Motor position encoders are widely used in industrial motor control applications such as servo drives, robots, machine tools, printing presses, textile machines and elevators. Connecting these encoders to other parts of your system with interfaces can cause some tough electromagnetic compatibility (EMC) problems. To help you meet these challenges, the author will begin with an overview of the various motor position encoders and their interfaces. The rest of this series will delve into how to design EMC standards for each different motor position encoder type. Industrial interface.
The required position/angle resolution can vary depending on the application of the industrial drive, from a few bits to 25 bits or more than 25 bits. Some drive applications even require angular rotation. The mounting distance from the frequency converter to the position encoder varies from a few meters (in a multi-axis drive) to 100 meters or more. Due to the long distance, the electrical interface needs to be designed to achieve robust data transmission with high immunity to electromagnetic fields, common mode voltages, and impulse noise.
Figure 1 shows several types of linear or angular position feedback encoders suitable for industrial applications.
Figure 1: Position feedback encoder and its corresponding interface
There are two types of position encoders: incremental position encoders and absolute position encoders. Incremental encoders provide information about incremental position or angular changes. They do not provide absolute position after power-up, but it is still possible to obtain the index signal after a mechanical rotation. Absolute encoders always provide absolute mechanical position.
The incremental encoder can display three differential signals: the A signal, the B signal, and the Z signal. The A and B signals can be coded for incremental position changes. The position resolution depends on the number of lines of the incremental encoder. A typical line number range is 50 to 10000 lines per revolution. The Z signal usually appears once per revolution and is the "home index" used to derive the absolute position.
The incremental encoder interface is a digital pulse train with transistor-transistor logic (TTL) or high threshold logic (HTL) compatible digital output levels or an analog sine/cosine output with 1 Vpp or 11 μApp amplitude. Encoders with analog outputs are often referred to as sine/cosine encoders, and the resolution allowed by such encoders is much higher than that allowed by encoders with TTL/HTL outputs, since you can use the measured sine The arctangent function of the cosine signal is inserted into its position within a line number. This interpolation increases the resolution by as much as 16 bits, with a possible total resolution of 25 bits or more. The product of the number of lines of the selected encoder multiplied by the rotational speed is proportional to the frequency of the output signal.
Absolute position feedback encoders provide absolute position (resolutions of 25 or more). Their electrical interfaces have evolved from serial interfaces based on analog and digital hybrid protocols to serial interfaces based on pure digital protocols. The standard for serial communication is typically vendor-specific and can utilize RS-485 or RS-422 differential signals through bidirectional data transmission. For example, EnDat 2.2 not only transmits absolute locations, but also allows data to be read from or written to the encoder's memory. The mode command sent to the EnDat 2.2 encoder via a subsequent electronic device (commonly referred to as the EnDat 2.2 master) allows you to select the type of data being transmitted - absolute position, number of revolutions, temperature, more parameters, diagnostics data.
Standards based on pure digital serial protocols such as EnDat 2.2, BiSS® and HIPERFACEDSL® compensate for propagation delays and support communication over cable lengths up to 100 meters. A pure digital protocol has a constant clock frequency that does not vary with the speed of rotation. For most protocols, you can choose the clock frequency/baud rate to accommodate external factors such as cable length.
Encoders with analog and digital hybrid communication interfaces or pure digital communication interfaces typically have a vendor-specific supply voltage range. Table 1 is an overview of the widely used encoder standards.
Table 1: Position encoder interface standard and supply voltage
When an interface is used to connect any of these encoders to a frequency converter for closed loop control, the position interface module contains the following functional blocks, as shown in Figure 2:
Physical analog or digital interface.
Electromagnetic compatibility (EMC) in accordance with IEC 61800-3.
power supply.
Signal processing for position decoding and/or digital protocol master stations.
Figure 2: Simplified block diagram of the position feedback interface module on an industrial drive/drive
Incremental digital HTL/TTL encoders and absolute digital encoders with RS-485 or RS-422 interfaces require less hardware interface operation, while analog sine/cosine encoders require dual analog-to-digital converters. Analog signal chain. You need to design physical interfaces to meet EMC immunity requirements such as electrostatic discharge (ESD), electrical fast transient (EFT) bursts and surge immunity requirements - the relevant standards specified in IEC61800-3 are as follows:
ESD: Voltage is ±4kV (when direct contact discharge) or ±8kV (at air discharge).
EFT: The voltage is ±2kV and the frequency is 5kHz, which is coupled through a capacitive coupling clamp.
Surge: The voltage is ±1kV and the source impedance is 2Ω, which is coupled through the cable shield.
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