Sometimes a PLC is the right solution

[Richards]

Lance,
Thanks for your interest in using a PLC. First of all, a PLC is probably overkill for most users, not because it wouldn't add possibilities, but because the Acorn does a great job on its own. The ClearPath/DMM issue was what caused me to look for other possibilities. Normally, I just hack together something on an Arduino or Raspberry Pi, and then have some proto boards made. Even easier is to use a few transistors to invert signals, but even that requires having a circuit board made (or keeping the cabinet door closed so that nobody ever sees my rat's nest of wires).

One of my sons is an electrical engineer specializing in process control for oil refineries. His job takes him all over the United States. In that field, PLCs are used to facilitate implimentation of changes and improvements at reasonable cost. He just happened to send me some texts containing photos of PLCs that he uses. That was enough to have the proverbial lightbulb moment. A few minutes later, late in the day, just before AutomationDirect closed for their Christmas break, I ordered a CLICK PLC with a power supply and two 16-circuit I/O modules. I figured with their 30-day money back guarantee, that it was worth taking a chance. I'm sure that almost anyother PLC could do the job. The CLICK (ethernet model) with the extra I/O is overkill for this particular job.

As the screen shots show, it only takes 27 rungs to capture the data and then to act on it (less than 1 millisecond PLC scan time). I grab the state of the input signals, store them in memory and then combine a little logic to get the desired result. In this first test, I use the Acorn's NoFault output on Output 8 to signal when the Acorn is ready. Using that signal, I have the PLC issue Enable signals to the ClearPath motors to keep them energized (otherwise, I would have to use a brake or counterbalance to hold vertical position).

Normally, I use self-latching contactors for motor power supplies; however, it seems that both Teknic and DMM require that power to the servos be cycled if a fault occurs (to clear the fault), so the PLC controls sequencing of the power supplies. Because I have extra PLC I/O available, I'll probably use three-way switchs (Auto, OFF, ON) for the motor power supplies and because the PLC can easily perform latching signals, I won't have to hard-wire the latching circuitry.

Right now my test bench is a maze of wires. I tend to make things overly complex and then to simplify when each part of the control works as expected. Someday I'll post photos of a "finished" controller. One thing that I like to do is to separate all signals, i.e., the state of each servo's fault signal is captured and stored, then a signal is sent to the Acorn. The same applies to the E-Stop circuit. Each condition that should cause an emergency stop is sampled and stored and then a single signal is sent to the Acorn. With a PLC, I can sequence Power Supply power-up to keep surges to a minimum. I can also delay the DriveOK signal to the Acorn until all circuits have had time to stabilize. Currently, I use 5 of the 500 available timers in the CLICK to keep the start-up process orderly.

After watching Marty's videos, Gary's videos, Franco's videos and CNCKeith's videos (many of them many times), I'm still blown away with what the Acorn can do. Now that I know basically what IT can do, I needed an easy way to mix and match any component that I wanted to use to get the job done. In my case, I prefer the ClearPath servos, but I need signals that the DMM servos have. I needed a way to easily cycle power to any or all of the drives without over-cycling power supplies. I needed a way to keep the servos enabled. Add to that things that I would like to do and a PLC for my purposes became viable and almost a necessity.

p.s. It's good to know of that South Jordan is on your map too.

[Richards]

I normally build a test model of new designs before building a control box. It allows me to measure signals, make changes, and test ideas.

Here is my test model for the Acorn, PLC, ClearPath servos, DMM servo and two proximity sensors.

I'm using an old 75VDC linear power supply to power the ClearPath servos. The DMM servo has its own DYN4 power supply. The PLC controls power supplies, the motor enable signals, the DriveOK signal, and the homing signal.

[Gary Campbell]

NICE!!!
"You can teach an ol dog new tricks!!"

Keep up the posting Mike. Thanks for the pics

[burbingus]

Hi guys,

Thanks for posting your projects on here. I'm in the process of trying to decide on a new controller for my mill. I like the look of the Acorn, but I also will want to add some functionality later that will require additional I/O outside the Acorn board. I am having trouble understanding how the different parts of your design are connected and communicating. Could you help me understand:
-How is your separate PLC unit connected to the Acorn or CNC PC?
-How does your PLC communicate and synchronize with the Acorn? Is it through the control software on the PC?

Thanks guys! I'm trying to understand some of the fundamentals of how this control system work before I buy in.

Micah

[Dan M]

^ plus 1. I'm still trying to understand two ethernet cables. Are you running a second pc for the plc or some sort of data hub?

[Richards]

I'm not sure what's going on. A long post that I submitted this morning dissapeared. I'll try again.

The PLC is not tightly connected to the Acorn. It monitors the NoFault output from the Acorn. Upon receiving that signal, it turns on the auxilary power supplies to the DYN4 and the ClearPath servos. Depending on an On/Off switch, it might turn on a 40VDC power supply to some steppers instead of powering the DMM servo and the ClearPath servos. The PLC monitors the fault signals from the DMM servo and the ClearPath servos. That signal is LOW on the DMM when there is a fault. The ClearPath servos send a LOW signal when there is no fault. The PLC straightens out the logic so that the signal to the Acorn is valid whenever their is a fault. The PLC has signals connected directly to the Enable lines on the servos. It senses the Enable signals coming from the Acorn, and depending on another On/Off switch, it either uses the Acorn's signals or generates its own Enable signals. I prefer to keep the servos enabled as long as the Acorn is sending its NoFault signal so that I don't have to use a brake on the Z-axis. The PLC monitors various proximity sensors and generates the proper signal to the Acorn to signal a limit error. The PLC monitors various other conditions, including the E-Stop switch and notifies the Acorn if a Stop condition exists.

In short, the PLC takes nothing away from the Acorn, but it allows me to mix and match devices that have opposing signals without designing my own hardware to handle signal mismatches.

The PLC does interface seamlessly with the Acorn board. It massages signals only.

I have two ethernet cards in my PC. One is dedicated to the Acorn board. The other connects to my office network via a network switch. The PLC also connects to the office network via that network switch. The Click PLC is connected to another PLC via ModBus which will allow several Acorn based CNC machines to be connected together. I just purchased an Android tablet to be used as an HMI device so that I can montitor and control the PLC without using expensive switches (The AB Green/Red relay latch switch cost $75. Indicator lamps cost about $20 each. On/Off selector switches cost about $20 each. Before long, using a $125 tablet is much cheaper than using individual switches.)

The PLC does makes it easier for me to do what I want to do with the Acorn board without designing specialty boards. Very few people will need to add a PLC unless they mix and match parts that have conflicting signals. That's why I tried a PLC. It works. It allows me to enjoy full functionality from the Acorn without stressing over incompatible signals. It also provides a buffer between my "field wiring" and the Acorn. If I do something stupid, the PLC loses an input or an output. The Acorn is left undamaged. So far, I've only blown up one output on a 16 output PLC card. It was a stupid mistake, but the Acorn was protected.

[carbuthn]

Thanks for the PLC information.

I have decided to go the PLC route on my lathe and mill. It may be overkill on my part but I think it will be cleaner and easier in the long run.

I purchased the ethernet basic version with relay output (it appears that the ethernet one adds timers that the RS232 one does not have). From the price that you listed it did not appear to use relays. Are the relays not need with the Acorn board?

Chuck

[Richards]

Chuck,
I'm also using the Click Ethernet basic relay model, the larger power supply, a 16 point 12/24 source sink input module and a 16 point sinking output module. I also purchased three zip link cables and connectors. That's overkill, but for test bench work I wanted enough I/O and quick connect cables for easy wiring. The actual PLC that I will install on a CNC machine will only have the minimum I/O to do the job. I use the relays on the PLC to control contactors for the three auxiliary power supplies.

[Richards]

Another quick note:
Although the Click PLC uses 24VDC, the same as the Acorn, I decided to buy the larger of the two Click power supplies so that the "field" wiring is separate from the PLC's control circuits. That way, the opto-isolators separate everything.

[carbuthn]

I didn't get the zip-link cables (didn't know what they where), I will look into them.

I do have a 16 input module that I was going to send back (have RMA, ordered it based on a number that I found thinking it was a PLC). It was late at night/early morning. I may keep the input model at least until I am sure that I don't need it.

I purchased the same power supply so as to provide separation and to provide backup power (keeps memory in the clearpath servo) and to ensure that I didn't overload the Acorn power supply.

Thanks for the information,
Chuck