J105 STMC

Section 1. Introduction

Stepper motors are widely used in industry because they can be precisely control led by digital systems. Until recently, however, they have demanded considerable processing power to run at high step rates, which has limited their application in low-cost digital systems. Intelligent controllers designed for use with computers have now appeared, and have the potential to extend considerably the use of stepper motors.

The STMC is an intelligent stepper motor controller designed for use with the STEbus. It has a number of features which increase its usefulness and versatility, and has an on-board stepper motor controller IC which accepts commands from the STEbus and controls motor excitation, number of steps, step rate, acceleration and deceleration, and interaction with limit switches. These functions are all performed independently of the main STEbus CPU, which can perform other processing tasks while the STMC is controlling the stepper motor.

In addition, the STMC has an on-board 4-phase driver with heatsink, so that motors of up to approximately 2 amps per phase can be driven with no external hardware except for the current-limiting resistors for L/nR operation.

To increase the usefulness of the STMC, 30 digital I/O lines are provided. Of these, 8 are optional outputs for motor control, so that (for example) higher power drivers may be used, 6 are used for control and limit switches, 8 are general-purpose digital input and 8 are general-purpose digital output lines. All these I/O lines are taken to a 50-way connector which conforms to the Arcom standard digital I/O signal-conditioning pinout. This means that the lines may be buffered by optoisolators or high-current drivers, and are compatible with the wide range of digital signal-conditioning boards available.

The STMC controller IC accepts eight commands which are described in outline below, and are detailed in Section 5.

Command
number
Function
0 Emergency stop
1 Decelerating stop
2 Single step
3 Acceleration/deceleration
4 Constant speed operation
5 Move to limit switch at constant speed
6 Move to limit switch with acceleration/deceleration
7 Move to reference point

In addition, the controller may be initialised with parameters for 3, 4 or 5 phase motors, full and half-step operation, maximum step rates, and numbers of steps over which to accelerate and decelerate. These parameters are also described in detail in Section 5.