Misconceptions about PID Controllers

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Chuoiujk
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Joined: Fri Dec 30, 2016 1:09 am

Misconceptions about PID Controllers

Postby Chuoiujk » Fri Dec 30, 2016 1:23 am

I'm currently designing an equivalent of the mass-spring-dashpot system, and during the design phase I realized I don't fully understand how PID controllers physically work.

Below is an image, the top half of which is the physical setup. There's a mass hooked up to a spring and dashpot. There is some magical position sensor that outputs the position of the mass (ignore how it does this), and there is a motor attached to the mass so we can apply a force to it. The position sensor feeds into a computer, and the computer controls what force the motor produces.

The bottom half is what I imagine the control loop looks like. We have some set point for the mass, and the offset (error) is fed into the PID controller, which outputs a current which is fed to the motor, which outputs a force which acts on the mass, which thus changes the position.

My question is that the input to the PID controller is a position (namely x_set - x_actual), but its output is mysteriously a current. How is this possible? The PID controller computes integrals and derivatives of position, which in no way is relevant to amperage.

Am I missing a component in my control loop - is there something in between the controller BA6235 and the motor? Or perhaps between the sum block and the controller?
BA6235 datasheet

Any help is greatly appreciated. ^^
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