Achieving Positioning Accuracy Goals
- Friday, 01 December 2006
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.