1756-HSC...help

Recently, I have learned the hard way that the 1756-HSC loses its count when power is taken away from the module. Unfortunately, the customer must remove power for certain maintenance situations. I am wondering if anyone has experience with this module and can tell me if the logic I have come up with will solve the problem of losing count. The module is basically being used like a cam: an encoder sends pulses to the HSC and after one revolution 0-359 it is reset. I have two scenarios which I have developed to capture the value and load it upon power up situation.

Any insight you have will be appreciated.

Happy Trails...

Kieffer

Can you not use an encoder with binary output that ALWAYS shows the good position?

Do you have a DOG on this setup where you will verify if the count make sence?

It would be nice to have an absolute encoder, but an incremental is being used. And you will have to expand on the DOG, I am not sure what you mean.

Happy Trails...

Kieffer

I don't know the details of that HSC module, but here's a possible approach.

Mount a proximity switch so that it senses a target that presents itself once per revolution. Use the prox to reset the counter's accumulated count. I think this is what Pierre means when he uses the term 'dog'.

A couple of things to consider. The prox and target have to be mounted in a portion of the cycle where you don't need to make any decisions based on the angle because the count is not going to be increasing while the prox is sensing the target. Usually, you would reset the accumulated count to zero, but if you need to make decisions at or near zero, you might be able to reset to a non-zero value. That's one of the details that I don't know. You might be able to hard-wire the prox to an input on the module and have the module reset itself . That would be the most reliable and repeatable way, but it's another detail that varies among HSC modules, so you need to see if it's an option for you. If you have to wire the prox to a PLC input and reset the count using ladder logic, make sure that the target is big enough so that the PLC can detect it at maximum speed.

Force user to home axis after powerup.

That was my original thought and I tried to convince the customer that was the most robust way, they seem to think that removing the human element would be the best solution.

I did speak with Rockwell's tech support and they seemed to think that I had the correct logic to solve the problem. I am really wondering if anyone else is familiar with the 1756-HSC.

Happy Trails

Kieffer

Kieffer:

I've used very similar logic to what you have - basically on power up I send data to the HSC, otherwise, I send data from the HSC to the processor. As suggested in other posts here, I reset the counter (via a prox being made) once per revolution.

While this works 95% of the time, we have a periodic disconnect between the HSC and the processor that I have never been successful debugging, so I have a manual "homing" process that can be called by the operator to fix the problem.

Gives more of the functionality that your customer wants, but has (as a backup) the more robust "home" routine built in.

Good luck,

Marc

1. use absolute encoder
2. use incremental and force user to home machine upon powerup
(this can be automated if axis is powered).
3. try to use DOG to correct counter value periodically. Someone
suggested once per revolution (sounds too agressive to me).
I would try to do it once per cycle if possible.
4. use encoders that emulate absolute encoders (they get battery
backup and run own counters that are periodically copied to PLC,
now this will NOT work with 1746-HSC).
5. don't use encoder, forget about project and get another customer
6. let someone else handle it or change job
7. retire

as you can see, you still have plenty of options.