Motor position encoders are widely used in industrial motor control applications such as servo drives, robots, machine tools, printing presses, textile machines and elevators. Using interfaces to connect these encoders to other parts of your system introduces some troubling electromagnetic compatibility (EMC) issues. To help you meet these challenges, I will start with an overview of the various types of motor position encoders and their interfaces. The remainder of this series will discuss in depth how to design EMC standards for each of the different motor position encoder types. Industrial interface.
The desired position/angle resolution can vary from a few to 25 or more than 25 bits depending on the application of the industrial driver. Some drive applications even require angular degrees of rotation. The installation distance from the frequency converter to the position encoder will vary 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 realize robust data transmission with high immunity to electromagnetic fields, common mode voltage, 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 on incremental position or angle changes. They do not provide an absolute position after power-up but it is still possible to obtain an index signal after a mechanical rotation. Absolute encoders always provide absolute mechanical position.
Incremental encoders can display three differential signals: A signal, B signal, and Z signal. A signal and B signal may be incremental position change encoding. The position resolution depends on the number of lines of the incremental encoder. Typical line numbers range from 50 to 10,000 lines per revolution. The Z signal usually appears once per revolution and is a "home index" used to derive the absolute position.
The incremental encoder interface is either 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 these encoders allow much higher resolution than the encoders with TTL/HTL outputs because you can use a sine with a The arctangent function of the cosine and cosine signals inserts its position within a line number. This interpolation can increase 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 rotation speed is proportional to the frequency of the output signal.
Absolute position feedback encoders provide absolute position (resolution up to 25 bits or more). Their electrical interfaces have evolved from serial interfaces based on analog and digital hybrid protocols to serial interfaces based on purely digital protocols. Serial communication standards are typically vendor-specific and can utilize RS-485 or RS-422 differential signaling through bidirectional data transmission. For example, EnDat 2.2 not only transmits the absolute position, but also allows data to be read from or written to the encoder's memory. With the mode commands sent to the EnDat 2.2 encoder by a subsequent electronic device (often referred to as the EnDat 2.2 master), you can select the type of data to be transmitted - absolute position, number of rotations, temperature, more parameters, diagnostics data.
Standards based on pure digital serial protocols, such as EnDat 2.2, BiSS® and HIPERFACE DSL®, compensate for propagation delays and support communication over cable lengths up to 100 meters. Pure digital protocols have a constant clock frequency that does not change with the speed of rotation. For most protocols, you can select the clock frequency/baud rate to accommodate external factors such as cable length.
Encoders with analog and digital hybrid or pure digital communications interfaces typically have vendor-specific supply voltage ranges. Table 1 is an overview of the widely used encoder standards.
Table 1: Position encoder interface standard and supply voltage
When using an interface 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.
Complies with the IEC 61800-3 standard for electromagnetic compatibility (EMC).
power supply.
Location decoding and/or signal processing at the digital protocol master.
Figure 2: Simplified block diagram of the position feedback interface module on industrial drives/converters
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 two-channel analog-to-digital converters Analog signal chain. You need to design physical interfaces to meet EMC immunity requirements, such as immunity requirements for electrostatic discharge (ESD), electrical fast transient (EFT) bursts, and surges. The relevant standards specified by IEC61800-3 are as follows:
ESD: Voltage is ±4kV (for direct contact discharge) or ±8kV (for air discharge).
EFT: The voltage is ±2kV and the frequency is 5kHz, through the capacitive coupling clamp.
Surge: The voltage is ±1kV and the source impedance is 2Ω, coupled through the cable shield.
The TTL/HTL encoder requires minimal signal processing and requires only one direction quadrature pulse counter. Incremental sine/cosine encoders also require this quadrature counter; in addition, signal processing is also required in order to calculate arc tangent for interpolation. Standards based on digital serial interface protocols require more signal processing and are usually implemented on field programmable gate arrays (FPGAs), and more recently on innovative processors (such as SitaraTM AM437x, which can utilize Implemented on the Programmed Real-Time Units subsystem and the Industrial Communications Subsystem (PRU-ICSS) peripherals.
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