Circuit for polarity detection

[kautilya]

Can anyone suggest to me a simple circuit constructed using digital gates for polarity detection? The circuit should be able to detect positive, negative as well as zero polarity.

I need to check the polarity of a dc motor which is controlled by a bidirectional converter. The bidirectional converter has to be controlled in current mode. If any one has ideas regarding the same, i will be grateful.

But I am not implementing it in real time. I am trying to simulate the logic in MATLAB/Simulink. Can you suggest how I can do the same in Simulink using logic gates? or any other digital gates? The truth table is given along with the schematic for reference as to the expected binary output.

I have attached the schematic in pdf.

Thanks and regards

kautilya

 

[ronv]

Are you really trying to detect when the motor is stopped?

 

[kautilya]

Hi there

I am not trying to detect when the motor stops. The motor acts as a load when the polarity is positive. it acts as a source when the polarity is negative. there are times when it doesnt have any polarity. i am trying to simulate a circuit which detects the change in polarity of the motor. Can you help me on this?

Thanks and regards

 

[MikeMl]

The controller needs to know if the motor is still producing "back EMF" because it has not spun down yet.

You need two "differential comparators", like LM311s. They have to be set up with a small "dead-band" in the middle. Practically, the "common mode" input range of the comparators must span the peak motor voltages. Three of the four states will be useful: 10 = +polarity, 00 = stopped, 01 = -polarity.

Or, if you are using a ucontroller, with an ADC, use an OpAmp configured as a "diffamp", with an offset so that zero motor voltage is half-scale on the ADC.

 

[Dr_Doggy]

2 LED's back to back(in series with motor) should work for simulators