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Mr JJ

Allen Bradley PLC

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Hi, Looking for recommendations using an Allen Bradley PLC for a simple project 2 or 3 inputs ( 24Vdc NPN) and 2 or 3 outputs (24Vdc NPN or relay) and I want to use an encoder with an internal HSC. The speed of the application is slow, 2 second cycle time. Which PLC would probably be the best to use? I thought the ML1000 should do the job? Also how would one figure out what HSC one would need with respect to the speee of the system rpm and the HSC? Where would I find the calculations? Thank you for any help you have provided John

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An added advantage for the Micrologix 1000 is that the programming software is free. The use of a high speed counter is dependent on the timing of the input to be measured/counted. What are the minimum on and off times of the signal? What is the overall frequency?

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That was the second part of my question, How would I figure out what min. frequency i would need? How would I calculate that? The cycle is less than 2 seconds and the sensor would see it approximilty .25 sec and be on for a .25 seconds

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If the input is on and off for 250ms, then you don't even need a high speed counter instruction to count the pulses. With so few I/O, I would expect your scan time to be very fast, probably less than 10msec, so you should have no trouble detecting that input in both states.

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For your encoder, you will have to decide if you want to measure single channel or multiple channels. The next question is to calculate your expected encoder pulse rate. Motor to gearbox to physical motion...what is the input and output shaft speeds. This will depend on the mechanics of your system and the encoder you choose (typically 1024 PPR [pulse-per-revolution]). Once you have your expected pulse rate, you can then dig into the specifications of the HSC input of the PLC. Sometimes a HSC card or input seems overkill for only a few I/O, but I have found the HSC card/input can repeatedly measure an encoder input whereas sending the pulse train into a cheaper I/O module may have repeatability issues if the program gets bogged down or grows. If you can get by with a slower, less accurate speed or position feedback, I have done it before with a "poorman's encoder" (proximity sensors firing off of nuts on the back of a motor/gearbox coupling). Not the most accurate (+/- 10% or more slop), but cheaper than dirt if you need to do this for many axes and your accuracy is low.

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Another option for encoders is something you might not ever notice, the Electro Sensors (www.electro-sensors.com) SG Series Position Monitors. I know that the data sheet talks about grain silos and such, but give them a bit a of break...they are in the middle of the grain belt of the U.S. The SG1000C especially is literally an absolute encoder except that instead of the usual binary inputs or pulses, it outputs 4-20mA. Now the cheapest PLC I know of that has an analog input is the Micrologix 1100. Now it's not pretty but it has two 10 bit voltage voltage inputs. If you buy a cheap metallized film resistor of the appropriate size for <$0.10, you can convert the current loop output of the above device to a voltage input which can then be read by the Micrologix 1100. The Micrologix 1100 has 10 inputs, 6 outputs, an Ethernet port, serial port, LCD screen and buttons, and 2 analog inputs, and the "free" version of Logix 500 does support it. Edited by paulengr

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