Closed loop control DC Motor

[viplavmulka]

In the example below,

http://www.ni.com/tutorial/12944/en/ 

The inputs and outputs of the transfer function are voltage and angular velocity.

My questions are,  

1) When we are putting the setpoint in the PI controller is the output unit even in voltage for the angular velocity or in rad/sec?

2) What if the tachometer used is showing the speed in voltage as my board has frequency-voltage converter and i want to convert it into rad/sec for showing angular velocity?

Thanks in advance 🙂 

[MikolajM]

Hi,

I'm Nick, AE at National Instruments.

If you put a setpoint in PID controller, the output still has to be in volts. The transfer function unit is (rad/s)/V therefore any input other than volts would output angular velocity in incorrect units. It's the plant that transforms V into rad/s.

In the loop diagram you should consider every particular wire to always hold the same unit. A block will change unit and output a wire with a new unit.

For the tachometer output, have a look at this White paper: https://forums.ni.com/t5/Example-Code/Decoding-Tachometer-Signals-Using-CompactRIO-and-LabVIEW-FPGA/...

[viplavmulka]

On what basis the set point has been taken from -1 to 1? If the motor I'm using is a 5V DC Motor what are your suggestions on taking the set point limit as I'm using a potentiometer from hardware to give the setpoint?

[MikolajM]

Hi,

looks like I misunderstood you, the setpoint you mean is "Desired Speed", right?

If you have a look at the loop diagram, you can see that "Desired Speed" is a part of subtraction node with Speed. Therefore it has to be given in rad/s. 

The scaling on the control is purely a matter of choice, you can type in whatever you want on the control. Obviously, if you enter a setpoint higher or lower than the limits of the hardware, then hardware becomes the limiting factor.

[viplavmulka]

Is it possible to apply the setpoint in voltage and get output in voltage from the transfer function used in the model as the rotational speed?

Sorry for asking too many questons 😕

[MikolajM]

Hi,

You can, of course, it's just the question of maths. Consitering that the transfer function converts volts to rad/s (Imagine as [V]xTF=[rad/s]) you can just set everything in volts by dividing your rad/s setpoint or output by TF. Not, that you have to add code to compensate for the additional maths you are doing. The main body of the code must remain the same, you can only fiddle with numerics.