Speed and position encoders in variable speed drive systems
Abstract
Modern variable frequency drives often operate in speed-encoderless mode. It means that there is no physical sensor to measure the rotational speed of the motor. Although the encoderless operation is generally preferred, some applications require speed or position encoder. This article provides an overview of encoders utilized in variable speed drive systems: in which situations are they needed, on what principle do they work, what are the key parameters of an encoder and what other design options shall be considered when selecting an encoder.
Introduction
Most modern variable frequency drives (VFD) support encoderless operation of the motor. Speed and rotor position are not measured but estimated from electrical values (calculated by the motor model). The encoder less solution has several advantages: elimination of encoder itself (hardware cost), no need to lay an additional signal cable between VFD and motor, higher robustness of encoderless solution for motors operating in harsh environment (vibration, pollution) etc.
Nevertheless, certain drive systems require a speed and/or position encoder for their optimal performance. Next paragraphs are dedicated to motor speed encoders used in variable speed drive systems. Topics such as application areas, selection of encoder resolution, types of mounting or communication protocols are addressed.
When is an encoder required?
Encoderless operation is usually preferred over installing a physical sensor on the motor. Advantages of encoderless systems are:
- Elimination of hardware (encoder on the motor, processing module inside the VFD)
- Elimination of additional signal cable between inverter and motor
- Higher robustness against EMC disturbances
- Easier installation on site
- Decoupling from environmental challenges (vibration, pollution etc)
However, the encoderless algorithms have their limitations. Thus, in some cases an encoder is inevitable.
Typical situations where a speed encoder is required include:
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Types of encoders based on physical principle
a) Optical
Encoders work on a principle of sending and receiving light signal that passes through a disk with transparent and opaque lines equally placed along its circumference. Main components are:
- light emitting source (mostly LED)
- encoder disk of glass, plastic or metal
- photodetectors (phototransistors or photodiodes) on receiving side
- electronic circuit to shape and convert the signal
The encoder code disks can be realized either as opaque disks with slots or a disk from transparent material with opaque lines (e.g. metal disk with slots, glass disk with opaque lines). Optical encoders excel in high resolution and high accuracy due to many pulses per revolution. Disadvantage is their sensitivity to contamination. Thus, their performance is degraded in installations with inherent pollution common in industrial applications. Proper mechanical installation is also a key aspect for good performance of the encoder. The tolerances on the coaxiality and pitch shall be minimized. Optical encoders remain the most spread encoders in motion control applications.
b) Magnetic
The encoder works on principle of detecting changes in magnetic field using rotating magnetized wheel (disk with poles) or ring. Characteristic of magnetic encoder:
- magnetic gear tooth sensor (variable reluctance rotary sensor)
- magneto-resistive encoder (patterned thin-film resistors fixed along the wheel)
- Hall effect magnetic encoder (Hall effect sensors with a layer of semiconductor material)
- measuring frequency up to 50 kHz or higher
- single turn and multiturn resolution – square wave output signal
Advantage of magnetic encoder is a rugged design resistant to shock and vibration. They can be built very compact. Magnetic encoders are not affected by contamination such as dirt, dust or oil. However, these encoders are susceptible to magnetic interference caused by electric motor, especially when supplied with modulated voltage from an inverter. They achieve lower resolution than optical encoders.
c) Capacitive
Encoders use changes in capacitance to detect position of the shaft. Incremental and absolute encoders can be realized by using capacitive principle. The encoder consists of three main components:
- Encoder rotor
- Stationary transmitter
- Stationary receiver
The rotor contains a sinusoidal pattern and, as it rotates, the high frequency reference signal of the transmitter is modulated in a predictable way. The encoder detects the changes in capacitance-reactance on the receiver board and translates them, using a demodulation algorithm, into increments of rotary motion [3].
Such encoders are insensitive to contaminants such as dirt, dust or oil [4] that sometimes create issues for encoders based on optical principle.
Types of encoders based on measured quantity
Speed encoder
Speed encoder is mostly realized as a pulse encoder (also called incremental encoder, IE). The device is used for asynchronous and synchronous machines.
The incremental encoder can calculate the speed in two different ways:
- Use the counter of pulses (at higher speeds)
- Calculate the time between two pulses (at low speeds)
Position encoder
Position encoders provide a unique code for each sector of shaft position. The main characteristics are:
- absolute encoder (AE), gray encoder
- single turn absolute encoder
- multi-turn absolute encoder
- for synchronous machines only
- optional incremental output
- Gray code or SSI
- protocols HTL or TTL
Encoder output
Incremental encoder
(sometimes referred to as quadrature encoding technique). Such signals allow to determine the direction of rotation. Often, they are complemented with a third signal ‘Z’ that provides one pulse per revolution (called reference pulse or index pulse). This pulse can be used for two things: a) verify the correctness of pulse counter or b) determine the relative position with reference to this pulse. It also means that the resolution is 4-times higher than the number of pulses per revolution (PPR). Finally, the incremental encoder may include also inverted signals inv(A) and inv(B) if using a differential wiring instead of single wiring. Differential wiring has higher immunity against disturbance (noise).
Absolute encoder
An absolute encoder provides the information about rotor position with reference to a fix point. This encoder that divides the circle into number of sectors and uses a special binary pattern that does not repeat within the revolution (each sector is associated with its unique binary code). Well-known coding is so called Gray code (named after Frank Gray from Bell Laboratories). Advantage of Gray code is that only one bit changes when transiting from one sector into the next one. It allows a simple implementation or error-checking by using the
bit sum.
Selection of encoder
There are multiple criteria to be defined when selecting a suitable encoder. Besides the functional requirements, such as e.g. the resolution (accuracy), there are parameters related to the type of mounting, area classification etc. This section presents some of the selection criteria along with typical options.
- Feedback type: Incremental encoder / Absolute encoder
- Housing material: Aluminum / Cast iron / Stainless steel / Plastic / Alloy
- Orientation: Horizontal / Vertical
- Encoder size: acc. to flange size
- Interface: TTL / HTL / DLD / Push-Pull
- Duty class: Industrial duty / heavy duty
- Resolution: e.g. 512 / 1’024 / 2’048 / … / 20’000 pulses per revolution (ppr)
- Connector type: Cable / Conduit / Connector / Pin header / Terminal box
- Environment: Non-hazardous / Hazardous
- Protection class: IP40 / IP51 / IP52 / IP54 / IP65 / IP67 etc
- Certifications: ATEX / CE / Ul
- Mounting style:
- Max. operated speed:
- Hollow bore size:
- Sensing technology:
- Special features:
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Installation of an encoder
Now we look at the methods of installing the encoder on the electric motor. Basic options are:
- Shafted (solid shaft)
- Hollow-shaft
- Hub-shaft
- Bearingless
- Flange plus foot
Shafted
Encoder has its own solid shaft that is attached to the motor shaft through a flexible coupling and external flange.
Hollow shaft
The hollow-shaft options allows fitting over the motor shaft. The encoder is clamped in place using spring loaded tether. Advantage is a compact installation (no additional coupling, no extra flange).
Hub shaft
In this solution, the encoder is sitting at the end of the motor shaft. The motor shaft does not pass entirely through the encoder. Method is also called blind thru-bore encoder.
Bearingless
Encoder is ring mounted. The principle uses a magnetic sensor on the motor face (static) plus magnetic wheel mounted on the motor shaft. Such solution is beneficial for heavy duty applications.
Potential issues
An encoder might help to achieve better accuracy of speed and position sensing across the speed range and especially in the low speed range. However, an encoder can also be source of issues. Some of them are related to…
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Summary
Most of the modern variable frequency motor drive systems operate in encoderless mode. Nevertheless, high performance applications may require a speed encoder and, in case of synchronous machine, also a position encoder. Incremental encoder is used to obtain motor speed while absolute encoder provides information about rotor position. Based on the working principle the encoders can be optical, magnetic or capacitive. There is a large variety of products and configurations. The selection of encoder shall reflect requiements from the VFD (to ensure compatibility), project specific aspects as well as the environmental conditions and mechanical installation on the motor shaft. This article provides a brief overview and orientation.
References
[1] Encoders in motor drive applications, MB Drive Services, October 2019, available online, https://mb-drive-services.com/encoders_part1/
[2] Incremental encoder interface, Posital, available online, https://www.posital.com/en/products/communication-interface/incremental/incremental-encoders.php
[3] Building the Right Foundation with a Hollow Shaft Encoder, DYNAPAR, available online, https://www.dynapar.com/technology/hollow-shaft-encoder/
[4] Capacitive, magnetic and optical encoders: Comparing the technologies, same sky, available online, https://www.sameskydevices.com/blog/capacitive-magnetic-and-optical-encoders-comparing-the-technologies
