Putting the “Mecha” into Mechatronics

Motion control, machine automation, mechatronics, actuator technology, whatever you want to call it, the thing you have to keep in mind is that in motion control, tasking is mechanically bounded.  The boundary conditions of the system are defined by the mechanical solution.  It is generally impossible for the control system and power electronics to cause a large inertia mass to behave like a small inertia mass with high acceleration rate and high frequency dynamic response.

There is a lot that can be done from the control system to improve overall performance.  Some of that improvement takes the form of sophisticated control algorithms.  Some improvement may take the form of increased power electronics to provide added acceleration and deceleration capability.  And sometimes projects get scoped to do things that are marginally impossible.  This is because there is gap between our understanding of the difference between mechanical and electrical properties of the application.

The gap in understanding is partly due to the educational system that teaches mechanics and electronics separately.  With the newer mechatronic programs that are available at many schools this gap is decreasing.

But there is a deeper issue.  The issue is context.  Mechatronics combines electronics and mechanics.  The missing context is that everything we do in the electrtronics, is an  analog for something mechanical.  If you don’t know how the mechanical relationships work, it’s unlikely that you will get the best results in controlling the system.

There are all sorts of examples.  A belt and pulley reducer is a mechanical system that turns a high speed input to a lower speed output.  The analogy in motion control is electronic gearing.  We can arbitrarily program any given motor to follow a speed reference from an encoder, tachometer or other motor, and follow the input signal at a programmed speed or ratio.

Electronic line shafting is a similar application that seeks to operate several independent motors and loads as if they were connected on the same mechanical shaft.  This application requires very high angular precision between the following loads which requires high speed regulation between the motor and associated electronics, but mathematically is very simple.

From the mechanical standpoint moving something from point a to point b seems pretty simple.  But as the time requirement for the motion decreases, the forces acting on the system become very significant.  As the load is accelerated from rest, it gains momentum.  Overshoot at end of the acceleration profile is a property of the momentum or kinetic energy of the system.  Tuning the system properly involves understanding the mechanical properties of the load and specifically how the control systems relates to the mechanics.

Does overshoot even matter?  Not so much if there isn’t a following axis involved.  At the end of travel it may be a concern if the load is positioning under a fixture or a next mechanical operation.  In many systems, there is settling time required for the load to come to a complete stop.  In these applications, the “S Curve” is an ideal solution because it compensates for the mechanical properties of optimizing the acceleration so that little or no overshoot takes place.

Maybe it’s less about the tuning and more about intelligent trajectory planning.