Servo motors and drives utilize feedback to deliver precise position, speed, and acceleration control, making them ideal for demanding mechanical and robotic applications.
Servo motor systems represent a high-performance motion control technology capable of executing complex motion profiles while providing high torque output that surpasses the capabilities of typical DC and AC motors. Consequently, servo systems are widely employed in various industrial machinery, robotics, and consumer products. However, because servo systems require drivers, motors, and feedback components, and involve more complex configuration and control requirements, they are typically more intricate and costly than other technologies.
This article outlines key characteristics designers should understand when specifying and configuring servo motor control systems.
Servo Fundamentals
The primary purpose of AC servo systems is to precisely control the motion of a load. As we will see, systems intended for industrial applications also incorporate various specific features that distinguish them from non-industrial products.
Servo motors can be installed to directly operate a load, but they are typically used with gearboxes or other mechanisms to modify the effective output speed and torque—often to increase available torque.
Unlike other motor types, servo motors cannot be operated simply by applying voltage. Instead, a drive (also called an amplifier, and sometimes a controller) supplies power to the motor and monitors an encoder, which identifies the motor's position. Using this information, the drive operates the motor as needed to precisely achieve the desired motion or maintain a target position. Industrial servo motors typically incorporate built-in feedback devices, usually encoders. Using this encoder, the servo drive can obtain shaft position information within a few arcseconds.
Amateur and consumer-grade servo motors feature integrated designs that operate at low DC voltages. These compact drive/motor/encoder/gearbox assemblies are driven by pulse width modulation (PWM) signals, providing a convenient solution for operating lightweight products.
However, for demanding industrial applications, the most typical system consists of a drive (powered by 110VAC, 230VAC, or 460VAC – single-phase or three-phase) that operates a brushless AC servo motor equipped with an internal encoder to provide feedback.
Basic servo drives receive commands from external logic controllers (such as Programmable Logic Controllers (PLCs)) via pulse signals, discrete I/O, analog signals, or more robust methods.
receive external commands from external logic controllers such as Programmable Logic Controllers (PLCs). Digital communication methods enable comprehensive configuration, monitoring, and diverse control options.
More advanced servo drives incorporate built-in motion controller logic, significantly simplifying and enhancing functionality with practical features including:
Registration: Synchronizes the drive with the product/material.
Electronic Cam: Synchronizes the drive with other relevant drives on the equipment.
Fly Cutting: A common function synchronizing a moving cutting mechanism with the moving product being cut.
Logic/Sequence Control: Capability to perform some functions that an external controller might execute.
Dynamic Control Mode Switching: Some tensioning applications switch between speed and torque modes to reverse the load direction (e.g., flexible saw blades).
Integration: Some controllers may offer features to facilitate easier integration with the drive.
Modern servo systems deliver the highest level of performance, enabling precise control of position, speed, and torque along complex, variable motion profiles. Early servo systems could be challenging for end users to specify and configure.
Applications of Servo Systems
The operation of servo drives and motors involves extensive theoretical knowledge, but designers typically only need to determine the overall physical and performance characteristics required for the servo system. Yuma Precision offers free servo selection software to help users choose the appropriate motor and drive for specific applications. Key considerations include:
Reflected load inertia.
Torque and speed requirements.
Physical and environmental constraints.
Reflected Load Inertia
Servo motors are categorized as low-inertia models, capable of speeds up to 6,000 rpm, and medium- or high-inertia models, capable of speeds up to 3,000 rpm. Servo tuning also determines how aggressively the drive controls motor response.
Any rotating system is affected by the inertia of the driven load and its intermediate transmission components. All load inertia must be determined and converted to equivalent inertia, as if the load were directly connected to the motor shaft (driven load). Mechanical transmission significantly impacts the inertia of the driven load reflected to the motor shaft. Therefore, a suitable motor must be selected to accommodate the reflected load inertia.
To achieve optimal system response speed, designers should strive to maintain low reflected inertia, as high-inertia systems exhibit slower response speeds or bandwidth. For systems requiring peak responsiveness, the load-to-motor inertia ratio should be minimized—ideally below 10:1—to benefit the application. Systems with ratios as high as 200:1 are feasible, but response speed will correspondingly decrease.
Several effective methods exist for determining the reflected inertia of a given system. Many CAD software packages can calculate inertia with considerable accuracy—provided the mechanical model is precise and appropriate material densities are assigned to each component. Estimates can be made based on the load's rough geometry, and simple equations exist to determine the inertia of single or multiple typical geometries (e.g., cylinders or spheres) combined.
Note that estimation suffices, as it provides only a rough understanding of inertia to determine whether the mismatch ratio is below 50:1 or 10:1, and so on.
Torque and Speed
Servo motor systems carry a continuous and an intermittent operating torque rating. With knowledge of the desired motion profile, the mechanical transmission, and the load, it is possible to determine the required continuous torque, peak torque, and motor speed. The required torque values must fall within appropriate ratings of the system.
Mechanical transmission devices—such as leadscrews, rack & pinion mechanisms, gearboxes, pullies, and timing belts—are used to change the motor speed and torque transmitted to a load, and they can also translate rotary motion to linear motion. Transmissions acting as a speed reducer provide a mechanical advantage, so a motor can operate a load at a lower speed but higher torque, while being able to reject load disturbances. Gear reduction has the added benefit of lowering a portion of the reflected inertia (the portion of the load inertia attached to the output shaft of the gearbox). The inertia of that portion of the load is reduced by the square of the gear ratio! Just be sure to add-in the inertia of the gearbox itself (or other gear reduction system).
SureServo2 and LS Electric servo motors feature keyed shafts that can be used with keyed couplings, or with “servo-grade” clamp-on compression style couplings. The former are lower-inertia, but the latter offer better stiffness. AutomationDirect recommends that coupling safety factors be chosen as at least 1.25 over the peak torque requirement of the application.
Physical and Environmental
Servo motors can be mounted in most any orientation, and transmissions can allow the use of a smaller motor. Transmissions also provide options for repositioning the motor to a more advantageous orientation, whether that is in-line, right angle, or parallel mounting.
Selecting the Right Servo Control System
The high-performance nature of servo systems can intimidate users seeking to specify and configure them. Yuma Precision has worked hard to make servo products flexible and easy to use, so the technology is accessible for all types of end users and applications.
