I have a question about intravenous flow rate controllers, which are controlled using microprocessors. For example: 
I want to control the IV flow rate, the flow is in range from 0 to 300. So, I just put a number between 0 drips/minute to 300 drips/minute in a 10 scale from the computer interface, and the microprocessor would control the flow rate to the desired number. For example, when I put 60 drips/minute on the computer interface, the microprocessor controls the mechanism until the flow rate reaches 60 drips/minute. The tubing used by the IV dispenser is a form of PVC plastic.
I've been trying to use a pinching mechanism like in the picture below. I'm using that kind of mechanism because it is easier to insert and remove the tube from the device when required. But, the mechanism still doesn't give the precise output that is specified, also it is unstable.
So, how can the flow rate be controlled just by using motors such as a servo motor or a motor DC and microprocessor?
