Working principle and application of photoelectric encoder

An encoder is a device that compiles and converts signals (such as bit streams) or data into a signal form that can be used for communication, transmission and storage. The encoder converts angular displacement or linear displacement into electrical signals. The former is called a code disk, and the latter is called a code ruler. According to the readout method, encoders can be divided into contact and non-contact types. The contact type uses brush output, and a brush contacts the conductive area or the insulating area to indicate whether the state of the code is "1" or "0"; the non-contact type receives sensitive elements, which are photosensitive elements or magnetic sensitive elements. When using photosensitive elements, the light-transmitting area and the opaque area are used to indicate whether the state of the code is "1" or "0". The collected physical signals are converted into electrical signals that can be read by machine code through the binary encoding of "1" and "0" for communication, transmission and storage.

Encoder working principle:

A measuring element that uses the principle of electromagnetic induction to convert the relative displacement between two planar windings into an electrical signal, used as a length measuring tool. Induction synchronizers (commonly known as encoders and grating rulers) are divided into two types: linear and rotary. The former consists of a fixed ruler and a sliding ruler and is used for linear displacement measurement; the latter consists of a stator and a rotor and is used for angular displacement measurement.

In 1957, RW Tripp and others in the United States obtained a patent for the induction synchronizer in the United States. The original name was the position measurement transformer, and the induction synchronizer was its trade name. It was initially used for positioning and automatic tracking of radar antennas, guidance of missiles, etc. In mechanical manufacturing, induction synchronizers are often used in positioning feedback systems of digital control machine tools, machining centers, etc. and in measurement digital display systems of coordinate measuring machines, boring machines, etc. It has low requirements for environmental conditions and can work normally in an environment with a small amount of dust and oil mist. The period of the continuous winding on the fixed scale is 2 mm. There are two windings on the sliding scale, and their periods are the same as those on the fixed scale, but they are staggered by 1/4 period (the electrical phase difference is 90°).

There are two working modes of induction synchronizer: phase detection type and amplitude detection type. The former is to input two AC voltages U1 and U2 with a phase difference of 90° and the same frequency and amplitude into the two windings on the slider respectively. According to the principle of electromagnetic induction, the winding on the fixed scale will generate an induced potential U. If the slider moves relative to the fixed scale, the phase of U will change accordingly. After amplification, the displacement of the slider can be obtained by comparing the phase with U1 and U2, subdividing, and counting. In the amplitude detection type, the AC voltage with the same frequency and phase but different amplitude is input to the slider winding. According to the amplitude change of the input and output voltage, the displacement of the slider can also be obtained. The system composed of induction synchronizer and electronic parts such as amplification, shaping, phase comparison, subdividing, counting, and display is called an induction synchronizer measurement system. Its length measurement accuracy can reach 3 microns/1000 mm, and its angle measurement accuracy can reach 1″/360°.

With the rapid development of industrial automation, encoders are more widely used in the field of industrial control.

1. Q: What should I pay attention to when selecting an incremental rotary encoder?

Three parameters should be noted:

1. Mechanical installation dimensions, including positioning stop, shaft diameter, installation hole position; cable outlet method; installation space volume; whether the protection level of the working environment meets the requirements.

2. Resolution, that is, the number of pulses output per revolution when the encoder is working, whether it meets the design accuracy requirements.

3. Electrical interface, encoder output modes commonly include push-pull output (F-type HTL format), voltage output (E), open collector (C, C is commonly NPN tube output, C2 is PNP tube output), long-line driver output. Its output mode should match the interface circuit of its control system.

2. Question: How to use the incremental encoder?

1. Incremental rotary encoders have different resolutions, which are measured by the number of pulses generated per revolution, ranging from 6 to 5400 or more. The more pulses, the higher the resolution; this is one of the important bases for selection.

2. Incremental encoders usually have three signal outputs (differential has six signals): A, B and Z. They usually use TTL level, with A pulse in front and B pulse in the back. A and B pulses are 90 degrees apart. A Z pulse is sent out every circle, which can be used as a reference for mechanical zero position. Generally, A leads B or B leads A to determine the direction. Our company's incremental encoder is defined as clockwise rotation of the encoder from the shaft end, which is positive rotation, A leads B by 90°, and counterclockwise rotation is reverse rotation, which is B leads A by 90°. There are also differences, depending on the product description.

3. When using PLC to collect data, you can choose a high-speed counting module; when using an industrial computer to collect data, you can choose a high-speed counting board; when using a single-chip microcomputer to collect data, it is recommended to use an input port with a photocoupler.

4. It is recommended that pulse B be used as a forward pulse, pulse A be used as a reverse pulse, and pulse Z be used as a zero pulse.

5. Set up a counting stack in the electronic device.

3. About outdoor use or use in harsh environments

The equipment is used in the field, the environment is dirty, and there is a fear of damaging the encoder.

It has an aluminum alloy (stainless steel material can be made upon special request) sealed protective housing and a double-bearing heavy-duty encoder. It is not afraid of getting dirty when placed outdoors and can be used in steel mills and heavy equipment.

However, if there is space for the encoder installation, it is recommended to install a protective shell on the outside of the encoder to strengthen its protection. After all, the encoder is a precision component, and there is still a certain gap in the value comparison between an encoder and a protective shell.

4. From proximity switches, photoelectric switches to rotary encoders:

The application of positioning, proximity switches and photoelectric switches in industrial control is quite mature and easy to use. However, with the continuous development of industrial control, new requirements have emerged. In this way, the application advantages of rotary encoders are prominent:

Informatization: In addition to positioning, the control room can also know its specific location;

Flexibility: Positioning can be flexibly adjusted in the control room;

Convenient, safe and long-lasting on-site installation: A fist-sized rotary encoder can measure distances from a few μ to tens or hundreds of meters, and n stations. As long as the safe installation of a rotary encoder is solved, many proximity switches and photoelectric switches can be avoided from mechanical installation troubles on site, easy to be damaged, and troubled by high temperature and moisture. Because it is a photoelectric code disk, there is no mechanical loss, and as long as the installation position is correct, its service life is often very long.

Multifunctionality: In addition to positioning, it can also remotely transmit the current position and convert the movement speed, which is particularly important for applications such as frequency converters and stepper motors.

Economy: For multiple control stations, only the cost of one rotary encoder is required, and more importantly, the installation, maintenance, and loss costs are reduced, and the service life is increased, and its economy is gradually highlighted.

Due to the advantages mentioned above, rotary encoders have been increasingly widely used in various industrial control situations.

5. About power supply and encoder and PLC connection:

Generally, there are three types of working power supplies for encoders: 5Vdc, 5-13Vdc or 11-26Vdc. If the encoder you bought uses 11-26Vdc, you can use the PLC's 24V power supply. Please note that:

1. The current consumption of the encoder is within the power range of the PLC.

2. If the encoder is a parallel output, when connected to the I/O point of the PLC, you need to understand whether the encoder's signal level is a push-pull (or push-pull) output or an open collector output. If it is an open collector output, there are two types: N type and P type, which must be the same polarity as the PLC's I/O. If it is a push-pull output, there is no problem with the connection. 3. If the encoder is a driver output, the general signal level is 5V. Be careful when connecting, and do not let the 24V power supply level be connected in series with the 5V signal wiring to damage the encoder's signal end. (Our company can also make wide voltage driver output (5-30Vdc), please indicate this requirement when ordering)

6. In many cases, the encoder is not broken, but it is just due to interference, which causes a bad waveform and leads to inaccurate counting.

The encoder is a precision component. This is mainly due to the serious interference around the encoder, such as whether there are large motors or welding machines frequently starting up to cause interference, whether it is transmitted in the same pipeline as the power line, etc.