The task of choosing the correct mix of motion control components for a successful servo positioning system involves a combination of art, science, and experience. It also includes a little luck, but luck is not needed when you fully comprehend the principle of operation, accuracy, resolution, and repeatability of each component in the system.
The first factors to consider are speed and torque. They determine whether the system should host a stepper motor or a servomotor. Steppers usually are superior for systems that operate at speeds lower than 1,000 rpm and less than 200 watts. By comparison, servomotors are preferred for speeds above 1,000 rpm and power levels above 200 watts. Each has a unique set of parameters that contribute to its accuracy, resolution, and repeatability.
The next issue concerns feedback devices. Steppers do not require feedback; servomotors use feedback devices by definition. Servo systems require one or more feedback signals in simple or complex configurations, depending on the specific needs of the motion system. Feedback loops include position, velocity or speed, acceleration/deceleration, and sometimes “jerk,” the first derivative of acceleration.
Motion control systems typically employ one or a combination of four basic components: belts, ball screws, lead screws, and linear motors. Belt-drive systems are the least expensive and are often used for high speed, relatively light-load applications. They tend not to be very accurate or repeatable and run at about 60% duty cycles. Ball and lead screws are the next most often used components. Lead screws typically cost less than ball screws but also are less accurate. The intelligence for determining the speed, torque, and direction is contained in the controller. Some servo systems now include stepper-like control functions. Some servo-like positioning is being seen with stepper-like motors creating Switched Reluctance or Variable Reluctance motor systems.
Error Budgeting: Resolution
The electrical resolution for steppers relates to the step size, while the resolution for servomotors relies more on the encoder’s resolution. Physical limitations may be due to the transfer mechanisms such as couplings, belts, or lead screws and their associated windup, backlash, dead-band, or hysteresis. (Screws have their own set of errors to be figured into the system’s error budget.)
The step size is predetermined for stepping motors, but the actual displacement depends on the capability of the drive and the motor itself. The synchronization of the system can come into question as well. A stepper may become desynchronized based on load, inertia, step rate, and the characteristic resonance of the load to motor ratio. Once in motion, it can lead or lag several steps from that which was commanded and remain there. Thus, the electrical resolution is better than the actual resolution that the physical system can achieve. The manufacture of the stepper comes into play with the accuracy of the rotor to armature without a feedback device to confirm the load’s position. The best way to decide the resolution errors in this system is to actually measure its response over the expected operating temperature range, expected loads, and the frequency range.
The physics constraints of a stepper must be realized as well. At 200 steps/revolution, the motor’s individual commutations create harmonic disturbances with a high frequency content. These have been known to excite or destroy couplings. Micro-stepping either binary or decimal may cure the noise, but if the intention is to accelerate at 1,000 rad/sec2 and achieve a final velocity of 6,300 rad/sec, then a frequency of 3.2 Mhz would be required with a setting at 1/16th steps. This application would favor a servomotor.
Repeatability and Accuracy
Motion systems are usually specified to perform any one or a combination of three different types of moves with consequential backlash and hysteresis. These include unidirectional, point-to-point bi-directional, and contouring modes. Unidirectional moves require point-to-point repeatable moves with the destination point being approached from only one direction. But, compared to repeatability, accuracy for these same systems is more difficult to achieve, because it must also serve as a measurement system. However, highly repeatable, unidirectional systems usually are also highly accurate.
Bi-directional repeatability is more difficult to achieve because of backlash and hysteresis. Many motion controllers can compensate for highly repeatable gear backlash, but fail to handle other components that have less predictable backlash. Supplying a secondary encoder for the position information can compensate for backlash. High accuracy/ repeatability systems often use a position sensor outside of the motor. Care must be taken with these systems since the hysteresis or dead-band would be enclosed within the position loop.
Hysteresis is evident when the system is commanded to reach the same destination from opposite directions. A rotary encoder coupled to the motor would indicate that the load reached the same destination, but the actual position difference is larger than the backlash alone. This hysteresis is caused by unseen clearances and elastic deformations. A linear encoder can compensate for backlash and hysteresis in a screw-driven positioning system. The hysteresis must still be minimized to avoid the aforementioned control problems with oscillation, and systems with hysteresis potentials must have friction minimized.
Some of the less obvious but critical errors come from the support structure (machine base), certain types of bearings, and couplings. The errors are specified as roll, pitch, yaw, straightness and flatness, straight-line accuracy, and resolution. A machine base must be machined to critical straightness and flatness specifications and ground to eliminate angular errors and ensure the drive and encoder accuracy. Aluminum, steel, stainless steel, and cast iron are typical materials used. Aluminum’s advantages include easy fabrication, having a light weight, and a relatively low cost. Stainless steel is used for applications needing high corrosion resistance. Cast iron is best for damping vibration in machine tool applications.
Two major types of errors must be considered in multiple axes systems: orthogonal and stack-up. Orthogonal errors come from axes that are not truly perpendicular. Stack-up errors occur when one motion axis supports another, such as the X-axis riding on the Y-axis. Although the angular errors of one axis affect the other axis, they can be compensated because they are highly repeatable. The critical thing to know is that they are present.
Another factor to consider is the effect of one axis on another. In a three-axis system, with the X-axis components riding on the Y-axis and Z-axis, numerous displacements between them arise from torque, skew, and lead screw errors. The errors in each axis may be as little as five or six microns, but they add up. The mechanical structure of the system supporting all the components must be rigid enough to prevent excessive distortion and consequential inaccuracies.
When specifying multiple axes, collision avoidance must also be dealt with. Many times, interacting multiple axis systems can have an overlooked error that the path to two specific “safe” positions can create a collision as well as the standard collision avoidance of ensuring that two axes cannot simultaneously occupy the same space. It may require an undertaking of the machine software, the motion controller software, or both.