Mechatronics is a field made up of paradoxes. It can be inclusive of so many different technologies that its hard to define what it is and what it isn’t. Primarily mechanical in nature, as the name suggests, it is incredibly ironic that there are no actual sensors for some of the major physical properties of the motion control system. How do you measure the physical dynamics of mechanical motion?
This is especially ironic since most mechanical components in the load can be characterized to 3 or 4 decimal places of accuracy. More than enough information to define things very precisely. Yet, little information actually flows into the control system model.
There are obviously plenty of ways to sense velocity or position. The old school solution was a tachometer generator puts out a voltage that is proportional to velocity. Although the voltage type feedback closely matches the velocity command signal sent to the motor, using a tach to get position requires some fancy integration of the voltage over time to get decent accuracy.
The most popular modern technology for position is the encoder. It’s great because it’s digital, it’s simple, low cost and very reliable. But the digital data flows as pulses and must be counted. So in order to get a velocity measurement you have to add up the number of pulses and divide by the unit of time, which can result in latency since you are trying to measure something in real time. Not so easy as it would seem at first glance. The update rates for encoders are now measured in nanoseconds in order to deal with this problem.
There is a sensor that helps in motion, the accelerometer. The accelerometer is a silicon strain device that outputs a voltage that is proportional to the rate of displacement and actual force produced by a moving part. This is an unprecedented opportunity to gain real control over the motion because there is typically no sensor that can give precise feedback about the moving parts. But there is a difficulty in implementing accelerometers, you need wires, which makes it difficult since motion control involves stuff that is moving.
However, there are some excellent devices on the market. two and three axis models, some with wireless interfaces. But best of all, prices are falling. All of the iPods and Ouii user interface devices use accelerometers. This is fueling mass production of the devices and lowering costs. So we should expect to see an increase in the use of accelerometers in motion control applications in the future.
This new opportunity for superior control system performance should be approached with a moment’s pause to consider the implications. Most trajectory planning is done based on manipulating the velocity command to the servo motor. But the forces involved are torques which create momentum in the moving parts. And the forces are changing in relation to time, the rate of change of force is very important in controlling the loads. A more complete control environment has to be created, and a lot of software innovation will be required to take fullest advantage of the implications.