STEPPER-MOTOR BOARD IN DETAIL

Transcription

1 Appendix A STEPPER-MOTOR BOARD IN DETAIL The schematic for the stepper-motor controller board discussed in Chapter Eight is shown in Figure A-. Its V power is supplied by a wall-transformer power supply. The Schottky rectifier, D, is present to reduce the risk of burning out the board circuitry by inadvertently plugging in a wrong wall transformer having its polarity reversed. The. V voltage regulator, U, derives the logic supply voltage used by the controller chip, U, from the V motor supply voltage. (If a higher motor supply voltage is used, it must not exceed the 0 V maximum input specification of this voltage regulator.) The input and output capacitors, C and C, are included to meet the stability requirements of the voltage regulator. The 0. µf ceramic capacitor, C, provides an RF bypass for the Allegro chip s logic supply voltage. The intent of using a supply voltage of. V (rather than.0 V) is to ensure that the DIR (Direction) and STEP inputs from the Qwik&Low board do not exceed this supply voltage. The motor current can be monitored by connecting a digital milliammeter between the two pins of the H header labeled V m I and opening the power switch, TBY. As the motor steps, the DMM will display the average current. This current is set by the values of the two current-sensing resistors, R

2 Appendix A Stepper-Motor Board in Detail CONX Wall transformer for 0. A regulated power supply Components mounted on stand TBX LED B R TBY Toggle Switch S S V m DMM (Measure motor current) I. V TBX Stepper motor R B W Y CON. mm barrel connector D Schottky rectifier TB Terminal block R 0 kω U. V Voltage regulator H C 0. µf C µf I motor + TB Terminal block C 0. µf C µf + C µf 0 U Allegro Microsystems ASLB-T (-pin SOIC) DIR STEP REF MS RESET SLEEP MS PFD VLOAD RC RC VLOAD ENABLE SENSE OUTB OUTA OUTA OUTB SENSE 0 Input protection circuit R0.00 kω R. kω D H Shrouded header R. kω R.00 kω K E Y Input header Cuttable links MS MS 0 0 Full-step operation(default) 0 Half-step operation 0 Quarter-step operation Eighth-step operation Cuttable link MS MS H (Unpopulated header) POT 0 kω Slow-mixed-fast decay control (Default is slow decay) C 0.00 µf C 0.00 µf R. kω R. kω POT 00 kω POT 00 kω frequency control R R. Ω C 0.0 µf H Cuttable link Motor current control R R. Ω C 0.0 µf FIGURE A- Stepper motor driver board

3 0 Appendix A Stepper-Motor Board in Detail and R, shown in the lower-right corner of Figure A-. The Allegro chip is designed to be able to drive each of the two stepper-motor windings with a voltage of up to 0 V and a current of up to ±0 ma. When full stepping (the default stepping mode), the current in each winding is approximated by the equation Iwinding = ± 0.0 V REF Rs = ± ma Rs with VREF =. V. With R s =.Ω, Iwinding = ma. When operating in the fullstep mode, both windings are energized with this (±) current at each step position for a total load of ma for the wall transformer. The Allegro chip carries out full stepping by alternately reversing the current in one winding and then the other winding. The stepper motor listed in the parts list of Figure A- steps 00 full steps per revolution. Even finer resolution (i.e., 00 s/r, 00 s/r, or,00 s/r) can be achieved by adding a pin header in the H header pattern on the board, cutting the two links on the back of the board as shown near the upper right of Figure A-, and then adding two jumpers to select the stepping mode. For example, if both center pins are connected to the bottom pins of H, eighth-step operation will result. While the total current drawn from the wall transformer for full stepping is constant, for any of the other modes, the total current varies with the step position, between a maximum equal to that found for full stepping and a minimum equal to 0.0 times that value. The pulse-width-modulation () control circuit defaults to the nominal RC values suggested by Allegro in the data sheet for this driver chip. Each motor winding is subjected to a current that alternates between ramping up and decaying down. When one of the winding currents is low, the V power supply voltage is applied across the winding until the voltage across the current-sensing resistor crosses its threshold voltage. At that point, the power supply voltage is cut off from the motor winding. The winding current decays for a time determined by C R (for one winding) or C R (for the other one). The rate of decay is determined by the voltage on the PFD pin of Figure A-. With this pin voltage defaulting to. V, the current decays relatively slowly, with minimum current ripple. The maximum stepping rate is evidently achieved with the fast decay setting of 0 V on the PFD pin, albeit with increased audible noise and vibration. In contrast to the Qwik&Low board, the stepper-motor board is a simpler board to build. Figure A- shows the front and back artwork. Even though the board employs size 0 surface-mount resistors and capacitors, these are relatively large (for surfacemount parts). The resistors are stamped with their resistor values, a help for checking the board. Check with for the price and availability of complete stepper-motor assemblies, ready for lab use. Check with com for information on obtaining the bare stepper-motor controller board. Allegro Microsystems Technical Paper STP 0-, pp.

4 Appendix A Stepper-Motor Board in Detail Part Number Footprint Part Description Manufacturer Unpopulated Quantity Distributor Designation Cost Each Total Cost Stepper motor driver board PCBCART Digi-Key C, uF 0V XR ceramic capacitor Yageo Digi-Key C, 0 0.0uF 0V XR ceramic capacitor Yageo Digi-Key C, 0 0.uF 0V XR ceramic capacitor Yageo P Digi-Key C,, 0. Dia. uf V electrolytic capacitor Panasonic KFRCT Digi-Key R0, 0.00K /W % thick film resistor Yageo KFRCT Digi-Key R 0 0.0K /W % thick film resistor Yageo KFRCT Digi-Key R,,, 0.K /W % thick film resistor Yageo P.ASCT Digi-Key R, 0. ohm.w % thick film resistor Panasonic x P.ASCT Digi-Key R, 0. ohm.w % thick film resistor Panasonic x 0 P.ASCT Digi-Key R, 0. ohm.w % thick film resistor Panasonic x 0- Digi-Key POT 0. x0. 0K turn, side adjust trimpot Murata x 0- Digi-Key POT, 0. x0. 00K turn, side adjust trimpot Murata 0.. x S0E- Digi-Key H, x pin header Male straight row x pin header Sullins ($./pins) S0E- Digi-Key H x pin header Male straight x header with,, removed Sullins ($./pins) HRP0H Digi-Key H Male shrouded 00 mil header, x pins Assmann Elect HKC0H Digi-Key HX Mating female ribbon cable connector Assmann Elect AE0G- Digi-Key HY 0-conductor ribbon cable, feet Assmann Elect N Digi-Key D DO- 0V,A Schottky rectifier Vishay BATC-FDICT Digi-Key D SOT- Dual Schottky diode Diodes Inc MCDTRK-OSCT Digi-Key U DPAK.V voltage regulator ON Semi Digi-Key U SOIC- Motor driver, ASLB-T Allegro.. - Digi-Key TB, 0. x0. -conductor terminal block, right angle Phoenix CP-0A Digi-Key CON 0. x0..mm barrel connector for power CUI Stack T-PP Digi-Key CONX V@0.A regulated wall wart CUI Inc... Stepper driver board + power = 0. Jameco TBX Bipolar stepper motor,.v, 0mA/phase Applied Motion.. 0- Digi-Key TBX mm Dia. Panel-mounted blue LED Dialight.0.0 EG Digi-Key TBY mm Dia. SPDT toggle switch E-Switch.. Aluminum stand SJ-00BK Mouser Four rubber feet for aluminum stand M K Digi-Key / round Round spacer for -0 screw, / long Keystone H Digi-Key -0 x / machine screw Building Fasteners H Digi-Key -0 hex nut Building Fasteners Bolt Depot Internal tooth lock washers, Bolt Depot Socket head screws, - x -/ Bolt Depot Internal tooth lock washers, Bolt Depot Hex machine screw nuts, Parts for motor and stand =. Total =. FIGURE A- Stepper Motor Driver parts list

5 Appendix A Stepper-Motor Board in Detail B R S S R B W Y TB TB Rev R H R U C C C D CON + + Stepper Driver R C C U C R R H C + MS MS R H R C R D POT R C R Vm I. V Ground POT POT R0 H Ground FIGURE A- Front and back of Stepper Driver board