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.
By Chip McDaniel December 5, 2014

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. 

Selection tools

Online tools for selecting servo systems and compatible gearboxes are provided by some suppliers, simplifying the design effort. Online selection guides aid in design and provide specific recommendations for closely matched components that can be purchased as a system.

These selector tools typically let designers enter their speed and torque requirements, and then automatically provide a list of available motor-and-gearbox combinations. Engineers can enter torque data in metric or imperial values, or the designer can select a particular servomotor size. Designers enter speed data as discrete values or pick a gear ratio. Finally, the engineer can choose a preferred physical orientation: inline, right-angle gearing, or both.

The resulting list of available systems includes pricing information, a factor often critical to the selection process. After choosing a motor/gear combination, the designer can view full specifications for the selected servo system, the gearbox, and the combination.

More advanced selector tools are available to assist with servomotor selection. One application allows designers to select the optimum motor for a specific application. Software can calculate required speed and torque and verifies many other system requirements such as top-end speed, acceleration, and inertia mismatch.

Using software, a design engineer can model the mechanical systems including lead screws, timing belts, gearboxes, and so forth. Once modeled, the software can recommend the optimal motor for a given mechanical arrangement. Software can calculate the torque, speed, and inertia requirements according to the user’s application specification; process motor data from a database; and create a list that matches the requirements. The software will point to the optimum motor, and allow the user to choose from the list. 

Internal or external control?

Most servo systems can accept traditional motion commands from external controllers such as programmable logic controllers (PLCs) and programmable automation controllers (PACs), and some drives also have the ability to provide their own internal motion control. With a servo drive and an internal "indexer," up to eight index moves can be pre-defined and stored in the drive, and then selected and executed using discrete input signals from a PLC or PAC.

These pre-defined index profiles can also be initiated or even changed by sending commands from an external controller using a serial communication protocol. The motion can be incremental or absolute (homing routines are available in the drive), and acceleration can be linear or S-curve. Many applications benefit from adjusting the acceleration and selecting an S-curve motion profile as this can reduce positional overshoot during a move.

There are multiple ways to control a servo drive from an external controller. RS485/RS422 and Modbus serial communication are typical protocols used to send motion commands to the drive. Commands are executed and the position control loop is closed internally at the drive. Drives can also be controlled by high-speed pulse and direction signals, or by an analog voltage proportional to velocity or torque. The drive and servomotor can also follow an external encoder signal.

Each of these methods can affect servo system accuracy. The old-school analog voltage command may not be as accurate as the newer digital control methods such as internal control, or external pulse and direction signals. The system resolution can be calculated with digital drive command schemes and should be checked to ensure it meets overall system accuracy requirements.

When using an external motion controller as the master, multiple drives can be daisy-chained and addressed separately using the drive’s serial port. This allows for very simple yet powerful control of multi-axis processes not needing precise path control, but only exact starting and stopping points (Figure 3). 

Servo system programming

Figure 3: The servos and actuators in this three-axis system provide accurate coordinated motion for continuous motion assembly equipment. Online tools for selecting servo systems and compatible gearboxes include the AutomationDirect SureGear online selecWhether it’s an external motion controller or an internal motion control drive, the motion software and its features and functions have a significant effect on overall system accuracy. Drives that feature on-board indexer and adaptive tuning modes must be configured properly, as must external motion controllers.

When high dynamic responses are required, careful tuning of the servo system is needed, preferably with loads attached. Whether the system is tuned using adaptive auto-tuning software or manually adjusted by the programmer, accurate tuning provides faster moves and minimizes spongy motion, while reducing overshoot or ringing after moves are complete.

Drive configuration software can provide configuration of drive parameters and automatic tuning algorithms and tools to help find the optimal settings for most applications.

Once the drive is configured and tuned, the type of motion profile can be programmed. S-curve acceleration profiles, as opposed to trapezoidal (linear) moves, provide better positional accuracy and less overshoot, especially in servo systems with marginal tuning. The capabilities of these standard motion profiles offer improved accuracy, and less system shake and vibration.

Sometimes the control sequence of the actuator affects accuracy. For example, a frequent method used to minimize backlash and its related positional error is to approach all target positions from a common direction. In the event a reversing move is required, some designers move the load past the desired position and then return to that position from the common direction.

Servo tuning for maximum performance

Properly selected servo systems, gearboxes, and actuators can be used to solve a wide range of precision automation challenges. 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. When properly specified, designed, installed, tuned, and programmed, servo systems can provide accurate, repeatable operation for many years.

– Chip McDaniel, technical marketing, AutomationDirect; edited by Mark T. Hoske, content manager, Control Engineering, mhoske@cfemedia.com.

Key concepts

  • 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.

Consider this

Are you considering all variables when considering a servomotor system? Could software help?

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More about the author

Chip McDaniel works in technical marketing for AutomationDirect and graduated from Georgia Tech with a degree in industrial design. His 30 years of experience in the industrial automation field include designing, building, and commissioning multi-axis servo systems.

See the motors and drives page here and related articles below.