34 n PRECISION ENGINEERING October 2023 www.drivesncontrols.com Specifying motors for precision applications When choosing a motor for a machine design, even one that demands precision control, the initial considerations are based on speed and torque characteristics. For applications such as robotic joint control, kinematic accuracy that relates to position and control velocity is also fundamental. Providing that the speed and torque requirements have been calculated, these criteria, along with inertial acceleration, can be selected through motor manufacturers’ specifications. However, in many cases, precise system power and accuracy requirements cannot be calculated until a prototype mechanical assembly has been tested. Initial motor selection can rely on a knowledge of motors used in legacy machines of similar functionality, or motors can be oversized during prototyping, then downsized later when precise requirements are known. Speed control Stepper motors are often the first choice when specifying motors for precision control because of their attractive costs. However, their suitability depends on speed requirements, because the maximum speed of a stepper motor is limited by its high pole count. This can be an advantage compared to servomotors though, if a high torque density is needed. While stepper motors can provide sufficient positioning for many applications, their accuracy depends on system loading as a proportion of the motor’s torque rating. At 10% loading, their positional error is approximately ¼ of a whole step – or 0.5 degrees. Servomotors offer much faster top speeds. High-speed applications, including those above 5,000 rpm, typically involve rotating a balanced inertia without any external loading. As the system accelerates, radial bearing forces become the dominant bearing load, and their impact is proportional to the system’s eccentricity. Generating a model of the radial bearing forces to determine the scope of torque requirements typically occurs during prototype testing. If, instead, a servomotor is accelerating and decelerating with an unbalanced inertia – for example when controlling a joint in an articulating robot arm – inertial properties dominate the motor torque demand. Torque requirements for prototyping can be estimated from a model with inertial and kinematic properties of the robot/load system. Position control In terms of control accuracy, servomotors with position feedback are the optimum choice. In most cases, a servo can settle within ±10 encoder counts, but this requires an encoder with sufficient positional resolution. The servomotor’s response is also critical. In theory, the kinematic response of the motor should be linear with torque, but static friction makes a linear response impossible when starting and stopping a move. Therefore, high-accuracy systems need additional, specialised mechanics to limit this effect. Brushless DC (BLDC) motors can also be used for position control in combination with a feedback device. An additional encoder adds its own footprint and cost, but a BLDC motor is more efficient than a servomotor and offers a higher torque density. They can also enable simpler, more flexible integration that can aid machine designs. Frameless BLDC motors can have a hollow shaft, allowing components to be inserted via their centres, and their design also reduces footprint sizes and weight. These motors are often direct drive, connecting to the load without needing transmission, which provides high dynamic and high-speed operation. Whatever motor technology you choose, matching the inertia of the motor to its load is crucial to optimise response times, and to avoid challenges such as vibration. It’s possible to achieve a large inertia ratio with the advantages of a smaller motor, yet still meet torque/speed requirements. However, this increases demand on the power input, and careful selection is essential to avoid mechanical instability that can cause motor oscillation at higher frequencies. Ultimately, optimising motor specifications requires a thorough understanding of the application. n To ensure precise control of applications such as robot joints and centrifuges, the choice of motor is critical. Options include servo and stepper motors, and brushless DC designs. As Gerard Bush of Inmoco explains, choosing the best motor depends on an understanding of parameters ranging from inertia to speed of kinematic response. Direct drives such, as Celera’s recent Omni+ models, can deliver high torque densities and smooth motion
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