Speed measurement

By using linear potentiometer how to measure the speed of the movement in S7-200 plc.
Linear potentiometer is fixed in a carriage actuated by Hyd. cyl. which is controlled by servo valve. Servo valve command and linear pot. is interfaced to S7-200 PLC.
Is it possible to measure the instantaneous speed by using the linear Pot. How to write the PLC program for this application.

sathya:
i installed same devices as yours but with AB plcs, in order to measure displacement and speed of the hydraulic cylinder i use a temposonic device, it uses magnetostrictive technology and it is specifically designed for this purpose. try www.temposonic.com
Using a linear pot to get speed, i think it can be done, not sure about precision.

sathya_max said:

By using linear potentiometer how to measure the speed of the movement in S7-200 plc.
Linear potentiometer is fixed in a carriage actuated by Hyd. cyl. which is controlled by servo valve. Servo valve command and linear pot. is interfaced to S7-200 PLC.
Is it possible to measure the instantaneous speed by using the linear Pot. How to write the PLC program for this application.

Click to expand...

Hi,
I would do like this:
1. make one pulse(timer) generator it will be for sampling data, cycle will depend on what your applications is doing (tgen: 10ms or 100ms should be enough)
2. you'll need one FB for scaling data of the position from lin. potentiometer (go http://www.oscat.de/ -here you have librarys with various FBs) just to scale from raw data to engineering data like millimeters, meters, or percent of the position.
3. Every cycle of the first tgen you calculate difference from last and current position and this is your dL/dt!
4. Equation will be for ex. 100ms samp.rate and reading in 1s:
v=(dL1/dt1+dL2/dt2+...+dL10/dt10)/10 [your_scaling_data_unit per 1 second]

This is in fact averaging speed reading in 100ms for 1s, but if you use 10ms sampling rate, you'll get more precise measuring, but also bigger number, keep that in mind when accumulating position difference.

Also, personaly I don't programm S7-200 so I can't write you down code.

sathya_max said:

Is it possible to measure the instantaneous speed by using the linear Pot. How to write the PLC program for this application.

Click to expand...

The key word is instantaneous speed.
This can't normally be done for a few reasons:
1. As pointed out above you can only measure the difference between two samples. This means the speed is an average over time.
2. The analog input module probably has long filter which are good for temperature readings but not good for calculating positions or velocity.

The best you can do is to make a derivative circuit board that finds the derivative of the signal coming from the pot and connect the output of the derivative circuit to another analog input. This is a lot of work but since the derivative circuit is before the analog input cards the value should be accurate after it is scaled.

Now if you want a filtered value over time then you can do as suggested by other above but the filtered value over 100 milliseconds is often too slow.