More encoder help

Hello everyone, I have learned a lot about incremental encoders today. But I am down to one last issue and it's a big one. I have 8 incremental encoders hooked up to a powdered metal press. The encoders are just keeping position of certain functions on the press. When the press powers down and back up I loose my position on my encoder and the press is not homed. Is there any way to adjust my plc program to work around this issue? I know the correct fix is an absolute encoder but that's not what I have.

Is there any way to detect "home" external to each the encoder, so could you basically do a calibration to get the encoder initialized?

On power up you must home all encoders
that may mean that you need to run a homing program
that may mean that you have to home each axis in a sequence to ensure they don't crash
you should have a at least a home switch i would recommend that in addition that you also
have travel limits on each axis to ensure tat you don't crash

As Gary said you need a homing process. Don’t allow any auto operation until it is completed, you will need to track this. You can program a routine to do it, or if it is simple enough you can have an operator do it after a power cycle.

Another thought, many brands of encoder will rollover at zero, meaning anything above zero is a positive value, anything below zero is a negative value. You need to make sure you account for this.

Most incremental encoders will rollover from zero to -1. Most absolute encoders will rollover to the top of the scaled resolution. Meaning if the scale is 0 to 10000, if it goes below 0, your next value will be 10000. This can cause obvious unintended movement. Make sure you know how your encoders will react in the case of +|- 0 position.

I would strongly suggest that you come up with a homing subroutine, this allows you to prioritize the motion of your various "certain functions" to ensure there are no collisions amongst the moving parts of your press.

I would also suggest you consider using proximity sensors as opposed to mechanical limit switches. Prox sensors are far superior in repeatable positioning applications.

Keep in mind that a homing operation does not require that you arrive at a zero position, it merely requires you to arrive at a known location. This can be useful if you have long distances to travel to reach a safe mechanical limit.

I recommend you do some reseach into how a typical servo controlled axis is homed. What you have, is for all intents and purposes, 8 servo axises.

Is there any way to adjust my plc program to work around this issue?

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As my colleagues explain, the most effective and probably safest way is to implement a homing routine.

There are many different methods of homing an axis. Homing to a switch, or to a Z-pulse on the encoder within a switch range, or driving the axis to a hard-stop or independently measuring the position of a mechanism are all different ways to determine the absolute position of an axis that doesn't have an absolute encoder.

Harder-to-interrupt power supplies that keep the PLC and encoders powered during power losses are another way.

I've seen some folks try to build systems that have enough UPS capacity to detect a power-down and save the encoder positions to nonvolatile memory, and rely on the machine to have not moved while the power is off. That is seldom a reliable or dependable method.

Your method is very probably going to involve a power-up procedure for the press that involves the operator commanding each mechanism to go to its home or defined position so that the machine's kinematics can be re-established after a power shutdown or maintenance of any of the mechanisms.

there are always alternatives to everything with each you have gains and loses
you and you corporate managers must way things out and see what my work for you.
One option you may want to consider is forget the encoders and us a laser measuring system.
they will read the position every time even after a power cycle they don't need homed
so there is no stored positions to keep track of.
I have used them in the past and they run for years with out any problems.

Cody, can you tell us more about the mechanisms in this press ?

This is the same system you were talking about the 1734-IK encoder module, correct ?

Are you controlling hydraulic cylinders ? Air cylinders ? Motors driving belts or chains or screws ?

Is this a powdered metal press you are rebuilding the controls for, or that your company is building from scratch ? Who designed the control system ? Did they do a functional safety analysis ?

Metal forming presses are a specific category of machine tool that you need to have a minimum level of experience to work on the controls, so you can be sure they comply with industry standards like 29 CFR 1910 Subpart O and ANSI B11.16.

They're outside of my area of expertise: I don't work on them even though I have a professional license in control systems engineering.

I don't want to discourage you from your efforts, but do want to remind you that one of the hardest-won pieces of experience in my career was how to know when I'm really in over my head.

I realise this isn't a perfect solution, but couldn't he use retentive memory for the current positions. The values would be stored through a power cycle. He would then have to add a reset function to zero the system when needed. As Ken said if the machine is moved during power down, you are stuffed.