present invention; Figure 10 shows the outputtorque of a stepper motor in relation to the step rate ofthe motor and the percentage ON time ofthe provided control pulses; Figure 11 showsthe motorcurrentof a stepper motor in relation to the step rate and the percentage ON time of the provided control pulses; Figure 12 shows a flow chart to illustrate a data entering program forthe stepping motorcontrol apparatus in accordance with the present invention;; Figure 13 shows a flowchartto illustrate a control program for the stepping motor control apparatus in accordance with the present invention; and Figure 14 shows a variable friction device used for the load coupled with the stepper motor.

In Figure 1 there is shown a priorarttransitvehicle 10 including a firstpairofclosed passenger doors 12 and 14 and a second pair of closed passenger doors 16 and 18.

In Figure 2there are shown the priorart passenger doors 12 and 14 of a vehicle and operative with a connecting drive cable 20 that is looped around an idler pulley 22 and a similar idler pulley 24 and connectedthroughfasteners 26 to move the passen gerdoor14and is connected to passengerdoorl2 through fastener 27 to move the passenger door 12 such that in this way the passenger doors 12 and 14 can be opened or can be closed together, as described in U.S. Patent 4,142,326 of W. E. Schmitz. A drive pulley 28 is coupled with a direct or alternating current drive motor 30 and operated by a suitable door control apparatus 32 in response to door open and door closed input control signals from a well known vehicle control apparatus 34.A drive belt member 36 is connectedthroughthedrivearm38forapplying a door movement and door unlatching force to the passenger door 14. The drive belt 36 provides a linear translationforceto movethe door 14through its desired travel to the rightforthe purpose of separating oropening the passenger doors 12 and 14. The same force that moves the door 14 open and closed also unlocks and locks the lock mechanism coupled with the drive arm 38. When the motor 30 is not energized and the passenger doors 12 and 14 are closed, the doors 12 and 14 cannot be opened by applying an opening force directlyto eitherdoorl2 or14.

However, when the door opening force is applied by the drive member 38 to the lock mechanism, the latter will unlock and the doorl4will movetothe rightwhile the door 12will move to the left in conjunction with the movement ofthe door 14.

In Figure3there is shown a priorartcoil energization sequence for a well known four-pole stepper motor. The respective stator coil currents in general follow the illustrated waveforms shown in Figure 3.

Each state change of the energization causes the rotor to incrementally rotate a fixed angle, which is dependent on the motor construction, and typically can be 1.8 , 3.6 , 7.5 or 15 . The currentwave 50 is ON at time T1 andthen goesOFFattimeT3andthen goes back ON attimeTS and goes OFFattimeT7.The current wave 52 goes OFFattimeTl,then goes ON attimeT3 and then goes OFFattimeT5and goes ON attimeT7.

The currentwave 54 goes ON at time T1 and goes OFF attimeT4goes ON attimeT6 and goes OFF attimeT8.

The currentwave 56 goes OFFattimeT2, goes ON at timeT4, goes OFF at time T6and goes ON attimeT8.

In Figures 4,5,6 and 7there is shown an illustrative sequential rotational movement of a simplified rotor of a prior artfour-pole stepping motor. When the field coils 60 and 64 as shown in Figure4 are energized at timeT2 as shown by the arrows and corresponding to the currentwave 50 energizing the field coil 60 and the current wave 54 energizing the field coil 64, the permanent magnet rotor 68 is aligned with the south rotor pole between the two north field poles 70 and 72 and the north rotor pole between the two south field poles 74 and 76.When the field coils 62 and 64 are energized attimeT3 as shown by the arrows in Figure 5, the rotor 68 aligns itself as illustrated with the south rotor pole between the two north field poles 72 and 76 and the north rotor pole between the two south field poles 70 and 74. When thefield coils 62 and 66 are energized attime T4 as shown by thearrows in Figure 6, the rotor 68 movesto the position with the south and north rotor poles as illustrated. When the field coils 60 and 66 are energized attimeTS as shown by the arrows in Figure 7, the rotor 68 is positioned with the south and north rotor poles as illustrated.When the rotor 68 is again positioned as shown in Figure 4 the rotor 68 wili have rotated through a total of 3609 It should be understoodthatan actual stepping motor rotorwoulo require 200, 100,48,24 orthe like steps to make one 360 rotation.

In Figure8there is shown a microcontroller 100 coupledthroughthedrivers 126,128,130 and 132to energize the respective stator coils 60,62,64 and 66 of a stepping motor 92;The shaft 142 ofthe motor 92 is coupledwithaload 94,which could be the drive pulley 28 ofthe door control apparatus shown in Figure 2. A program terminal 148 is operative to provide and modify the control programs supplied to the micro controller 100. Atorque indicator 150 is coupled with the motor shaft 142 for indicating the motor torque. A speed indicator 152 is coupled with the motor shaft 142 for indicating the motorspeed.Astop signal device 158 provides a stop control 160 signal to the microcontroller 100. A power supply 96 is connected through a resistor 1 56to the field coils 60 and 62,and is connected through a resistor 158 to the field coils 64 and 66.

Figure 9 illustrates the stepper motortorque control operation ofthe present invention, and shows the generai coil energizing sequence, corresponding to that shown in Figure 3. For providing torque control of the stepping motor, the waveform 50 when ON, as shown in Figure 9, is pulse width modulated by an adjustable ON time as a percentage ofthe provided ON time period desired for speed control of the motor 92, and between the state of changes as shown in Figure3, such that additional OFF time periods are introduced in the ON time control pulse between the times T1 and T3 when the currentwave 50 energizing the statorcoil 60 is ON.These OFF periods 80 and 82 control the percent ON time such thatthe average currentthrough the stator coil 60 between the times T1 and T3 is reduced and it is the magnetic strength of the stator coil 60 as determined by the average coil current in conjunction with the magnetic strength ofthe stator coils 62, 64and 66thatdetermines the torque atwhich the rotor 68 misses step movements.In Figure 9 OFF times 84 and 86 are provided for the ON control pulse of currentwave 52 energizing the stator coil 62, OFF times 88 and 90 are provided for the ON control pulse of currentwave 54 between times T2 and T4 energiz- ing the statorcoil 64, and OFFtimes 92 and 94 are prdvidedforthe ON control pulseof current wave 56 between times T4and T6 and energizing the statorcoil 66.

The stepping motor 92 has a predetermined minimum ON time period pulse to which it will respond by making a step movement to move the load 94 one increment of distance. By pulse width modulating the ON time control pulses as shown in Figure 9 below that minimum time period or length, the speed ofthe motor92 does not respondto such modulating operation butthe average currentsupplied to the motor changes accordinglyto control the motor torque.

To obtain torque versus speed curves for the motor 92, the speed of the motor 92 was changed by modulating the width of the ON time control pulse above that minimum time period and was measured with the speed indicator 152, and the torque ofthe motor92was changed by modulating the ON percentage ofthe ON time control pulse below that minimum time period and was measured with the torque indicator 150. The operating speed ofthe stepping motor 92 which is the stepping rate was determined bythe microcontroller 100 including an application software program using a selectable time delay between changes ofthe stepping states.The motor 140 was run ata pluralityofpredetermined stepping rates and the load 94was adjusted by using a variable friction deviceforthe load such as shown in Figure l4to require a plurality oftorquevalues,with the highesttorque value sustained being noted before the slipping ofthe requested motor step operations occurred.

As shown in Figure 14,the load 94 coupled with the motor 92 can include a variable friction device 250 including a disc member 252 coupled with the motor shaft 142 and friction pads 254 and 256 that are applied to the disc member 252 with an adjustable force determined bythethreaded bolt and adjustable nut 260 operative between frame members 262 and 264 including a pivot connection 266.

The following table shows the results ofthe operation of the stepping motor 92 at stepping rates of 75 steps per second, 100 steps per second, 150 steps per second,200 steps persecond and 250 steps per second. The table also shows the provided percentage of time of the energization ofthe respective motor statorcoilsto be 100%,85%,70% and and 55%.The corresponding motor current in amperes is shown in relation to each ofthe stepping rates and percentage of ON time, and the corresponding sliptorque in inch pounds is shown for each of the stepping rates.

TABLE 1 Program Input Motor Percent Motor Slip Total On Step Rate On Current Tongue Delay Delay Step/Sec Amp In m 238 238 75 100 27 75 202 85 22 64 167 70 18 53 131 55 13 38 177 177 100 100 25 68 150 85 20 55 124 70 16 43 97 55 12 37 117 117 150 100 21 67 100 85 17 60 82 70 13 36 64 55 9.5 23 88 88 200 100 17 53 75 85 14 54 62 70 10.5 39 48 55 8 21 70 70 250 100 13 54 60 85 12 42 49 70 9 23 38 55 6 17 The numberin thefirstcolumn ofthe above table was set in a first counter and then decremented by the clock of the microcontroller 100 to establish the time delayofthe ON time and OFF time control pulses as shown in Figure 9 to determinethe step rate ofthe motor 92. The numberofthe second column was set in a second counter and decremented to determine the percent ON time ofthe ON time control pulses.

The variable friction device 250 was then used by the operator to determine the highest motor torque value as shown bytorque indicator 150 at which slipping of the requested motor step operatio ns occu rred, a nd this established the slip torque in column six and the corresponding motor current in column five as shown by the current indicator 97.

In Figure l0thetorque in inch pounds as shown in the above table is plottedforeach of the motor steps per second and for each ofthe percentage ON times.

In Figure 11 the motorcurrentin amperes as shown in the above table is plotted for each ofthe motor steps per second and in relation to each of the percent on times.

In Figure 12 there is shown a flow chartforthe control program which establishes the desired step time delay orthe step rate, and the ON time percentage for each energization ofthe stator fields entered by the operator from the terminal 148 shown in Figure 8.Within the limits ofthe time delay subroutines, this allows the stepper motor 92 to be run at any desired speed and with any reasonable desired percent ON time. At block 200 the desired step time delay period is entered. At block 202 the step time delay is stored in memory. At block 204 the ON percentage time delay is entered, and at block 206 the percentage ON time delay is stored in memory. At block 208 the drive subroutine shown in Figure 13 is called.

In Figure 13 aflowchartforthe drive subroutine is shown. At block 210 the microprocessor controller registers are initialized for counting the steps per second and which establish the percentage ON time ofthe stator coil energization. At block 212 the stepping motor 92 is rotated one step. At block 214 a first counter is loaded with the desired step time and a second counter is loaded with the desired percentage ON time. At block 21 6the second counter is decremented. At block 218 a check is made to see if the second counter is zero. If yes, turn OFF all the motor stator coils at block 220. If no, decrement the first counter at block 222. At block 224 a check is made to see ifthe first counter is zero. If not, the program returnsto block 216.If the first counter is zero at block 224, at block 226 a check is made to see if the stop signal switch 158 shown in Figure 8 is operated to provide the stop signal 160. If not, the program returns to block212. If yes, at block 228 all ofthe stator field coilsareturned OFF. In operation,thefirst counter controlsthe stepping rate and the second counter controls the percentage time, such that the statorcoils are energized ON which controls the average current and the torque ofthe motor. To stop the motorthe stop pushbutton 158 is operated. After each movement step the program checks to see if the stop button is pushed. If it is the motor coils are turned OFF and the program returns to the data program shown in Figure 12.

The present motor torque and motor current control method and apparatus can be used with a load, such as a transit vehicle door, to control the torque atwhich the motor rotational step missing operation occurs to provide desired load obstruction detection, with the sensitivity of that detection dynamically modified as desired. For example, the load motor 92 can be run normally with about 70% ON time and if a load obstruction is detected, when that load is a passenger door apparatus, the door can reopen and then reclose at about 75% ON time with a small increase in the motortorque applied to move the door. Each consecutive obstruction ofthe load canoperatetoincreasethepercentageONtimeand the motortorque.If the door is obstructed by inanimate debris inthe doortrackthe resulting higher torque levels will likely clear that debris away and no longer obstruct the door.

For a load that is controlled to move to a predetermined desired position and then hold that position, this torque control operation can be used to diffe rentiate between a desired full torque and full current run operation ofthe motorasccnparedwith a desired lower torque and lower average current holding operation ofthe motor in relation to that load.

When in the run mode of operation the motor needs the full developed torque and when the motor is stopped and in the holding position modethetorque required by the motorto hold the load inthatposition is lower. The present control apparatus and method enablethe motor current to be lowered to a reduced level adequateto hold the load position and provide increased motor efficiency by reducing the required power during the holding operation ofthe load, such as would be the operation for a stepping motor coupled with acamcontrollerofavehiclepropulsion control system as described in above-referenced patentapplication Serial No. 500,217.

A practical embodiment of the present control apparatus and method was made using the following components.

Motor 92 Superior Electric M112-FJ-327 52.5 in-*, 2.1 volts, 9.2 amps/ phase .242 ohms/phase, 2.1 mhj phase class B insulation Drive Relays 126, Douglas Randall Solid State Relay 128, 130 and 132 Model K12B, 12 amp 55 vdc, TTL input Micro Controller 100 Micromint Z8 controller with 28671 single chip micro Program Developer TRS-80 Model III running Micromint & communication aXZ8 cross assembler and Omniterm Terminal 148 Power Supply 96 Sorensen DCR40-40B, 0-50v, 0-40 amp (set at 24 vdc)

Claims (10)

1. CLAiMS 1. In aparatus for controlling the torque applied to a load,the combination of means including a stepper motorcoupled with the load for providing an outputtorque to the load, and means coupled with the stepper motorfor providing sequential ON time period and OFF time period control pulses having a predetermined time delay to energize the stepper motorfor providing said output torque, with the percentage ON time ofthe ON time period control pulses being modulated to determine the outputtorque provided to the load.
2. 2. The control apparatusofclaim 1, with the stepper motor having a predetermined minimum ON time periodto which the motorwill respondforchangingthespeed ofthatmotor, said control pulse providing means modulating the percentage ON time period ofthe control pulses below said minimum time period.
3. 3. The control apparatus ofclaim 1, with said modulation being provided by introducing OFFtime periods within each ON time control pulse.
4. 4. The control apparatus of claim 1, with the ON time control pulses being modulated to includeOFFtime periodsforadiustingtheoutput torque ofthe motor such thatthe remaining ON time period is greaterthan a predetermined minimum ON time period.
5. 5. The control apparatus of claim 1, with the control pulse providing means providing a second time delay for the ON time control pulsesfor modulating the ON time ofthe ON time control pulses.
6. 6. The control apparatus of claim 1, with the control pulse providing means establishing at least one OFF period to modulate each ON time control pulse.
7. 7. The control apparatus of claim 1, with the control pulse providing means establishing the percentage ON time to determine at least one OFF period for each ON time control pulse, such thatthetime delay establishes the stepping rate ofthe motor and the percentage ON time establishes the output torque ofthe motor.
8. 8. In the method of controlling the output torque of a stepper motor coupled with a load; which stepper motor is energized by sequential ON time and OFFtime control pulses, the steps of providing a firsttime delayto determinethetime interval of said control pulses for establishing the stepping rate of said motor,and providing a second time delay to determine the percentage ON time of the ON time control pulses for establishing the outputtorque ofthe stepper motor.
9. 9. The method of claim 8, with the second time delay determining the ON timeofsaid ONtimecontrol pulse.
10. 10. The method of claim 8, with the second time delay providing atJeast one OFFtime modulation forthe ON time control pulses to determine the actual ON time energization of the stepper motor by said ON time control pulse.