Improving servo system accuracy

Servomotor systems: Careful selection of servomotors, machine actuators, and related components provides precise accuracy in motion control applications. Designers and machine builders should carefully select and review the required components, and online selection tools can help define choices. To achieve maximum performance, all servo systems need tuning at initial power-up along with proper software configuration and programming. See 8 critical components for precision motion control.


Figure 1: This servomotor and drive provides accurate positioning for a wide variety of motion control applications. With an AutomationDirect SureServo drive and its internal “indexer,” up to eight index moves can be pre-defined and stored in the drive, aIn motion control applications where precision is required, a host of servomotors and machine actuators are available to fit the bill—but selection, design, and integration of the complete motion control system are critical for repeatable and precise motion. While servomotors have many advantages over competing technologies, a number of factors must be considered.

Typical applications for high-precision servo system motion control include press feeds, auger fillers, rotary tables, robots for pick-and-place, test or assembly operations, boring, drilling, cutting, tapping, and similar applications using simple index moves for single or multi-axis motion.

Servo systems are used to control a load. For the servo to do this properly per the system requirements, it must be appropriately sized. The servo system needs to be able to provide the required torque, speed, and accuracy for the entire system to perform as designed.

The type of load, the mechanical transmission, the duty cycle of the system (how often it starts and stops), how fast the system needs to go during operation, and how accurate the system needs to be all play a part in deciding what system should be selected. 

Servo control advantages

For applications requiring precision control of motion, a variety of options are available including ac and dc motors with variable speed drives, stepper motors, and servomotors (Figure 1).

Servo systems provide the highest possible level of performance for precise control of position, velocity, and/or torque. Compared to lower cost stepper motor systems, servo systems provide more torque at higher speeds, up to 5,000 rpm. With stepper motors, the maximum torque is at zero speed, but servomotors have maximum torque at higher speeds. Typical servo systems for machine control also provide a broader range of power, up to 3 kW or more, than stepper motors.

Perhaps the most notable difference between steppers and servomotors is that servomotors improve positioning with closed loop control. Although some stepper packages take advantage of closed loop control, accurate and high-speed motion profiles without the motor stalling and the related position error is a common advantage of servomotors. Closed-loop position control, higher torque and higher speeds of the servomotor all confer benefits in high-accuracy applications.

Compared to ac and dc motors operated with variable speed drives, servo systems have a clear advantage with respect to speed, high peak torque, and acceleration. Servos operate accurately at speeds up to 5,000 rpm or more. Their closed-loop positioning capability also far exceeds typical positioning capabilities of variable speed motors and drives. Servo systems can also operate in a pure-torque mode where the system provides a specific amount of torque without regard to position or speed. This is a common requirement in various winding operations. 

8 critical precision motion components

When replacing an existing servo system, the same power size can usually be selected, even though the motor may be a different physical size. When deciding on a servo system for a new application, sizing software is often used. This software contains the mathematical formulas used to determine the inertia of the load, a critical parameter when selecting a servo.

Many components are specified, designed, installed, and tested to create a servo system. Eight critical components for precision motion control are:

  1. Servomotor
  2. Encoder feedback
  3. Motor drive
  4. Gearbox
  5. Actuator
  6. Motion controller
  7. Drive communication hardware
  8. Control and tuning software.

The servomotor, encoder feedback, and servo drive, sometimes called an amplifier, must be designed to work together as a package, and carefully matched to the motor and load.

The type of actuator and even the actuator material selection must be carefully considered. In some applications, for example, an aluminum lever actuator might flex too much for accurate motion, so stiffer materials or structural bracing should be considered.

For high-performance systems, the reflected inertia of the load including any gearbox and actuator inertia should be kept as low as possible—ideally a 1:1 match to the inertia of the motor—but often acceptable performance can be achieved with inertia mismatch as high as 5:1 or even 10:1.

With a suitable servo system and actuator selected, a motion controller and related software for tuning and control can be specified. Whether it is a single-axis or multi-axis system, the requirements for the motion profile such as maximum velocity, acceleration, jerk (change in acceleration), total distance, and deceleration must all be carefully reviewed for a successful application. 

Gearbox, actuator selection

If gear reduction is required, a precision planetary gearbox provides better accuracy and repeatability compared to most other gear reducers, and its high efficiency lets it deliver the maximum power available from the servo system. Gearboxes also lower the reflected inertia of the load by an amount equal to the square of the gear ratio.

If an application can handle the reduction in top speed, a gearbox can be a great way to improve the overall performance of the system. In some applications, using a gearbox to multiply the system's available torque can allow the use of a smaller motor and drive—a big cost saver.

But the gearbox will add some reflected inertia of its own to the system, and also introduce a certain amount of backlash. Most precision gearboxes have very low backlash, but designers need to be aware and plan for resulting positional errors.

Figure 2: Planetary gearboxes and servomotors work well in applications where precise positioning or high torque is required. Drive configuration software such as AutomationDirect’s SureServo Pro provides configuration of drive parameters and automatic tuA servo system coupled with a planetary gearbox (Figure 2) will provide accurate motion when connected to a wide range of actuator types, but only if all components are carefully specified and matched. Although it's possible to buy the servo drive, servomotor, and planetary gearbox from different suppliers, it's not recommended as this requires a great deal of research, design, and analysis to ensure all components work together properly.

Purchasing the components from one supplier, especially one that has carefully matched the components and will stand behind the specific combination of parts, offers several advantages.

The supplier has done all the research and will assure the customer of compatibility. Most suppliers will extend a more favorable warranty on such a purchase, and the supplier can also provide the approved mounting hardware and cabling required to connect the components. 

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