Step Up To the FX PLC

Do you have a small automation project coming up? Don't say the word positioning. Oops, you did! Now it's going to cost you. Let's add it all up. Small PLC, $500.00, intelligent stepper card, $1,000.00, stepper Amp. $500.00, and motor, $50.00. If you picked a transistor output FX PLC or a relay output with a transistor extension unit you can skip the intelligent card and the amp.! You can hook a small 24 volt stepper motor directly to the PLC. The transistor output can replace the Amp., and ladder logic software can replace the intelligent card.

Any motor with more than two wires hanging out of it scared me, but after some study, I have removed the mystique. You have a motor with two center taped coils. This adds up to 6 wires hanging out of the motor. The two taps are tied to the 24 volt DC power supply and the ends of both coils are tied to their own transistor switch, in this case 4 transistor outputs of the PLC. The outputs are fired in an overlapping sequence. Each step will rotate the motor shaft 1.8 degrees for exactly 200 steps per revolution.

That's all you need to do for continuous rotation. The step frequency will determine the speed, and scan time determines range of speed possible. With FX it is EZ to have 0-100 RPM, 1-32K step pulses, + or - 10 micron accuracy with a lead of 20 revolutions to 1.0000 inch of linear movement. Each step pulse will move the load (probe, weld tip, tool, hand) 0.00025 inches. That's 4000 steps per inch at up to 330 steps/sec. Rotational position with ratio 5:1 will get you 180.0° in 1.5 seconds.

With a direct PLC to motor hook up, working loads and speeds limit application. But, the FX has a special pulsed output command that is not influenced by scan time! Higher speeds and/or more power require you use a stepper amp. These amps drive the 4 legs CW or CCW.

Stepper amps only need a pulse train input and a direction flag which can come directly from FX - a multi tasking smart card!

Here is an axis program example using the PLSY command and power fail protected registers to allow pulse rates for Y7 to be up to 2000 steps per second for up to 32 thousand steps!

-----| M5 |---------------------------------[PLSY D500 D501 Y7]-----

That's it! When M5 goes true the PLC will start to pulse output transistor Y7. The total number of pulses is stored in D501 and can not be changed while M5 is on and Y7 is pulsing to reach this goal. If you turn M5 off before the pulse train is finished Y7 will stop pulsing and the count will reset to zero.

With M5 on, when the goal has been reached Y7 will stop pulsing dead on the mark. You have to turn M5 off and back on to start another pulse train. While M5 is off you can change the pulse total in D501.

Motor speed is stored in D500. This can be changed at any time even if M5 is on so you can put in a staircase ramp to get a motor with a higher inertia load to accelerate up to max speed and allow travel at this efficient rate till just before the stopping point and then staircase down to a perfect stop.

I can hear the "faint pinking sounds" of those light bulbs coming on.