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What’s the Difference? – Servo Motor vs Stepper Motor


One of the most common Final Control Elements (FCE) of an Industrial process are Motors. It is used in a wide variety of applications such as a pumping and material transfer, as prime movers of agitators, conveyors, dampers, louvers, fans as well as lifting devices. With the wide variety of applications, multiple control methods that can be implemented to it — ON/OFF control, Speed Control, Torque Control as well as Motion and Position Control.

Each method requires use of specific components or equipment to implement the necessary control. For ON/OFF control of motor, simple combination of Magnetic contactors and relays are used; for Speed and Torque control, Variable Frequency Drives (VFD) or Variable Speed Drives (VSD) are coupled with an AC motor while for Motion and position control, Servo and Stepper motor control systems are utilized.

Out of these control methods, it can be said that the more complex to implement is Position and Motion control as this requires high accuracy, precision and response time. Additionally, while Magnetic contactors / Relays for On/Off control and VFD for Speed and Torque control can be easily understood, the difference between a Stepper and Servo Motor for Speed and Motion control adds to the complexity of the control method.

To help simplify and understood motion and position control, we must differentiate first the motors used for this application, Servo and Stepper motor.

General Principle of operation


A Stepper motor from the word itself, controls motion and position through fixed, discrete motor rotation or steps. Most stepper motors are open-loop by default as the rotation command sent by an external motor controller equals to the motor rotation in steps or specific degree of rotation. Recent developments provided the release of a closed loop stepper motor with improved Torque, Acceleration, Speed performance among others.

A Servo motor, meanwhile, is a closed-loop system. It implements control through a series of pulses or electrical signals which is processed by a Servo Drive to drive the motor to a particular degree of rotation.

Open Loop vs. Closed Systems


Understanding Open Loop and Closed Loop control systems contributes to better understanding on the use of Stepper and Servo Motors.

An Open loop system is a control system without feedback. Most of the time, this control method is linked with Manual control as the operator, or the initiator of the command is left to manually adjust the output should the control command is not sufficient to maintain the desired Setpoint.

Figure 1: typical block diagram of an Open Loop system


A Closed loop system is a control system with feedback. The feedback device communicates to the controller and compares the operator to the defined Setpoint against the control feedback. The larger the difference between the Setpoint and the feedback, the larger the correction. Conversely, smaller deviation leads to smaller adjustment. The main difference of having a feedback is that adjustment is Automatically implemented rather than manual input. 

Figure 2: Typical Block Diagram of a Closed Loop System

By default, Open loop systems costs lower to implement and maintain however, offers lower control efficiency and accuracy compared to a Closed loop control system.


Control System Elements

Servo motors and Stepper motors operates when provided with an external reference rotation signal, however, control system elements vary depending on the motor type.

Figure 3: Stepper motor block diagram


A Stepper motor simply consists of a controller and the motor itself. Capable of operating in Open-loop configuration, a feedback device is not necessary. The type of controller needed may be a stand-alone electronic controller or a Programmable Logic Controller (PLC) that appropriately provides the signal voltage in the form of pulses to operate the motor. A Servo motor requires more components to function properly. A typical Servo motor control system requires an Encoder, a Servo drive and a controller. The Encoder is typically mounted along the servo motor which serves as the inherent feedback device. On the other hand, the Servo drive receives feedback from the encoder and external signals coming from electrical components and controllers. Controllers may be in conventional or modern type such as discrete signaling devices or Programmable Logic Controllers (PLC).

figure 4. Servo Motor Loop

Motor Construction and accuracy


Both the Servo Motor and Stepper motors are generally composed of a Stator winding and Rotor winding. The Stator is the stationary part while the rotor is the rotating part of the motor.

A stepper motor’s Rotor is made of a permanent magnet with high concentration of poles. The poles are arranged following the principle of magnetism where opposite region attract. To achieve high-accuracy or generate as many steps needed, the number electromagnet poles are increased. Additionally, the poles also provide the natural motor stopping point. To compute for the Stepper motor’s resolution or the number of steps a certain stepper motor is capable of, the full degree of rotation which is 360O is divided by the number of poles. For instance, a stepper motor with 200 poles is capable of 1.8O per increment (360 / 200 = 1.8) while a stepper motor with 400 poles can provide 0.9O (360 / 400 = 0.9O) per increment.

A Servo Motor is basically constructed the same way however, it does not depend on magnetic poles to rotate accurately. Instead, the encoder’s feedback to the servo drive continuously reports the current degree of rotation of the motor. When the intended position or home position is reached, the motor, through the dynamic braking resistors or regenerative braking immediately halts motor operation. The rotation accuracy is dependent on the resolution of the encoder, sometimes expressed in Pulse Per Revolution (PPR) and No. of bits, depending on the type of encoder.

Applications


Both motors are generally used for Motion and Position control. As previously mentioned, Factory Automation is the main niche of these motors. In line assembly applications, Servo or Stepper motors may be used. Typically, they are the joints of an industrial Pick and Place robots and other more sophisticated unmanned equipment and devices.

Since Servo and Stepper motors have identical application compatibility, how do we select which device is appropriate for an application?
There are some considerations that can be factored in.

A. Torque Control If the application requires varying Torque output and high level of Torque during start-up, a Servo motor fits this type of application. For constant Torque applications and high Torque at low speed, a Stepper motor should be enough to drive the system.

B. High speed / High acceleration applications – Since construction of Servo motors closely resemble those standard AC motors, the acceleration and operational speed of Servo motors are more superior compared to a Stepper motor.

C. Stopping accuracy – Stepper motors exhibit higher stopping accuracy even without a feedback device. Having fixed, defined steps, the stepper motor’s poles provide convenient stopping areas. Additionally, each movement of a stepper motor is directly proportional to the pulse input therefore, stepper motor operation halts according to the input voltage. A servo motor when properly sized and tuned, offers high stoppage accuracy as well, however, there are cases that a servo overshoots by a small margin. With the feedback encoder, automatic correction is applied but this may cause some jitter which is not ideal in some applications.

D. Ease of use Stepper motors are generally simple to operate. As stepper motors does not require use of feedback device, it does not need sophisticated tuning. Most configurations are done to the external controller that operates the stepper motor. Servo motors, with the independent servo drive and feedback device, needs detailed tuning and configuration.

E. Cost Effectiveness – In terms of direct and upfront cost, Stepper motors costs lower while Servo motors are more expensive by default. This is due to the number of control system elements required by a Servo motor to operate.

F. Ease of maintenance – Requiring fewer number of components to implement motion and speed control contributes to simple maintenance. This is true with Stepper motors. In the case of servo motors, common problems encountered by maintenance personnel using this equipment is a defective encoder. Though replaceable, the encoder device may offer some challenges during installation and replacement. 

G. Closed Loop control When it comes to motion and speed control, a closed loop control system preferred as it provides “true feedback” which is useful data for operators when making real-time plant decisions. Therefore, a Servo motor is more ideal.

H. Operational Efficiency – Stepper motors are known to generate high current typically, near maximum value while Servo motors can be programmed with start-up, ramping and acceleration characteristics similar to Variable Frequency Drives (VFDs) as Servo motors are coupled with Servo drives.

Figure 2: Some difference and advantages of Servo and Stepper Motors. Tick marks refer to the more ideal device. It does not indicate that the motor is not capable of such feature.

Conclusion


Servo and Stepper motors are position and motion control devices, compatible for use in rotary and linear control requirements. Each equipment is largely utilized in a wide variety of industrial applications, capable of implementing accurate Speed and Torque control with accurate stopping characteristics. Though these equipment serves similar automation niche, identification of the main differences between each equipment help users decide which one to use. In this field of automation, selection is not always answered by determining, which is better instead, in terms of application suitability.   

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This entry was posted on May 7th, 2021 and is filed under Automation, Hardware Comparison, Technology. Both comments and pings are currently closed.

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