The only problem with that is the TS wants less than 500Hz switching. Which specifies an upper limit but no lower limit. If the rates are realistic a mechanical relay could accomplish the job.

 

Interesting. A PLC (Programmable Logic Controller) works with timers and relays. All you need is a set of timer circuits using say sets of RC Timers with each timer going to a comparator with some hysteresis along with a simple threshold detection limit (preferably with some Hysteresis. These drive the relays. Then you just use ladder logic. Is that what the TS is asking for?

That is not exactly "programmable" in the same sense that a PLC is.

The only problem with that is the TS wants less than 500Hz switching. Which specifies an upper limit but no lower limit. If the rates are realistic a mechanical relay could accomplish the job.

Depending on load, the life of a mechanical relay at 500Hz could be measured in days or hours.

 

That is not exactly "programmable" in the same sense that a PLC is.

Depending on load, the life of a mechanical relay at 500Hz could be measured in days or hours.

Then we need to ask the TS what the lower boundary is and also make sure he really needs to go to 500Hz

 

What is the upper and lower boundary on the frequency? Do you really need something at 500Hz?

 

The fact that a CNC package has a section that can be programmed with ladder logic, and may even have similar I/O, does not make it a PLC. It is still a CNC device that can run ladder logic while it is doing CNC magic stuff, like driving multiple axis to produce curves.
Of course, with those systems that add motion control to standard PLC functions, they do blur the line between the types. But the CNC packages I have seen all included a panel for entering the control values, which I have not seen on any PLC that I have used so far. BUT I do mostly use the less expensive PLC models.

You still don't get the point, the CNC side and the PLC side are two different entities which are capable of communicating with each other by passing info over a common/shared buss.
The PLC side just runs the general machine control. As in the case of Fanuc, Mitsubishi and others, they are identical to the Descrete PLC made by these companies. just imbedded in a dual unit.
Also to me, it full fills the original concept developed by Dick Morley at the behest of General Motors requirements . back in 1968.

 

For the switching speed, the TS clearly meant "Up To 500Hz switching rate." That makes perfect sense to me, at least. No, not a relay, but probably a TTL compatible signal as would command a stepper driver. Or possibly a PWM controller.
A package with a timer based on RC timing is not even close to being equal to a PLC timer, nor probably anywhere nearly as suitable for controlling a machine that is intended to run production.

And @Max: If the assembly is intended for CNC operation, then it is not a PLC, even if it also contains a PLC function in some portion of the same package. Also it costs a lot more.

 

Then we need to ask the TS what the lower boundary is and also make sure he really needs to go to 500Hz

What is the upper and lower boundary on the frequency? Do you really need something at 500Hz?

I mean, he did clearly state 500Hz, which is well within the capabilities of any high speed PLC I/O and possibly even doable with ordinary (non-high-speed) I/O on some PLC platforms if scan time is <2mS. It isn't a crazy requirement and I'm not sure why we should be suspicious of it.

 

I mean, he did clearly state 500Hz, which is well within the capabilities of any high speed PLC I/O and possibly even doable with ordinary (non-high-speed) I/O on some PLC platforms if scan time is <2mS. It isn't a crazy requirement and I'm not sure why we should be suspicious of it.

Good points. I was under the impression the TS wanted to use a standard set of mechanical relays (i.e. SPST,SPDT,SPTP etc.)

 

@Dragonoid Like I mentioned, Opto-22 has schematics details for their various I/O .
e.g. here is one such example.

 

Good points. I was under the impression the TS wanted to use a standard set of mechanical relays (i.e. SPST,SPDT,SPTP etc.)

I would not expect to use any mechanical relay at even close to 500 Hz. Some PLC systems are much faster than others, which is reasonable because applications vary widely. PLC response times vary quite widely, as do PLC prices.
For a frequency range, the understood minimum is zero, so why get excited about that not being stated. The TS was not asking about a frequency generator.
What I see is a request for an isolated I/O module able to function with kilovolt common voltages. A fiber optic connection can easily allow that, it will be the module power supply that will be the challenge.

 

I would like to know if there is a optimal design for PLC Output stage with 0.5A Output current and under 500Hz switching.

BTW, what is it intended to switch and also the voltage level? AC? DC?

 

If the assembly is intended for CNC operation, then it is not a PLC, even if it also contains a PLC function in some portion of the same package. Also it costs a lot more.

So what would you rate the first CNC systems that used an outboard/stand-alone off-the-shelf PLC which had a communication via I/O card to the CNC controller?
All they did for later versions, was to merge the two into one unit. every thing about the PLC was virtually identical to the externally implemented one. processor, memory, I/O etc.
For me, one main criteria for a PLC is that the Boolean logic is displayed in ladder format and all functions show their status, This was due to the ground breaking creation by Dick Morley that fulfilled the requirement laid down by GM. And has been a boon to shop floor maintenance personnel ever since.

 

I recommend the following:


The following truth table shows this is possible:

View attachment 303649

Thus all the TS conditions are covered. We still may need some external electronics (Transistors, etc.)
The commons need to be tied all together as well. There are 4 relays on the board each on SPDT (I am not sure why we think there is 6 relays on the board when there is clearly 4. We may still need some external electronics like transistors, switches, etc. But IMHO this should be workable.

 

There a

I recommend the following:

View attachment 303648
The following truth table shows this is possible:

View attachment 303649

Thus all the TS conditions are covered. We still may need some external electronics (Transistors, etc.)
The commons need to be tied all together as well. There are 4 relays on the board each on SPDT (I am not sure why we think there is 6 relays on the board when there is clearly 4. Per the following truth table:

I simplified the truth table:

 

 

By tying the input negative side to the output side common all isolation is prevented. Isolation needs to include the common side, otherwise it is not isolation.

 

View attachment 304270

I am not sure how this accomplishes the goal of 4 SPDT relays along with three devices, each with 3 states : "charging, on and off'. Can you elaborate?

 

I am not sure how this accomplishes the goal of 4 SPDT relays along with three devices, each with 3 states : "charging, on and off'. Can you elaborate?

Unless I missed something, this goal of 4 SPDT relays is your own goal and TS never said anything like it. I have been puzzled as to why you have been in such dogged pursuit of it.

 

Several of these posts belong in a different thread. #33,#34,#37, and probably #35.
Or am I the only one seeing this???

 

Unless I missed something, this goal of 4 SPDT relays is your own goal and TS never said anything like it. I have been puzzled as to why you have been in such dogged pursuit of it.

I understand where you are coming from but the TS did provide this information. If you look closely at the board, there is 4 SPDT relays and each relays is labeled with a common connection and text stating 'OFF' for the NO contact and "ON' for the normally closed contact. Which also implies the coils providing power to those relays is coming from the board. That is not a surprise to me. He also provided a table with 3 devices having 3 states (Charging, On, and Off).