Absolute Encoders

What is a rotary encoder?
Rotary encoders detect or specify angular positions and convert this information into electrical signals. They are angle sensors that transmit their measured value to electronics without contact. And it is precisely this characteristic that differentiates them from potentiometers, which are passive components. Basically, each encoder consists of a housing, electronics with sensor as the heart of the measurement and the electrical connection. Depending on the design, a shaft with a shaft bearing is also part of the sensor to produce the angle measurement mechanically.  There are numerous related terms for the word rotary encoder, such as rotary angle encoder, rotary angle transducer, rotary angle meter or rotary angle sensor, but also angle encoder or angle coder. However, if the angle is provided as a complete value (i.e. as an absolute value) with a fixed reference to a zero position, then the encoder is referred to as an absolute encoder. If only the angle change is provided, i.e. the output signal provides only the relative information, then it is an incremental encoder. In this guidebook, only contactless technologies with magnetic or optical measuring principle are described.

What does "contactless" mean?
Contactless means that the measured value transmission between measured value recording and measured value acquisition takes place without contact. For example, a rotating part of the application is connected to the encoder's shaft, and the measured value is recorded by the electronics. However, there is no direct mechanical connection between the two components. The measured value transmission is therefore contactless or non-contacting. In all MEGATRON encoders, the contactless transmission of measured values is based either on magnetism or on an optical measuring principle. With magnetic encoders there is practically no wear on the measured value acquisition (electronics) and with optical encoders only the optical detection unit has a limited lifespan. The only significant wear on contactless encoders occurs via its mechanical components for measured value recording when there is a shaft and shaft bearing.

 Magnetic rotary encoders with Hall effect
The Hall effect, named after Edwin Hall, describes the occurrence of an electrical voltage, the so-called Hall voltage, in a current-carrying conductor (Hall element) located in a stationary magnetic field. If a circular diametrically magnetized (north pole/south pole) permanent magnet is placed over a Hall element, this magnet is subjected to a rotary motion and the voltage is measured at the output of the amplifier circuit, then a sinusoidal output voltage curve is measured. External magnetic fields can in principle interfere with this technology. So-called gradient-based Hall sensors are predominantly used, which are largely insensitive to these disturbances.

Advantages:

• The best choice when longevity is required
• Less sensitive to vibrations
• Less sensitive to changing environmental influences, such as temperature fluctuations, high relative humidity
• High actuation speeds (rpm) possible
• Suitable for operation in oily, polluted environments (machine shops, construction sites, etc.)

Optical encoders
Optical encoders are based on a contactless, optical scanning principle. Light is generated by means of a light-emitting diode, that shines through a coding wheel onto a photodetector. The photodetector generates an electrical signal that is processed by the electronics and used to output the measured value. In the case of contactless encoders, the light-emitting diodes are subjected to a continuous aging process during operation. In addition, suspended particles that settle on the optical system in the form of fine dust contribute to the aging of the sensor.

Advantages:

• Angular encoders with a high optical resolution enable position measurement with very high accuracy
• High repeatability of the measurement result
• Very high actuation speeds (rpm) possible
• Very stable against external magnetic fields (immission)
• Excellent suitability for exact speed determination
• Very long lifetime
• Low signal jitter

Incremental encoders
Incremental encoders output a certain number of rectangular signals instead of information proportional to the angle (cf. absolute encoders). They are also referred to as pulses. Incremental encoders are therefore also called rotary pulse encoders and the number of pulses per revolution is always given (unit imp./rev.). One pulse corresponds to one period duration. The term "one increment" is also used for one period duration. This also explains the term incremental encoder. For the evaluation of the measurement result of an incremental encoder, an external evaluation unit is always required, for example a counter.
The following points should be noted in particular:

• For an angle measurement, the number of pulses must be counted in an external evaluation unit and the sum of the pulses must be converted into an angle.

• - If the operating voltage of the counter is interrupted, the counter information is usually lost. If the absolute value of the angle relative to a reference point is to be measured or calculated, referencing must be performed by passing through the zero position.
• For a speed measurement, the number of pulses per time is calculated.

Incremental encoders are available with different numbers of pulses per revolution. If, for example, 360 pulses/rev are specified, this means that 360 pulses (360 signal periods) are output per full shaft revolution (360°). If, for example, 1024pulses/rev are specified, then 1024 pulses (1024 signal periods) are output per full shaft revolution (360°).

Absolute encoder
Absolute encoders output an analog or digital signal proportional to the angle. There is therefore a fixed reference point for the angle measurement. Rotary encoders with analog output provide the measured angle as output voltage, output current or pulse width (PWM).

Singleturn encoder
Singleturn encoders are absolute encoders that can only measure the angle of one full revolution.  After one full revolution, the output signal shows the same value every 360° as for 0°. Most contactless singleturn absolute encoders measure the full angle range from 0° to a maximum of 360°. Only a few products measure angles in a restricted angular range, for example +/- 45°.

Multiturn encoder
Compared to single-turn encoders, multiturn encoders are capable of measuring angles beyond 360°. The measuring system is capable of counting the number of revolutions and it is usually programmed so that the signal increases continuously over the maximum electrically effective angular range. For example, certain multiturn absolute encoders from MEGATRON are capable of measuring angles up to a maximum of 72000° (up to 200 shaft revolutions). Without special precautions, such encoders lose their position information when the supply voltage is interrupted. One class of multiturn encoders are true power-on encoders. Such an encoder provides a correct output signal even if the angle of rotation changes arbitrarily during a temporary absence of voltage.

Actual value device and setpoint device
The two terms "actual value device" and "setpoint device" are defined by the purpose of use in the application. Some encoder models can be used for both purposes. For a setpoint encoder, a value is set by manual input. A setpoint value is specified via a manual rotation of the encoder shaft (usually via an adjustment knob mounted on the shaft). These adjusters / manual adjusters are used in control panels, for example to navigate through menus or to specify various parameters for measuring instruments. The actual value encoder is used as a synonym for an angle sensor or rotary encoder when an angle is simply measured and not manually specified by hand. Since this does not necessarily correspond to the target value in an application, the two terms are used to distinguish between them. Setpoint and actual value encoders can be parts of control loops; this application is what gives the two terms their meaning in the first place.

Mechanical angle of rotation
The mechanical angle of rotation is the total angle at which the encoder can be mechanically operated. The mechanical angle of rotation is not mechanically limited in most non-contact rotary encoders. I.e., the shaft of the encoder can be continuously actuated clockwise and counterclockwise without being stopped in the rotational movement. With a few exceptions, there is the option of mechanical end stops. These are particularly useful for setpoint devices (manual adjusters). An example is the ETAM25 series from MEGATRON which has mechanical end stops.

Electrically effective angle of rotation
The electrically effective angle of rotation is the angular range in which the output signal changes. The following illustrations show exemplary signal output functions of singleturn absolute encoders. In both cases, the mechanical angle of rotation is 360°.

Sense of rotation
When programming the output signal curve, it is important to specify the direction of rotation of the desired output signal curve. The direction of rotation must be specified in the description of the desired output signal curve so that an unambiguous relationship is established between the signal and the direction of rotation of the shaft. The direction of rotation of the shaft is specified when the encoder is viewed from the front. That is, when the observer is looking at the shaft bearing and the shaft end. In the case of a kit encoder (without its own shaft), the observation is made on the housing side facing the magnet. For the direction of rotation, a distinction is made between clockwise and counterclockwise. The abbreviations CW for clockwise and CCW for counterclockwise have become established. The CW or CCW direction of rotation can be selected for almost all absolute encoders by the customer when configuring the encoder.

Supply voltage
All contactless angle encoders require a direct voltage (DC) as supply voltage (VSUP) for operation. A distinction is made between encoders that have a supply voltage change in a defined range if there is a ratiometric relationship to the output signal or if no ratiometric relationship exists, i.e. has no influence on the output signal. With a ratiometric relationship between supply voltage and output signal, the output signal changes in the same multiplicative ratio as the supply voltage. This option is only available for absolute encoders with analog signal output.  Furthermore, not all available supply voltage ranges can be combined with every output electronics. When selecting the supply voltage, it should therefore be checked whether the desired output circuitry is available for the desired supply voltage. The data sheet of the encoder provides information on the possible combinations.

Redundancy
Some applications require redundancy of the sensor signal. The following goals are often the motivation for using redundant encoders:

Increasing the availability of systems
The double design of the sensor reduces the probability of system failure. If one of the two lines fails, an error is logged. However, the machine or plant can continue to run until the next maintenance interval, where the sensor is replaced without loss of machine time.

Increasing operational safety
When operating safety-critical machines (e.g. vehicles, aviation, etc.), a failure can be fatal. Redundancy enables a safe, controlled shutdown of these machines or systems until the sensor is replaced. Redundancy is mandatory for many applications of this type.

If two encoders cannot be installed in principle, it is possible to implement encoders with two separate supply voltages and separate grounds (GND) for encoder operation, which provides galvanically isolated additional electronics.
With magnetic angle encoders, the magnet is always installed at the shaft end. Therefore, it is not possible here to guide the shaft through the housing to another sensor. The magnetic sensor element itself is double/redundant and, in some models, galvanically isolated as an option. In the case of optical angle encoders, it is possible to realize tandem versions which mechanically only have the shaft in common but are otherwise completely double.

Signal outputs

The following signal outputs are available for contactless absolute encoders.

Analogue:
Voltage output (different ranges, ratiometric/non-ratiometric).
Current (0...20 mA, 4...20mA)
Pulse width modulation (PWM)

Digital:
SPI: Serial Peripheral Interface
SER: Special form of SPI format
SSI: Synchronous Serial Interface

The following output circuits are available for contactless incremental encoders:
OC (Open Collector, Pull Up resistor not integrated in the encoder)
Voltage Output (=Open Collector circuit incl. pull-up resistors integrated in the encoder housing)
TTL (Transistor Transistor Logic)
PP (Push Pull)
Linedriver

Cabling
For electrical connection cables (tolerances) of angle encoders, other guidelines apply than for housings and shafts of rotary encoders. Note: If the cable tolerances are not explicitly mentioned in the data sheet, they are traceable to IPC/WHMA-A-620. For MEGATRON angle encoders with metal housing, the connection cable is shielded by an external cable shield. For all angle encoders with plastic housing, the connection cable is not shielded.

Designs of rotary encoders
Encoders are offered in many designs. They can be divided into kit encoders (without shaft bearing) and encoders with shaft bearing. For the latter, a distinction is made between variants with plain bearings or ball bearings, and solid or hollow shafts.

Installation and mounting
The options for mechanical mounting of the encoder in the application depend on the design of the encoder housing. MEGATRON offers a total of five different mounting options for its contactless encoder families. The mounting can be done by means of central thread (bushing), flange, threaded holes, servo flange (with synchro clamps), mounting ring or spring plate.

Product adaptations
For over 60 years, MEGATRON has been a reliable partner for your design-in needs. In addition to the wide range of options offered by our sensors, we also offer specific designs tailored to your application requirements, even for small quantities. Whether it is an early-stage project or series production - we are happy to accompany and support you.