Doctor of Philosophy
School of Electrical, Computer and Telecommunications Engineering
Simpson, John William Lee, Force and low speed control in mechatronic systems with friction, Doctor of Philosophy thesis, School of Electrical, Computer and Telecommunications Engineering, University of Wollongong, 2003. http://ro.uow.edu.au/theses/1947
A mechatronic system is one in which mechanical, electrical and intelligent computer control are integrated. Two mechatronic systems were used in this thesis. One was a four axis industrial robot and the other a grinding machine. Friction exist in almost mechanisms with moving parts and depending on the arrangement of the mechanical components may have a significant influence on the mechatronic system's overall performance. Both the mechatronic systems used in this thesis have large friction components. An understanding of friction and subsequent application of this understanding has been used in force estimation and control schemes and to develop control schemes that lead to improved low speed position tracking.
At low velocities friction has highly non-linear characteristics. In many instances the friction at rest (the static friction) is higher than the friction when the mechanism moving at low velocities. This in many mechanical systems leads to a phenomena referred to as "stick slip". Stick slip is a periodic cycle of alternating motion and which limits the velocity and position accuracy of many machines. An impulsive control scheme for low velocity position tracking which reduces the stick slip limit cycle was developed in this thesis.
A force control scheme in which the applied force is estimated from servo motor information was also developed. The torque applied by the motor is made up of the torque accelerating the mechanical components, the torque required to overcome friction and the torque required to overcome an external force. For many mechanisms the largest force that the motor has to overcome is the friction component. Therefore accurate friction models of the mechanical system were used to extract the required information about the external force from the motor torque signals. The friction models used here include viscous, coulomb and position dependent friction components.
The force estimation and control techniques developed were applied to the grinding process. A series of experiments were conducted in which grinding forces were calculated from servo motor information and a control scheme to control grinding forces without using force sensors was implemented.