lab,
Mentioning light curtins can open a can of worms, as you probably notoced from the responses.
Firsly, you should determine what your light curtin is to do. Is it a safety device or is it simply a measurement device? If it is only for measurement and not safety, just wire it to a PLC input as you would any other device.
If it is for safety, then stop what you are doing, take a few deep breaths, and think for a bit.
The rest of my post will deal with the safety aspects.....
1, Know the rules....
Your local authorities will have a set of safety regulations and standards that you will have to follow. I would strongly advise against designing any safety related equipment unless you are familiar with these rules. Also note, if you are designing equipment for export to another reagion, you may have to check the rules for this reagion, since they are often different.
2, Risk assessment......
Most safety standards require different designs for different levels of risk. For low risk equipment, it may be acceptable to wire through an ordinary PLC. For equipment that can potentially cause injury or death, you must use some safety rated equipment. These are safety rated PLCs available, although if the one youv'e got is not safety rated I doubt you will be changing it. Instead you may have to run your light curtain through a safety relay.
3, Feedback......
Your safety circuit must be checked to ensure it has operated correctly when it operates. Light curtins generally have this built in, and using double contacts for electrical equipment with cross checking is normally well understood. The difficult part is isolation of air circuits. Generally you should use two solinoids in series with position feedback from the spools. without these, you cannot determine if a solenoid has failed or is correctly operating. You also must determine what ha[pens when you dump the pressue from the air lines, will anything drop?
4, Consultation......
You must also consult with your management team on your risk assessment and safety equipment.
Designing good safety systems needs careful planning and attention to detail. I would look carefully at your system before proceeding.
Doug
