Optimizing Motion

Optimizing a motion control system is not easy.  There are many tradeoffs that need to be considered.  The strategy needed for each situation tends to be unique based on the problem that is being considered.

One strategy involves speed, torque and time.  These three variables are a connected system that is defined as the mechanical work to be performed.  Speed torque and time must be optimized according to the priority of each variable.  The difficulty is in exploring three variables simultaneously.

Two variable tradeoffs are no problem.   So having a two variable strategy for three variable systems would be really handy.  And as it turns out you can consider any one variable as a constant for the purpose of optimization.  So its easy enough to consider time as constant and optimize torque and speed.

This strategy turns out to work really well.  The speed and torque requirement for a given actuator system was very well defined and had been prototyped with good performance.  But the customer needed to optimize the cost of the design.

The cost of servo systems primarily follows the power requirement.  The more torque needed, the more expensive the servo system.  So the first place to work on cost optimization is the motor drive combination.  If the torque requirement can be reduced, the cost goes down.

So how do we reduce torque? You can either trade off time, or look for ways to reduce the load inertia. If time is a priority, then reducing the load inertia is the way to go.  And reducing the load is most easily dealt with by material substitution.  Steel is pretty strong, but may not be needed.  Aluminum may be a suitable substitute at one third the density of steel but half the strength.  Strength is absolutely required and reducing the weight is going to generate big cost benefits, titanium may turn out to be the best choice.  I have had applications where titanium was the used, and it was the right choice.

Sometimes the materials substitutions can be significant.  It is possible to use plastics like polycarbonate as an alternate to aluminum.  Again, the material strength is lower, but polycarbonate is so light, that more material can be used volumetrically to achieve the necessary strength, and still end up reducing the weight of the load.  So this is a viable option.

In the project I am currently working on, we have reduced the weight of the actuator system by about 25%.   What was really amazing, and unexpected, was that the weight reduction and rearrangement of the load structure resulted in a 56% reduction in the torque required.  The motor and drive has been reduced from a Nema 34 motor to a Nema 23 motor.   And further improvements are expected.

Which is why I like my job.