One of the most difficult challenges to machine builders is understanding the notion of positioning and how it is reflected in component selection in their equipment design. Positioning an object based on a rotary or linear input can be really simple or really complicated. Everything you need to know is based on the work statement of the load.
There are 3 key elements of any motion control problem that need to be identified;
Mass – how much weight is the payload that is being moved? Sometimes it’s not just the weight, but the center of gravity may be a requirement, even in some relatively simple situations. Consider moving filled, labeled and sealed ketchup bottles down a conveyor on the way to putting them in a carton. Different bottle sizes have different centers of gravity. The move profile of the conveyor will require considerable tweaking to adjust for the momentum and torque arm presented by the different bottle sizes.
Time – how fast is the cycle time? This one is pretty obvious except for the fact that the mass of the object to be moved forces certain constraints on the acceleration required to move the part. As the time requirement decreases the torque required goes up arithmetically. It ain’t pretty when you are shaving milliseconds off of move times. The control system is not capable of making up the time required once the mechanical design is frozen. Simulation of the motion can reveal a great deal of information about where mechanical trade-offs can provide impact to the high speed goal.
Accuracy – what unit of feedback is required? Are we polishing at 50 millionths, or positioning a pallet full of breakfast cereal in an automated storage and retrieval warehouse? Positioning accuracy in these two systems are orders of magnitude apart. Accuracy considerations are also tied to speed. As the velocity increases the rate of feedback goes up in proportion. If the motion requires high speed, high resolution can become a problem for the control system bandwidth. In the real world the position accuracy depends on the sensor, so selecting a feedback device is something you want to spend some time doing carefully.
What is not obvious is that there are trade offs that can be very helpful. Understanding the real world motion and the rate at which things must happen is crucial to decision making in specifying motors, drives and controls. More feedback and higher acceleration capability may not be the right thing for your project. A 1200 pound roll of paper can only accelerate at a certain rate. To go beyond the natural frequency of the load, the motor would have to be bigger than the roll. Not gonna work out well.