Modelling and Position Control of a DC Motor

The aim of the lab was to create and evaluate a model of a DC motor, and use a PIDfb controller to meet given specifications. It was found that increasing  and  helps to speed up the response, while increasing  helps reduce overshoot and increases damping, and so they had to be considered in conjunction to create a fast, stable system. Broadly, the experiment agreed with the theory, however due to errors such problems with reading, and possible wear and manufacturing variances, the design specifications were not met exactly. Minor discrepancies were evident between the theoretical and experimental data in multiple stages of the calculation, and the compounding of these errors likely led to the system straying away from the specifications. It was also essential to stay below the saturation limit for the signal to give accurate responses.


DC motors are a popular way of converting electrical potential power into mechanical actions. They are often used in conjunction with control systems to affect their behaviour, and so understanding the interactions between the two are essential for many electromechanical systems. For this experiment a DC Servomotor was connected to a controller (figure 1). The basic principal of how a low-cost servomotor operates is a motor is connected to a potentiometer, and as the resistance changes as the motor rotates, a control circuit can determine its position from its magnitude[1].

Using the manufacturers given data (see figure 13) and equations [1,2,3,4], it is possible to construct a system model, and compare to the real life output data. Combining this with a controller (calculates an error between the desired setpoint and its actual process variable) it is possible to attempt to set positions (using whichever of the proportional, integral and derivative terms are activated[2]). The controller will also try to oppose any changes caused to it, and so will try and retain a desired setpoint. This means that this kind of setup is often used for rudders on small planes and boats, where unbalanced forces will tend to try and change the angular position[3].

Figure 1: Annotated diagram of the DC Servomotor and Controller (Dr. Roger Ngwompo, University of Bath)

[1] Frances Reed. (n.d). How Do Servo Motors Work? Retrieved November 22, 2016, from Jameco website:

[2] National Instruments Staff. (March 29, 2011). PID Theory Explained. Retrieved November 21, 2016, from National Instruments website:

[3] Petter Blix. (n.d). RC Servos. Retrieved November 26, 2016, from Building Model Boats website:



Due to the lab being repeated for other years, the full report has not been uploaded. It is however available on request, using the contact details available.