| SRS Home | Front Page | Monthly Issue | Index |
|
|
This article describes the hardware and the software used to control two stepper motors suitable for a robot differential drive.The circuit accepts two 2-bit words that command each of the motors to go forward, backward, hold a position, or to idle. It also provides an output signal when a step is being taken. It is a flexible design allowing for unipolar and bipolar motors and being microcontroller based the winding excitations, motor speed, control word format, and other parameters can be tailored in software. The software is provided in C and in assembly language.
The circuit consists of three ICs, a PIC16F84 and either two L293D H-bridge drivers for bipolar steppers (shown in the figure below) or two ULN2803 for unipolar steppers. Aside from the power supplies, the only other components are a 4 MHz. resonator, a 10K pull-up resistor, and some connectors. In the current implementation a pack of 6 x 1.2V batteries, supplying 7.2V, is linearly regulated to 5V to supply the logic voltage and the raw unregulated power is applied to the 5V steppers. The 7.2V do not overdrive the motors due to the voltage-drop across the bipolar transistors in the drivers.
The control word is broken into two 2-bit control words: the two most significant bits of the control word which are bits 2 and 3 of PORTA control the left motor and the two least significant bits, bits 0 and 1 of PORTA, control the right motor. For the present control word values 00, 01, 10, and 11, the motors can be commanded to go forward, backward, hold a position, or to idle respectively. This sequence can be easily changed (see the software below).
As designed, the circuit provides the speed control; in particular, it does not require a periodic control pulse to step the motors. Rather, the circuit provides an output signal, on bit 4 of PORTA, when a step is taken. The main controller can monitor this signal to determine when the control word should be changed. For instance, to proceed forward a certain distance, the main processor calculates the number of steps necessary to achieve the goal and thus commands the motors forward. Once that number of steps is taken the command words can be changed to stop or redirect the motors. This task, of counting steps, can usually be relegated to a background task on most processors.
There are two versions of the software available; one is written in C2C, a C dialect, and the other in PICmicroŽ assembly language with the help of some macros that provide high level programming constructs
The main program simply repeatedly reads PORTA looking for an updated control word.This process is periodically interrupted by TMR0 whenever the motors require a new excitation; the task controlling the motors, which are excited only very periodically, in effect runs in the background.
The software allows for easy changes to the motor winding excitations, the control words, and the motor speed. The excitation table can be easily modified in content and in size thus allowing for half-stepping.The control words are simply an enumeration so that one can choose their own sequence. The speed can be changed by factors of two by changing the TMR0 prescaler. Finer changes can be made by changing the initial value of TMR0.
The software is presented below in pseudo-code.
|
A general, albeit rudimentary, micro-controller based stepper motor driver has been
designed and successfully built. Since only 124 of the 1024 words of PIC16F84 memory is
currently being used (in the assembly version) features could be added such as
acceleration and deceleration. All 13 I/O lines are currently being used and so additional
external control signals may require a PICmicroŽ upgrade. The PIC16F876 with Pulse Width
Modulation and Analog to Digital conversion may also allow a chopper-drive design.
Author: J.- L. (John) Girard
Website: John Girard's Embedded Systems