Nine things to consider when specifying a servo system

Cover story: This tutorial will help with selecting a servo system that will perform at the optimal level for the selected application. Ensure these areas are addressed.

02/16/2016


There are many things to consider when selecting the right servo system for a particular application. Look at these nine areas for the best results.

1. Selection of motor size

The three most important criteria when starting with the motor are:

  • Speed
  • Torque
  • Rotor inertia.

The first two are pretty obvious and are typically calculated using "sizing" programs that are offered by various manufacturers. In these programs you can enter the types of moves in the application, and the software will calculate the torque and speed requirements based on the load and the type of transmission (gearbox, belt, rack and pinion, ballscrew, etc.).

The inertia is less intuitive but no less important. Because there is some compliance in the majority of couplings and the mechanical transmissions to which they are connected, it is vital that the reflected inertia of the load is not too large when compared to the motor's rotor inertia.

Tuning becomes increasingly difficult as this ratio rises. As tuning algorithms become more advanced and microprocessors get faster, the allowable ratios have increased, but the traditional 10:1 load-to-motor inertia ratio is always a safe bet.

Figure 1: Software can help sizing a servo motor system. This typical load/mechanism entry screen from Yaskawa SigmaSelect Servo Sizing Software shows a diagram calling out external force, load mass, side mass, pitch, and inertia of the ball screw, fricti

2. Load connection

Should the motor be coupled directly or not? There is also a class of servo that allows for the load to be directly mounted onto the rotor of the motor. With this minimal compliance and low acceleration and deceleration rates, inertia ratios of over 100:1 are possible. The sizing software provided by the manufacturer is the best place to manipulate these design criteria to ensure that the system's inertias are within the proper bounds. 

Figure 2: Torque/speed curve from Yaskawa SigmaSelect Servo Sizing Software plotting position (m) and velocity (m/s) on the vertical axis and time in seconds, horizontally. Courtesy: Yaskawa

3. Regeneration energy processing

Figure 3: Design of the Yaskawa SGMCS (Direct Drive) Servomotor allows for fewer mechanical components than earlier models. Courtesy: YaskawaAnother consideration in many applications is the ability for the servo system to handle the regenerative energy created by the motor and the load. This leads to the amplifier section of the servo system. When the motor's torque is applied in one direction but the rotor is moving in the opposite direction, the motor will regenerate energy back to the amplifier. This is because the basic designs of a motor and a generator are the same.

Applying energy to the winding of a motor will create a magnetic field that will cause the rotor with its permanent magnets to rotate. Likewise, when the motor rotor turns, the permanent magnets will induce energy into the windings. This regenerative energy is handled by the drive in many ways. Some smaller drives will use the bus capacitors to absorb this energy while larger drives have an internal resistor through which the energy is dissipated as heat. The sizing software from the manufacturer will typically report the need for regeneration energy processing.

4. Active line regeneration

Larger systems have converters that can allow for the energy to be returned to the power line that supplies the power for the system. The amount of regenerative power that the drive can handle is another concern when designing the allowable inertial mismatch between the motor and the load when selecting a servo system.

Figure 4: Calculations may result in showing that an external regeneration resistor is required when using the Yaskawa SigmaSelect Servo Sizing Software. Courtesy: Yaskawa

It is common to connect power resistors external to the amplifier when dealing with excessive regenerative energy applications. These applications are typically vertical where gravity is a factor and when decelerating large inertial loads quickly. Besides the regen capacity, the amplifier obviously needs to be able to provide the right voltage and current for the motor to reach the required speed and torque. Sizing the power requirements for the motor and amplifier is just the beginning. [subhead]

5. Control system requirements

Figure 5: Yaskawa SigmaLogic single-axis motion sequencer with AOIs compatible with Control & CompactLogix PLCs from Rockwell Courtesy: YaskawaNext, the requirements on the control side need to be addressed. Let's start back at the motor. A servo system will always have a feedback device by definition. Today, it is typically a high resolution encoder or a resolver. It is important for the feedback device to have the appropriate resolution for the repeatability and accuracy that match required positioning tolerances. The amplifier also must be compatible with the type of signal coming from the feedback device on the motor. 

6. Positioning resolution

Figure 6: Yaskawa MP3300iec Machine Controller with Mechatrolink-3 Motion Network can serve up to 32 axes and communicate with various Ethernet devices via OPC, ModbusTCP/IP, EtherNet/IP, and/or custom communication interfaces. Courtesy: YaskawaEncoders have traditionally used a pulse train on two channels to transmit the information of position, speed, and direction back to the amplifier. However, since the resolution of these encoders has increased, some manufacturers now use serial encoders that send packets of this information at a high rate instead of a pulse train. These serial encoders can transmit higher resolution signals that are significantly more noise immune and also can provide other types of information. This additional information can include the motor temperature and even the motor part number.

In servo systems where the motor and the amplifier are provided by the same manufacturer, the ability of the feedback device to identify the motor to the amplifier allows it to setup internal parameters for the optimal performance and the protection of the motor automatically. 

7. Communication connectivity

Once there is a matching motor and amplifier that has the power and a feedback device with the necessary resolution for the required application, it's time to consider the command signal between the amplifier and the motion controller. For single-axis applications it is common to have the controller built into or attached to the side of the amplifier. Some of these applications are integrated into a large controls system. A programmable logic controller (PLC) or programmable automation controller (PAC) may be responsible for controlling the entire cell or line and communicating to the single-motion axis via digital input/output (I/O) or a communication protocol, such as EtherNet/IP or ModbusTCP/IP.

Figure 7: It can be useful to see rated and peak torque application information overlaid on motor rated/peak torque curves, when using Yaskawa SigmaSelect Servo Sizing Software Courtesy: Yaskawa

8. Control interface selection

In a multi-axis application the controller is usually separate. Traditionally, the command signal provided by the controller was a +10 V signal that either represented a torque or a speed command. Today, most manufacturers provide network-based solutions. These network-based solutions require less wiring, handle higher resolution feedback, and gather more diagnostic information from the amplifier. There are many motion networks available, and each provides their advantages and disadvantages. 

Figure 8: Available motor results for a representative application include the capability to filter and sort in various categories when using Yaskawa SigmaSelect Servo Sizing Software. Courtesy: Yaskawa

9. Motion control network selection

Many newer networks are based on Ethernet hardware and take advantage of its ever increasing speed and decreasing cost. However, just because there is an RJ-45 connector on the amplifier, it doesn't mean that it is compatible with the associated controller or the rest of the network. The protocols running on those networks define the compatibility. EtherCat, Mechatrolink III, and Powerlink are among a few high-speed deterministic Ethernet-based networks suitable for motion controls. Network-based servo systems typically provide other Ethernet ports with common industrial protocols like Modbus TCP/IP and EtherNet/IP for reporting diagnostic and production information to supervisory networks.

Jerry Tyson is the southeast regional motion engineer for Yaskawa America Inc. He has 26 years of experience in the motion control industry and has worked with Yaskawa for 25 years. He has a BSEE from Georgia Institute of Technology. Michael Miller is the regional motion engineering manager at Yaskawa America Inc. where he has worked for 17 years. He has a BSEE from Kettering University. Edited by Eric R. Eissler, editor-in-chief, Oil & Gas Engineering, eeissler@cfemedia.com

Key concepts

  • The three most important criteria when starting with the motor are: speed, torque, and rotor inertia.
  • Most manufacturers provide network-based solutions. These network-based solutions require less wiring, handle higher resolution feedback, and gather more diagnostic information from the amplifier.

Consider this

Servo selection requires mechanical, electrical, and network engineer expertise. Some universities offer degrees in Mechatronics, covering these multidisciplinary engineering fields.

ONLINE extra 

See related articles linked below.



No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Sensor-to-cloud interoperability; PID and digital control efficiency; Alarm management system design; Automotive industry advances
Make Big Data and Industrial Internet of Things work for you, 2017 Engineers' Choice Finalists, Avoid control design pitfalls, Managing IIoT processes
Engineering Leaders Under 40; System integration improving packaging operation; Process sensing; PID velocity; Cybersecurity and functional safety
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

SCADA at the junction, Managing risk through maintenance, Moving at the speed of data
Flexible offshore fire protection; Big Data's impact on operations; Bridging the skills gap; Identifying security risks
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
click me