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Types of Encoders Used in Servo Motors

A Servo motor is an electric motor used in a servomechanism, which means to automate a mechanical movement with a closed-loop control circuit, unlike the stepper motor, which follows an open-loop control mechanism. A servomechanism comprises a servo amplifier (servo driver), encoder, controller, and servo motor. Servomechanisms can be found in various industrial applications, including CNC machine tools, antenna rotators, and robotics.

Servo motors provide high-speed and precision control based on the speed and position commands. Servo encoders are among the hardware components that comprise the basis of a servo-based system. An encoder used for a servo motor is a device that measures the speed, angle of rotation, and distance traveled by an element of the machine. It also gives information on the location of the machine and its controller. The controller transmits the generated signals to the motor driver, which magnifies and transforms the signals for the motor. The motor responds to the servo driver signal and produces output to manage the speed and torque to achieve the desired location.

In most cases, the encoder is built into the servomotor or is attached to the motor. In some cases, it is an independent device removed from the motor. When an encoder is located at a distant location, it can be used to determine other related parameters and control the servomotor’s operation.

Servo motors featuring encoders are commonly used and are expected to become more mainstream as automation increases, energy efficiency rules become strict, and advances in servo system technology become practical. OEMs demand reliable and affordable encoders for servo motors in a small footprint as the industry is progressively becoming more competitive.

Importance of Encoders in Servo Motors

Contrary to stepper motors, which operate in sync with the controls output signal, mechanical servomechanisms require feedback on their positions to enable the control of speed and commutation and to take into account any interruptions in the operation of the motor or load.

Servo motors do not have to be equipped with an encoder always. For obtaining data on angular or velocity displacement, an encoder inside the servo motor is able to be substituted with a potentiometer, resolver, or Hall-effect transducer. But these alternatives show less robustness, responsiveness, and reliability in most cases.

A servomechanism can be used to examine the mechanical movement of the shaft that drives the actuator, the movement of the shaft, and the speed at which it changes. It converts mechanical input into an electrical impulse and then transmits the sequence of electrical impulses as a quadrature signal to the controller.

Allen Bradley Kinetix 350 2097-V31PRO-LM

Types of Servo Encoders

There are two types of encoders based on the measurement mechanism, incremental and absolute. These two measurement methods can be adopted to furnish various motor fittings and motions.

Incremental Encoder

Incremental encoders comprise electromechanical devices that monitor incremental advancements from an undetermined home position when the system is first started. They must be reset before starting operation if they shut down or fail to function. An incremental encoder could be a viable solution for applications that only require speed control for a processing line, like in a packaging facility or a warehouse.

Incremental encoders have a reference point for monitoring the measurement, and they always refer to the same point to measure the change in motion; this means that they need to move to record changes in an angular direction. These encoders can be highly flexible and utilized in various motion control applications, where rehoming upon startup isn’t an issue.

Absolute Encoder

Absolute encoders monitor position by using a specific part of code unique to each rotation location. The absolute encoders usually provide precise information about the position without needing to go home. Homing is a limiting factor for many applications, like surgical robots, where movements always happen arbitrarily.

Absolute encoders can assign a unique digital phrase to each angular point. The encoder will always report the angular location of the device being monitored when questioned, even at the initialization time. The most appropriate applications for absolute encoders comprise those that require rehoming at any time during the process and could cause injuries or dangerous circumstances. For instance, surgical robots, automotive robots, or other interconnected mechanics or axes could be destroyed if they are powered up following an error. In certain instances, the process of rehoming may adversely affect productivity and justifies the minimal cost advantage of an absolute encoder.

The main choice is between the absolute and incremental types of the encoder. Incremental encoders are designed primarily to measure speed as they do not precisely indicate the positions. Absolute transducers allow speed and angular displacement monitoring permitting the user to know precisely where the motor moved in response to the execution of the command.

Encoder Types Based on Movement 

Linear Encoder

Linear encoders can be utilized to gauge the movement of straight lines, like the laser scanner and cut-to-length machines.

Linear encoders installed on linear motors measure the speed and position in real-time and provide feedback to the controller. The encoders have an important role in motor control as they constantly monitor the motor movement, and the controllers re-adjust the motor voltage all the time. The linear encoders produce a low error in position measurement against an accelerating movement in a specified direction. The design is suitable for shafts with high speeds. The absolute positioning measurement works on superior sinusoidal increment signals or purely on digital data transmission.

Exposed linear encoders are a type of linear encoders that own a higher quality in terms of accuracy, greater traversing speeds, and no contact scanning. The exposed linear encoders can be used in a clean environment, for example, measuring or production equipment within industries like semiconductors.

Sealed linear encoders provide a benefit of a superior protection level and simple installation. These can be used in areas with high contamination, for example, in a machine, and on tools.

Rotary Encoder

Rotary encoders are used to give feedback on the speed of rotation of an instrument or object, for instance, the shaft of an engine. A rotary encoder transforms the direction of the shaft into a digital or analog output signal, allowing a control system to determine the shaft’s location or speed.  

Rotary encoders in motors with forced ventilation are usually placed on the motor housing or inside the housing. These rotary encoders are usually placed in the non-filtered forced-air stream. Protecting these encoders by a class of IP64 or higher is recommended. 

Multiple sensing or working principles are adopted to measure the motion or position through encoders. The widely adopted mechanisms and sensing technologies used in encoder sensing are optical, magnetic, and capacitive.

The rotary encoder is installed inside the motor housing for motors without forced ventilation. In this case, the encoder does not require a high protection class, though the temperature inside the motor housing reaches a high value. That is why the operating temperature of these types of encoders is high.

Encoder Sensing Technologies

Allen Bradley PowerFlex 4 22-SCM-232

Optical Encoders

Optical encoders are the most accurate sensors. A rotary optical encoder is composed of lighting sources, like an LED and an elongated disc covered by a set of transparent, opaque lines and slots. As light moves across the spinning disk, an optical sensor on the opposite side detects the light and produces a sinusoidal signal, representing the light from the slots. The slots are transparent and opaque, in contrast to light not emanating from the opaque lines. The sinusoidal electric signal is converted to an inverse square wave signal. The result is a series of frequency pulses with high and low amplitudes. The sequence of these pulses is given as an input to a controller circuit, which utilizes it to regulate the number of pulses generated as the encoder spins. Then utilize the information to determine the rotating shaft’s location or to control a move or position.

Magnetic Encoders

Magnetic encoders depend upon magnetic flux fluctuations to determine the position and movement. The magnetic rotary encoder comprises a magnetized disc with a few magnetic poles around the circle. The sensor is located near the disk, the disk turns, and the sensor detects variations in the magnetic field when the poles on the disk’s surface move towards the sensor. The magnetic field that changes can be used to produce an output signal with a sinusoidal pattern that is then converted into the form of a square pulse that an electronic control circuit can analyze. The sensors used in these encoders may use magneto-resistive devices that can directly detect changes in the magnetic field or utilize the Hall effect, a method of detecting changes in voltage.

Capacitive Encoders

Capacitive encoders are a modern technology for sensing in encoder design. The basic principle behind the operation is to detect changes in capacitance by employing a frequency-sensitive reference signal. With a rotating capacitive encoder as an example, you can employ an arrangement of three parts to encode signals. The encoder includes a stationary transmitter, a rotor, and a stationary receiver. The transmitter generates a high-frequency electrical signal or current that is transported through the rotor before reaching the receiver. When the rotor revolves, the sinusoidal pattern engraved on its surface alters the alternating current signal. The receiver transforms this altered sound into output signals that are utilized to generate the fluctuating magnitude of rotating motion. The rotor creates an erratic capacitive response to the output signal produced by the transmitter and the metal used to make the rotor. This results in constant and predictable distortion in the AC field.

Quadrature Encoders

Single-channel encoders can determine movements and motion but cannot detect movement direction. When using a rotary encoder, for instance, clockwise movements produce the identical output signal as the counterclockwise movement. Therefore, that encoder’s electrical output will not discern the direction of motion but rather the magnitude of the movement. This issue can be solved through the use of what is called a quadrature encoder. These encoders use two output channels, whose electronic output signals are out of phase.

Encoder Selection Considerations 

Many factors of a servo system combinedly decide the type of servo motor encoder to be used for a particular application. Selecting a sensor compatible with a servomechanism is a matter of determining the details of the system into which the component is intended to fit. The selection of an encoder demands knowledge of the surrounding variables as well as the desired performance levels. Before selecting an encoder, consider the following factors.

Operating Environment 

The environmental conditions under which the encoder for servo motors is to be used is an important aspect to consider. The temperature, humidity, and vibrations, as well as the presence of particulates, chemicals, or electromagnetic interference, could influence the performance of encoders. A closed (IP66) optical rotational encoder is suitable for highly demanding environments. Also, some applications require the miniature nature of the motors and encoders, so the servo encoder should be low-profile in that scenario.

Moisture, temperature shock, and vibration contamination are all dangers to equipment, including encoders, which often have fragile glass scales. When any of the above issues occur, it is recommended to look at sealed encoders or consider numerous mounting options. Inductive encoders, for instance, are generally bearingless and contactless options which use the change in the mutual inductance between the scale and sensor to generate an electrical signal. Inductive encoders are less prone to disruption such as moisture and dust and enhance their capacity to deal with shock and vibration.

Electrical Requirements 

The choice of the output driver is determined by the voltage requirements from the control and the driver. If there isn’t enough voltage, the signals will not be able to transmit across the network. A suitable encoder that offers adaptable electrical configurations and a range of communication options should be used.

All available feedback data will not be beneficial to the system if the data cannot be transmitted directly from an encoder to the reader. Absolute and incremental encoders produce different outputs. Therefore, their transmission protocols and wiring differ. Incremental encoders require that each channel be connected directly to the counter or control device.

Servo Motor Encoder Specifications 

Resolution

Resolution is the smallest distance measurement within a single shaft turn or one millimeter per inch. It is measured in pulses per revolution. In this case, we are talking of incremental sensors or bits when talking about absolute devices. Incorporating a high-resolution encoder may not necessarily increase the system’s overall quality. In some instances, controllers aren’t able to manage the rate from the sensor. The general rule is that the resolution should be at least twice to four times large than the equivalent equipment.

Accuracy  

Accuracy is the extent to which an actual measurement differs from the measured result. In arcminutes, arcseconds, and microns (linear), The magnitude is determined by various variables, primarily due to the mechanics of the detector or the control machinery’s properties. For instance, backlash in gear-driven servomechanisms could cause additional errors. Another important parameter, especially in systems that need precise positioning, is called integrated non-linearity (INL). It is the maximum deviation between the real and ideal output of the measurement from peak to peak.

Repeatability 

Repeatability is the ability to repeat the exact reading with the same accuracy. In general, the measurement can be supposed to outdo the accuracy claimed by 2-10 times. Remember that the ability to repeat the encoder is just one aspect of the overall system’s repeatability. Mechanical failures caused by gear backlash and hypertrophy significantly reduce the parameter value.

This entry was posted on September 13th, 2022 and is filed under Automation, Electrical. Both comments and pings are currently closed.

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