Engineers with a mechanical engineering background typically have the best grasp of motion control. This should not be a surprise to anyone. Motion control is the science, sometimes the art, of moving mechanical loads. So the premise of any motion control problem statement is all about the mechanics.
The basic mechanics of moving something boil down to inertia and friction. Those are the choices. Inertia is the resistance to change. Friction can be similar in the form of stiction, but is mostly experienced in the form of losses like efficiency.
Distinguishing between the two can be critical. Inertias are generally calculated based on the shapes and densities of the parts being moved. In general, once a piece of machinery is built, the inertias don’t change. Throughput, however, is inversely proportional to inertia. So it pays to spend some time on reducing inertia if the application requires speed.
Friction can vary depending on a number of conditions. For example, a ball bearing that becomes contaminated and fails creates tremendous friction and will cause abnormal wear from the load being out of round. As failure progresses, the friction increases and the torque required will increase as well.
The difficulty becomes controlling the situation. Unless the control system has provision for monitoring each load in a system, this type of damage can go undetected until something catastrophic happens. The good news is that current monitoring is commonplace in today’s AC motor drives. The bad news is that the information is rarely used.
Starting and stopping loads is also not simple. In the electric motor, the time and energy required to magnetize the motor is significant. The energy is often 1/3 of the running load and the time required can be several seconds before the motor reaches running speed. During this time the power factor of the motor is extremely low since little real work is being done.
Motion is work over time. Always work from the problem statement and understand the basic mechanics of the work to be done and most importantly, the time required to do it. As the time requirement decreases, the forces required increase arithmetically.
If the load is bigger, the time requirement expands. The physics doesn’t change.
Filed Under:Mechatronic Tips
