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PLCs IN MOTION CONTROL

Programmable Logic Controllers (PLCs) have traditionally been used as electrical controllers, using relay outputs to switch devices like solenoids, clamps, cutting tools, etc. However, in recent years PLCs have become smarter, lower in price and able to switch devices using solid state outputs instead of relays. Many PLCs are now available with high speed current sink outputs of upto 10 kHz, specially designed for direct connection to stepper motor drives. This is achieved by ordering a PLC with a dedicated pulse output or an add on module. This makes them ideal for small single axis control in machines. Some of the more advanced PLCs are available with +/-10V velocity command output and high speed encoder input for servo motors.

PLCs are also electrically more reliable and smaller in size than computer based controllers. Programs are written using a desktop computer and then downloaded to the PLC which then retains the program, even after the power and computer are disconnected. The added advantage is that the PLC program can also monitor inputs, control other devices on the machine and track motor position. If machine parameters need to be frequently changed, operator panels with keypads and LCD displays are available for many PLCs.

When selecting a PLC to control RTA stepper motor drives you should consider the following points.

Output Signals
The PLC should have at least 2 high speed NPN, current sink outputs, sinking to ground. One output is used to send clock (step) pulses to the drive. The other is used to define the direction of rotation. One pulse from the PLC corresponds to one motor step. Relay outputs and AC thyristor outputs are not suitable for stepper drives. If your PLC has a voltage output instead of current sink output, you can either use a drive with voltage input or convert the signal to current sink using an external transistor. Some of the later RTA drives like the HGD, SDC, SAC, MIND have voltage inputs with optoisolation and these would require a PLC with voltage pulse output rather than NPN current sink.
 

Control Mode
The method of controlling RTA drives is two signals (one for step and one for direction). PLCs with only dual pulse outputs (one for clock+ and the other for clock-) are not suitable. When running the motor in reverse direction, the direction bit must be on for at least 50
msec before the first step pulse is received and must stay on at least 30msec after the last pulse is received.
 

Pulse Width
There is a minimum pulse width for a step pulse going into a drive. Due to noise filtering on the drive inputs, if the pulse duration to quick, the drive may not recognise it and the motor won't step. Stepper drives vary but a minimum pulse width of 9 microsec is starting point.
 

Clock Speed
The maximum switching speed of the step output of the PLC determines the maximum motor speed. Low cost PLCs can usually provide upto 10kHz switching speed. Some more expensive PLCs can go as high as 100kHz. A drive with 800 steps/rev resolution (GMD, GAC) will run a motor at 12.5 rev/sec with a PLC of 10 kHz. When using a drive of 4000 steps/rev resolution (GMH, SDC) you would need 80kHz to achieve 20 rev/sec motor speed.

Trapezoidal Profile
Motor speed, acceleration (known as ramping) and deceleration must be programmable within the PLC program. Most PLCs are capable of operating in absolute or relative coordinates so move distance or end position can be requested. The most important move is a trapezoidal profile where the motor starts, accelerates to a plateau speed, runs at this speed for the necessary time and then ramps down to rest. If the total number of steps is requested the PLC calculates out the number of steps at plateau speed.
 

Datum Seek
In linear and rotary systems it is usual for moves to be referenced to a datum or starting point. Since it cannot be guaranteed that the motor will be in the correct position on power up, a PLC must be capable of outputting a continuous pulse train at a defined speed and direction until a datum switch is sensed. The motor will then decelerate and stop or preferably creep back in the opposite direction at very low speed to the exact switching point, thereby maintaining a repeatable datum point.
 

Jogging
In machines it is often useful to manually control tool position by pressing JOG forward and JOG reverse switches to move the motor at predetermined speeds. The PLC will have to track position while this is occuring.

Emergency Stop
At all times the PLC must monitor an emergency stop switch on a control panel. When this switch is hit momentarily, the motor must stop instantly to prevent operator injury or machine damage. The emergency stop switch should also be linked to the mains power supply to disconnect drive power in case the PLC program fails to respond to this switch.

Minimum Speed
When a trapezoidal profile is started, the motor should not start accelerating from 0Hz. Instead, it should start ramping from a minimum start/stop speed of between 100 Hz to 400Hz. This is because stepper motors, depending on load inertia, can resonate at their natural frequency which is usually between 0Hz and 200Hz, so it is good to avoid these speeds. Similarly, deceleration should occur linearly down to the minimum speed rather than 0Hz.
 

AUTOMATED MOTION SYSTEMS PTY. LTD. - WESTERN AUSTRALIA