Convert pulse to square wave

Hello Forum,

I have a machine being commissioned that takes a flat sheet of metal and forms it into a series of convolutions. Each convolution must be detected as it exits the press to ensure that the press is not jammed up. The sensor fitted seems to produce a very short pulse as the convolution exits the machine(to be confirmed).

I'm in need of a device that can convert a series of very short duration pulses (24Vdc) in to a square wave so that a PLC can see it. My first though was to use a D Flip flop to divide pulses by 2 and produce a square wave.

Has anyone come accross such a device that operates at 24Vdc? Can anyone think of another way to convert the pulses to a square wave signal?

Nick

The first two questions:

1. How long are the pulses? That will determine whether they can be read directly by a PLC Input.

2. Which PLC and which Input module are you using? That determines how long of a pulse can be directly read.

There are electronic devices called Pulse Stretchers that read a short pulse and convert it into a longer pulse.
http://www.ifm.com/products/us/ds/E80110.htm

http://www.analogmodules.com/admincenter/datasheets/611a.pdf

Lancie,

Thanks for your response.

Until I get someone on site (France) with an oscilliscope, I can't be 100% certain whether the pulse is too short for the PLC to detect (currently running a 30mS task) or if the sensor is simply not producing a pulse. The machine is cyclic and operates up to 140 cycles/min but the moving phase is less than one quarter of the cycle and not at a constant velocity. During this movement cycle, the sensor should see the passage of one convolution represented by a change of height and then a return to the previous height. Depending on the shape and pitch of the product being made, this may only be a very short period.

The PLC is RA Compact Logix with remote I/O on ethernet but I didn't design this control system so can't say what exact components were used.

The machine operates up to 90 cycles per minute no problem and on all products except one it runs at full speed without any problems. I will have an engineer on site next week to look at it and I want to make sure that his pockets are filled with all items that might be needed.

Nick

Nick,

You can find the specs on the CompactLogic Input modules, which will show the minimum duration that an input has to be on before being seen by the PLC. That takes care of the PLC end, but the pulse length is dependent on the sensor, so you will have to get the brand and catalog number to find anything about the sensor.

It could be that a High-Speed Counter instruction is being used in the PLC, which will mean that much shorter pulses can be read, so the PLC end may not be the problem. That would be a question to ask, or maybe you could look at the program to see if it uses a HSC instruction.

It probably would be smart to buy a spare sensor, and also one of the 24 VDC pulse stretchers and have on hand, just in case.

Lancie,

Thanks for your help. I will try and get the IFM module delivered this week and make sure that the PLC task is running at 10ms or faster.

Nick

Hello,
In the past for "pulse stretching" I've used this programmable timer:
http://www.balluff.com/balluff/MUS/en/products/catalogue/products_details.jsp?id=BAE002M-122422
Tom

Manglemender,

rather than use a flip flop, have you considered a Schmit trigger circuit?

I designed a jk flip flop circuit for detecting pulses and it oscillated. one of the engineers designed it with a schmit trigger & comparator to reset the circuit and it worked great.

regards,
james

Thanks everyone for your advice.

On further investigation, the Keyence Sensor/Amplifier used to detect the convolutions has built in timer functions for the output: On Delay, Off Delay, etc. It should be able to produce an extended pulse without the need for an additional component.

James, thanks for your input. I have used shmitt triggered devices in the past to clean up/buffer encoder pulses to great effect. That was 20 years ago but I might still have the circuit somewhere.

Nick