Experiment #26: Stepper Motor Control

Transcription

1 Experiment #26: Stepper Motor Control This experiment demonstrates the control of a small 12-volt unipolar stepper motor. Stepper motors are used as precision positioning devices in robotics and industrial control applications. New PBASIC elements/commands to know: ABS Building The Circuit StampWorks Manual Version 1.2 Page 157

2 ============================================================================== File... Ex26 - Stepper.BS2 Purpose... Stepper Motor Control Author... Parallax ... stamptech@parallaxinc.com Started... Updated MAY 2002 {$STAMP BS2} ============================================================================== Program Description This program demonstrates unipolar stepper motor control. The pot allows the program to control speed and direction of the motor. Revision History I/O Definitions PotCW CON 0 clockwise pot input PotCCW CON 1 counter-clockwise pot input Coils VAR OutB output to stepper coils Constants Scale CON $0100 scale for BS2 (1.0) Scale CON $0080 scale for BS2sx, BS2p (0.5) Variables Page 158 StampWorks Manual Version 1.2

3 speed VAR Word delay between steps x VAR Byte loop counter saddr VAR Byte EE address of step data rcrt VAR Word rc reading - right rclf VAR Word rc reading - left diff VAR Word difference between readings EEPROM Data ABAB Step1 DATA %1100 A on B on A\ off B\ off Step2 DATA %0110 A off B on A\ on B\ off Step3 DATA %0011 A off B off A\ on B\ on Step4 DATA %1001 A on B off A\ off B\ on Initialization Initialize: DirB = %1111 speed = 5 make stepper pins outputs set starting speed Program Code Main: FOR x = 1 TO 100 GOSUB Step_Fwd NEXT PAUSE 200 FOR x = 1 TO 100 GOSUB Step_Rev NEXT PAUSE 200 Step_Demo: HIGH PotCW 1 rev forward 1 rev back discharge caps StampWorks Manual Version 1.2 Page 159

4 HIGH PotCCW PAUSE 1 RCTIME PotCW, 1, rcrt RCTIME PotCCW, 1, rclf rcrt = (rcrt */ Scale) MAX 600 rclf = (rclf */ Scale) MAX 600 diff = ABS(rcRt - rclf) IF (diff < 25) THEN Step_Demo IF (rclf > rcrt) THEN Step_CCW Step_CW: speed = 60 - (rcrt / 10) GOSUB Step_Fwd GOTO Step_Demo read clockwise read counter-clockwise set speed limits get difference between readings allow dead band calculate speed do a step Step_CCW: speed = 60 - (rclf / 10) GOSUB Step_Rev GOTO Step_Demo Subroutines Step_Fwd: saddr = saddr + 1 // 4 READ (Step1 + saddr), Coils PAUSE speed RETURN point to next step output step data pause between steps Step_Rev: saddr = saddr + 3 // 4 READ (Step1 + saddr), Coils PAUSE speed RETURN point to previous step Page 160 StampWorks Manual Version 1.2

5 Behind The Scenes Stepper motors differ from standard DC motors in that they do not spin freely when power is applied. For a stepper motor to rotate, the power source must be continuously pulsed in specific patterns. The step sequence (pattern) determines the direction of the stepper s rotation. The time between sequence steps determines the rotational speed. Each step causes the stepper motor to rotate a fixed angular increment. The stepper motor supplied with the StampWorks kit rotates 3.6 degrees per step. This means that one full rotation (360 degrees) of the stepper requires 100 steps. The step sequences for the motor are stored in DATA statements. The StepFwd subroutine will read the next sequence from the table to be applied to the coils. The StepRev subroutine is identical except that it will read the previous step. Note the trick with the modulus (//) operator used in StepRev. By adding the maximum value of the sequence to the current value and then applying the modulus operator, the sequence goes in reverse. Here s the math: // 4 = // 4 = // 4 = // 4 = 0 This experiment reads both sides of the 10K potentiometer to determine its relative position. The differential value between the two readings is kept positive by using the ABS function. The position is used to determine the rotational direction and the strength of the position is used to determine the rotational speed. Remember, the shorter the delay between steps, the faster the stepper will rotate. A dead-band check is used to cause the motor to stop rotating when the RCTIME readings are nearly equal. Challenge Rewrite the program to run the motor in 200 half steps. Here s the step sequence: Step1 = %1000 Step2 = %1100 Step3 = %0100 Step4 = %0110 Step5 = %0010 Step6 = %0011 Step7 = %0001 Step8 = %1001 StampWorks Manual Version 1.2 Page 161