micrologix 1200 high speed input

I have a 1762-L24BWA microligix 1200 that gets an input from a proximity switch that is mounted near sprocket the prox picks up once per revolution it is only a 24 tooth #40 chain sprocket the motor is on a vfd that needs to run from 30 to 60hz 50-100 percent
the program reads the input up to about 70 percent is there any way to increse the speed that the input will read I see in the controler manual that there is a input signal delay Selectable: 0.025, 0.075, 0.1,
0.25, 0.5, 1, 2, 4, 8, or 16 ms

I can not find any infromation on selecting a input delay
or is there any other way you know of to reed at such a high speed
if there is no solution in the 1200 are there expansion cards that will read these speeds any help would be appreciated

Thanks

Are you using the embedded High Speed Counter feature of the MicroLogix 1200, or are you just using a Counter in the normal control program ?

Do you know how long the input pulse actually is ? In my opinion the only way to tell for certain is to use an oscilloscope.

Can your proximity switch change states fast enough to keep up with the target ?

Let's say for the sake of argument that the sprocket is 8 inches in diameter at the point where the proximity target is placed, and that the target is 1/2" in diameter.

An 8" circle is 8 * pi = 25.13 inches circumference, so you're sensing 0.5 inches of that, or about 1/50th.

At 60 Hz, let's say you're turning at 1800 RPM, so that's 30 revolutions per second, or 0.033 seconds per revolution (33 milliseconds).

The proximity switch is only made for about 1/50th of the revolution, so that's 1/1500 of a second, or about 0.67 milliseconds.

The high speed counter on the MicroLogix 1200 can handle input frequencies up to 20 kHz.

Even though you are sending it just 30 pulses per second, the duration of the pulse is as short as if you were sending it at 750 Hz.


Of course, all of the above is conjecture; maybe your trigger is half the sprocket.

The first thing you need to do is measure the geometry of the sensor trigger and figure out how long the proximity switch will be triggered at full speed.

Then you need to get an oscilloscope and determine if the signal is actually reaching the MicroLogix input as expected.

Only after you have done those two things should you start working on configuring and troubleshooting the MicroLogix.

And here is the manual in case you don't have it. Chapter 1 page 38 and chapter 5.

http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1762-rm001_-en-p.pdf

Ken and Mickey covered it all except one thing I can think of:

Sensor output speed, which is often assymetical with an inductive prox.

I have used flags shaped to replicate 150 to 180 degrees of a shaft rotation, made from half a lock collar...then you can usually count with a regular input. Trying to sense sprocket teeth is much better suited to optical sensors that can be placed at a safe distance, and have a very high frequency output, and very low latency. Sense the hub, unless your finest position requirements are too small for that. I have used keyed lock (clamp collars) with half of a larger one welded on the outside I would grind down the end where the capscrew went first because in my experience, the off switching times for standard inductive proximity switches was often longer that the switch on time.

But, with a regular DC prox to a DC input, you can get down to what Ken explained about the scan time of the PLC, and lest we forget, there can be another layer of delay time at the input card which should be considered.

If you are over halfway to your goal, you can probably benefit from tweaking the squarity of the signal wave (on and off times equal) I agree with the use of a scope to measure the input. It will save you so much time, it is worth it to beg borrow or rent one if necessary.

The above is much less applicable if you are using a purpose built high speed sensor, hall effect or other.

And, depending on other unknown factors like the calculated frequency of the top speed at the sensor, you may be able to use the immediate input instruction, which I rarely recommend.