Being engineers, at some point in our lives, some of us may have disassembled a small mechanical toy just to see what makes it work. Inside the toy is a wonderful collection of gears, cams and shafts connected to a windup spring or motor. All of the pieces are interconnected in such a way that, when the main power was applied, the gears and shafts interact with each other moving the toy in a predefined fashion.
This toy is an example of mechanical coordinated motion. As a spring winds down, a battery loses power or a hand inhibits motion, the toy slows down but still operates with the same motions. Early machine tools used the same mechanical techniques along with motors, clutches and brakes to usher in the industrial revolution.
Although these machines were very sturdy and produced the products for which they were designed, there were problems with this method of manufacturing. Gears and cams would eventually wear, and clutches and brakes would slip, requiring maintenance to continue producing high-quality products. A bigger challenge came if machine changes were required to make another product or due to modifications in the product’s design.
When servo and stepper motors and electronic control systems became available, pure mechanical motion control was no longer necessary. With the introduction of computerized controllers, it became easier to design one machine with the capability to produce multiple products and to compensate for mechanical wear.
Moving to coordinated motion
A coordinated motion control system has at least two axes, but not all multi-axis motion systems require coordinated control. Examples of multi-axis motion systems include a gantry crane, a drilling machine and a waterjet cutting machine.
A gantry crane has two axes of motion. The horizontal axis moves back and forth over the top line of machining centers and the vertical axis moves up and down to grip, lift and lower a part. The first operation is with the vertical axis raised, where the horizontal axis moves into position over a machining center. When in position, the vertical axis lowers and grips the part; it then lifts the part from the machining cell. With the part raised and clear of the machining cell, the horizontal axis then moves the part over a second cell. The gantry vertical axis lowers the part into the second cell tooling and releases it. The vertical axis then raises and waits for the next operation. This motion is not considered coordinated because, although there are two axes of movement, each axis is moved only when the previous axis has completed its motion.
The example of the drilling machine uses an x-y table that moves a part to a set of coordinates. The drill head lowers and proceeds to drill a hole. When done, the drill head raises, and the table moves to a second x-y coordinate to drill a second hole. This is not coordinated motion, even though the two axes are moving at the same time. Positioning or speed corrections to coordinate the x or y axis motion is not necessary. Each axis may finish its motion a few milliseconds or even a few seconds before the other axis without affecting the final position of the drill head.
A final example is a waterjet cutting machine where coordinated motion is required to cut a circle out of the workpiece. As the x and y axes move the workpiece under the cutting head, any position or velocity error in one axis must be compensated by the other axis. If the compensation does not occur, the round disc will end up as an oval or have jagged edges. Coordinated motion is used to produce quality parts by following an accurate motion profile.
Position error is the difference between the commanded position and the actual position of the axis or servo motor read by the feedback device. The coordinated motion control system combines the errors of all axes and adjusts the velocity of each axis to produce the programmed path, or trajectory, of the workpiece.
Most motion control systems operate with some degree of error between the commanded position and the actual position for every axis under control. These errors may be very small, or they may be significantly large depending on the controller performance, feedback resolution and motion actuator specifications. Additional position error or velocity errors are produced by a number of factors including imperfect tuning of a servo system, dynamic load changes, excessive acceleration or deceleration rates and underpowered servo motors or drives.