How to choose an encoder?

If your project requires the use of an encoder, then you need to choose the one that suits your needs. Today, the consumer is given an incredible range of encoders to choose from, but if you don’t know what you’re looking for, there’s a good chance you’ll end up with something suboptimal. Let’s take a look at what is on the market today for such components, and perhaps introduce you to some new options.

Encoder types or how to select the right encoder

Types of encoders

There are two most common types of rotary encoders: incremental and absolute.

Incremental (quadrature) encoder

Incremental encoders generate pulse signals when a wheel or shaft turns a certain angular distance. This type outputs two separate signals in a quadrature arrangement, so it can indicate both distance and direction of shaft rotation.

In addition to indicating direction and distance, some incremental encoders also have an indexing function at a single point of rotation. The indexing function allows the shaft to return to a known point. Other encoders have an enter button, adding a new user interface.

The main advantages of incremental encoders are:

1. reliability,
2. ease of installation,
3. relatively low cost.

In order to select an incremental encoder, it is necessary to determine the following main points:

• the required measurement accuracy,
• the shaft diameter and its model variety,
• the type of cable output connector and its length,
• electrical characteristics.

Incremental encoders are one of the most common sensors in modern complex technical systems. Their use is expedient in any powerful electric drives of precise movement systems or systems that are critical to high-speed and starting-braking modes.

Here are just a few examples of their uses:

• Medical equipment
• Automated test and diagnostic equipment
• Self-propelled robotic devices
• Access control systems

Absolute encoder

For the most part, incremental encoders inherently don’t “know” the position of the shaft (other than the index point). Absolute encoders, on the other hand, use an internal readout to measure the angular position directly and maintain this measurement capability even if the power is turned off.

In addition to determining the shaft position, multi-turn absolute encoders can also determine how many revolutions the encoder has made in one direction or the other.

Absolute encoders are an important link between the mechanical part of the machine and its control unit due to their precise and unambiguous measurements, which they can transmit instantly. This quality is successfully used in modern enterprises of mechanical engineering and robotics.

Absolute encoders are divided according to the following parameters:

• Single turn or multi turn
• Number of pulses per revolution or number of bits
• With shaft, with hollow rotor or with through hollow rotor
• Shaft or shaft bore diameter

When choosing an absolute encoder, you should pay attention to the following parameters:

• The number of bits per revolution. The accuracy of the system depends on this indicator.
• Type of shaft connection to the encoder.
• Encoder output type (The most common types of signal outputs are parallel code, SSI, DeviceNet, Profibus-DP, CANopen, LWL interfaces).
• Electrical characteristics. The operation of the system and the accuracy of signal removal depend on this indicator.
• Degree of encoder protection against dust and moisture penetration.

The absolute encoder is designed to be a versatile, configurable sensor for a wide range of applications. The following areas of use are distinguished: the medical field, alternative energy, telecommunications systems, packaging and food industries, and many others.

Optical and magnetic encoders

Depending on the application, the principle of determining the rotation is important. It can affect the price or how well an encoder can handle external interference or contamination.

The main types are:

1. Optical encoder. For optical reading, encoders have a light source that is progressively interrupted by a disc or other means attached to the shaft. This light transmits pulses for incremental encoders and transmits position data for absolute encoders.
2. Magnetic encoder. They are much cheaper than optical ones and are more compact. Most of them use analog Hall effect devices mounted on a printed circuit board. Hall sensors are driven by a two-pole magnet mounted on the end of a shaft. It produces two AC signals in antiphase with one cycle per revolution of the shaft.

Additional functions, features and characteristics of encoders

Among encoders that have the same class and rotation definition, there can be a large discrepancy between capabilities. Encoder prices often reflect these differences. As a result, the following parameters must be taken into account:

Encoder resolution.

Number of pulses or encoder positions per revolution.

The resolution of a rotation encoder corresponds to the maximum number of points it can measure in one revolution.

In the case of an incremental encoder, the resolution is directly related to the number of pulses it delivers per revolution.

In the case of an absolute encoder, resolution measures the number of revolutions the device can write before “rolling” to the zero position. For example, a 16-bit encoder would have a resolution of 65,536 points per revolution.

Encoder PPR is to be chosen according to the expected resolution, which must match the application and the accuracy of the mechanical components of the measuring circuit.

The sensor is connected to an electronic device, controller or meter that allows a maximum input frequency to be taken into account. Indeed, a high resolution incremental encoder generates more pulses per revolution than a low resolution incremental encoder. Depending on the speed set by the application, the encoder output may be at a higher frequency than the device connected to the encoder can handle. In this case, you will need to use a lower resolution encoder.

Speed

Encoders have a maximum mechanical speed in rpm as well as a maximum frequency response. Both must be considered, as well as the frequency response capabilities of the readout electronics.

Reliability

While many encoders require a relatively clean environment, others are hardened for industrial use. This characteristic may be related to the rotation detection method or device case type. Thus, when choosing, you need to take into account the working environment.

Eltra offers a wide range of heavy duty encoders. Also in some series it is possible to order a reinforced metal case.

Encoder electronic output types

Here some the most common output types of Eltra encoder.

1. Line Driver and RS422 (Differential output). The Line Driver can actively force the output low and high, allowing it to source and sink current from the load. As a result, a higher current is generated while maintaining longer transmission distances. As the name suggests, this type of encoder output is popular for a range of industrial applications, especially where the noise level is quite high.

RS-422 (TTL) output provides a constant 5 V signal level independent of the supply voltage.

2. Open Collector output. This type of output allows to receive a signal with a voltage level determined not by the encoder supply voltage, but by the voltage of the additional power source. To do this, it is necessary to connect an external resistor between the power circuits and the output, the resistance value of which is determined by the voltage value of the additional power source.

Eltra uses in its models NPN open collector and PNP open collector

3. Push-pull (HTL). This type of output is built on r-p-p and p-p-p transistors and has a low output impedance, which allows to increase the current delivered to the load. Thus, the load capacity is increased and the charging and discharging processes of the load capacity are accelerated, and hence the speed is increased as well.
4. Analog encoder. This is an absolute position sensor that has a 0-10 V signal at the output; 4 – 20 mA; 0 – 5V; 0.5 – 4.5 V; 0 – 20 mA. The device is used to measure angle, tilt, linear position and displacement. They have gained popularity in industrial automation, now they are being actively replaced by encoders with digital signals. But analog sensors continue to hold their niche due to high reliability, ease of signal processing, and unified dimensions.
5. SSI (Synchronous Serial Interface). It allows transmission of the absolute encoder position data by a serial line synchronized by a clock. The principle of operation of an encoder with an SSI interface is very similar to the standard one. Its main parts are a light source, disc with transparent and opaque windows, compare/trigger circuits, photoelectric receivers, parallel/serial converter, monostable circuit, input circuit for a clock signal, and an output driver for a data signal.