Analog input to my versamax!

[abhi6136]

hi all,
i am fresh wityh PLCs so need a lil help.
i am trying to give an analog input to the PLC and compare the analog input with a preset value.but the analog input fluctuates too much. how to go about decresing the fluctuations or if thats not possible how to compare the two?precisely!

 

[dchartier]

Hello abhi6136;

The most professional way to start with this would be examine why the 4 to 20 mA signal fluctuates (check connections, cable, transmitter sensitivity...).
If this is just for a quick fix, use a on-delay timer to filter the value for a determined period; the same trigger that resets the timer tranfers (MOVE) one present value of the analog signal to a register. The value in the register will be stable until the timer resets again and a new value is written to it. And so on...
Hope this helps,
Daniel Chartier

 

[abhi6136]

thanks daniel
ill try that..well actually i am trying to design a motor overcurrent protection scheme.
the problem is the current is alternating and so the input register value is also fluctuating..shall i filter the current and then give it to the comparator?

 

[dchartier]

Hello abhi6136;
This is far from by usual hunting grounds, so bare with me if the questions I ask are not useful.
But: if you are doing something related to overcurrent protection, shouldn't you be looking at peak values of the current? And their rate of increase?
And what reaction time are you looking for to protect your circuits? Motor ptrotection is usually based on protection relays with very fast, microprocessor-based circuits. Do you plan on implementing this with a PLC?

Hope thisd helps,
Daniel Chartier

 

[abhi6136]

the thing is..lets say we looking to cut off the relay, wen the current in the line reaches 9 amps so an alternating voltage signal corresponding to the current is generated using a simple resistor, then we rectify it using full bridge diode rectifier, now the problem is that the ripples produced in the rectified DC signal is too much to be directly given to the PLC so as to make a comparison with a pre set value. Even if we add a RC filter circuit after rectification, then also the level of precision is not per our satisfaction. As far as operating relays is concerned and rise time of current, we r dealing in range upto 100milli seconds. so the rate of rise of current is not a problem. the problem is if the current above a certain value persists for 100ms or more, then only relay should operate, and that kind of time precision can be obtained using PLC driven relays. The main problem concerning us is to reduce the ripple in rectified DC signal. Or if you have any other suggestion for conversion of AC signal to DC signal, do suggest. the precision we lookin for is say for 5V, the maximum peak to peak ripple should be 0.3 or 0.4 Volts.. so around 5%.
plz do reply.

 

[RussB]

You might try putting a small capacitor across the leads. This will dampen the changes, but also lengthen your reaction time.