Motion control using the PLC
Original article date: February 1999
The stepper and servo options provided on modern PLCs make networkable motion control a possibility without ever leaving the PLC rack. Robert Brooks from Omron explains.
While most of the servo applications that catch the headlines tend to be demanding tasks with extremely stringent dynamic requirements at the same time motion control is being adopted for simpler motion systems built around stepper and servo motors perhaps providing point-to-point or indexing control.
What is required in this instance is not necessarily a full blown stand-alone motion control architecture but rather one which is built around a much more user-friendly strategy based on existing hardware with a familiar software front end.
By adding stepper and servo control options to PLCs even the smallest PLCs can now be equipped with a pulse train control output suitable both for controlling stepper drives and – increasingly – servo drives.
Omron’s UE series servo drive is an example of the modern breed of servo which offers good dynamic performance without the cost and complexity traditionally associated with servo drives. Controlled via a pulse train input the UE range is ideally suited to applications such as point-to-point and indexing.
The pulse train output is becoming more common on PLCs. On Omron’s compact and micro PLCs such as the CQM1 pulse output for stepper and servo control is a standard function while on the mid-range C200H series of PLCs it is provided via an NC position control card.
Using the PLC in this way can make it very simple to develop motion control programs since motion-specific instructions have already been included in the standard instruction set. Fig 1 shows a section of an indexing program written for Omron’s CQM1 compact modular PLC. The program instructs the motor to move 25 0 pulses initially at a speed of 1kHz but slowing to 200Hz when a limit switch is actuated.
Combining with the ability of the CQM1 to link directly with any Omron Sysdrive inverter the stepper and servo options extend the capabilities of PLCs to allow complete highly functional machine control architectures to be built up simply around a single compact PLC.
PLCs of this size are also designed to link directly to man-machine interface products without the need for interfacing software making it easy to implement powerful control and diagnostics strategies. PLCs such as the CQM1 are also designed with communications in mind: an I/O card allows it to be connected to networks such as Omron’s Sysmac bus or to open fieldbus systems providing integration into a bigger factory automation network.
- Omron
- Robert Brooks
- 0181 450 4646
February 1999