Motion control and mechatronics technology have grown up as a control discipline at somewhat of a disadvantage. Mechanical engineering programs tend to ignore electric motors as being, well, electrical. And electrical engineering programs ignore what’s attached to the motors as being mechanical, which it usually is. This prevents the two disciplines from correctly understanding each other and more importantly, can prevent needed breakthroughs in performance for the companies that employ mechatronics.
This has been changing in recent years with a growing number of mechatronic programs at major universities and wildly popular programs like the First Robotics Competition. Interest in mechatronics has spawned a wave of contests sponsored by manufacturers to educate young people about the technology and make future customers in the process. The greater benefit is the number of creative individuals being exposed to the technology in grade school and high school. Undoubtedly, we will all be the beneficiaries of some new inventions that will be coming in the future.
But there are still some interesting subtleties that arise in motion control. A common problem is defining coordinated motion. This is because the precise behavior has to be described BOTH as mechanical objectives and correctly modeled in the control system hardware.
There can be many axes of motion in a particular machine. But they are rarely coordinated in the absolute sense. And this is an important distinction to keep in mind during design of the control system. Most of the axes, maybe 80% of them, will require a start signal to coordinate their operation while the equipment is operating. Rarely do the axes require time synchronous control.
Want to know the secret? Simple. Think of a machine that does “Tic Tac Toe” versus one that “Draws a Circle”. Tic Tac Toe can be done with simple Cartesian linear axes with no coordination, other than a start bit and a done bit. You can have a busy signal if you want to get fancy.
Ever try to draw a circle with an Etch a Sketch? It is harder than it looks. Because every tiny point must be coordinated between the two separate sources of motion.
And when you draw a circle, what happens as the time constraint is decreased? As you go faster the acceleration and inflection points of motion become much more critical. Generally, this produces increasing error in the actual trajectory.
Which leads to the “Stump the Band” question for would-be mechatronics engineers. What is the one variable that connects all aspects of mechanical motion and electrical control together? Time
And there is no end of importance in this fact.
When you try to Draw A Circle, time is absolutely essential. The incremental change in time, delta-t, will impact how precise the circle is. And the control system programming and execution will not be of much help in regulating this. Neither will servo tuning.
For those of you planning a multi-axis system, let me share one further time-oriented thought. When you have two truly coordinated axes, and they can be anything, a servo following an ac frequency drive (don’t laugh, I did this once and it worked great) make sure that if you are using a PLC that the coordinated axes are on the same control module.
Most PLC’s use a separate processor to run up to 4 axes of motion at a time. The slave axes have to all have to be on the same module or the backplane update will limit the performance of the motion. You will see perfect performance up to some speed and then synchronism will be lost because the new position update is going through the backplane and the servo is being commanded to follow old position information.
More on this next time.