Controling Position and Velocity of trolley

Hey,
I am working on an application for a engineering design project that requires me to control the position and velocity of a trolley on a linear track.
I have done some work in LabWindows before controlling the speed and position of a linear movement but am having some troubles shifting my knowledge from a small application to what I am working on now. This is what my plan is; I currently have a motor (DC 24V 88A), String potentiometer to determine linear distance travelled (there was risk of slip and need of high precission), sufficiently sized power supply and BNC-2110 DAQ.
My plan (which I am looking for some feedback on ... no pun intended) is to use LabView to create a PWM (which has already been done) to control the speed of the motor via the counter turning on and off a power mosfet completing the Motor's circuit. By varying the time on and off I hope to control the speed.
After this has been completed I hope to hook up my string potentiometer to the Trolley and see how it's position changes over time (i.e. determine the speed at which the trolley is moving at for a variety of different % time on and off). This will act as an open loop test, which I was hoping to then figure out a practical transfer function for the motion of the trolley and then somehow use the String Pot as a feedback sensor to complete the loop and create a PI controller. I am having a hard time wrapping my mind around how I am suppose to put the later bit into practice. It all sounds fine on paper but actually approaching an implementation of this idea is boggling my mind.
Any input or pointers would be greatly appreciated. Am I on the right track or am I looking at something completly wrong... being a student I guess I don't know exactly how things are done in practice in industry, just what is written in books so any help would greatly appreciated.
Thanks!
Mac

Mac,
starting with an open loop system, determining the transfer function and designing the control algorithm is a reasonable approach albeit a little bit academic.
A more empiric approach is tuning the closed loop motion control system by means of test functions like white noise or a setpoint step. Here you can find some information about tuning a motion control system by analyzing the step response of the closed loop system (Please note, that this document refers to NI motion control boards, but the general approach of the tuning process is universal).
This brings me to something very essential. Stable control systems require a deterministic real-time environment. You haven't provided explicit information about the environment that you are intending to use, but it sounds like you are planning to use a Windows based system with LabVIEW. This is not recommended, as such a system can't guarantee stable control loop timing, which is at least as important as the right set of PID parameters.
So my recommendation here is to use either a motion control board which runs the control algorithm onboard or to run the application with the DAQ board under LabVIEW RealTime (could run on a PXI-system or on desktop PCs that meet certain requirements). If you are going for the second approach, you may also consider using the NI SoftMotion Development Module.
If there is any kind of campus license of NI-Software available at your university, all the required software components should be already available for you.
Last but not least here are some considerations about the power circuitry. You have suggested to use a PWM signal that controls a single mosfet. This approach is not feasible in most applications, as it doesn't allow you to control the position and the speed of the motor accurately. To do this, you need to be able to drive the motor in both directions. You could do this by using an H-bridge circuitry, but this requires some more considerations when designing the control algorithm. In fact in most cases you would work with two cascaded control loops. The inner control loop is the current control loop, that generates the signals for the H-bridge, while the outer control loop is the position/velocity control loop.
Depending on the focus of your project, you may also consider using a commercial motion drive that provides the power circuitry and the current control loop. Those devices typically provide a +/- 10 V input for an external control signal that is provided by the outer control loop.
I hope this information provides some ideas to getting started.
Kind regards,
Jochen Klier
National Instruments

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