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 via interfaces can present some challenging electromagnetic compatibility (EMC) issues. To help you address these challenges, this series will begin with an overview of various types of motor position encoders and their interfaces. The remainder of this series will delve into how to design EMC-compliant industrial interfaces for each different type of motor position encoder.
The required position/angle resolution can vary depending on the industrial drive application, ranging from a few bits to 25 bits or more. Some drive applications even require angular rotation degrees. The installation distance from the inverter to the position encoder can vary, from a few meters (in multi-axis drives) to 100 meters or more. Due to such long distances, the electrical interface needs to be designed to achieve robust data transmission with high immunity to electromagnetic fields, common-mode voltage, impulse noise, etc.
Figure 1 illustrates 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 upon power-up, but it is still possible to obtain it after a single mechanical rotation via an index signal. Absolute encoders, on the other hand, always provide absolute mechanical position.
Incremental encoders can display three differential signals: A, B, and Z signals. The A and B signals encode incremental position changes. Position resolution depends on the number of lines in the incremental encoder. Typical line counts range from 50 to 10,000 lines per revolution. The Z signal typically appears once per revolution and is used to derive the "home index" for absolute position.
Incremental encoder interfaces are digital pulse trains with transistor-to-transistor logic (TTL) or high-threshold logic (HTL) compatible digital output levels, or analog sine/cosine outputs with amplitudes of 1Vpp or 11μApp. Encoders with analog outputs are often called sine/cosine encoders. These encoders allow for significantly higher resolutions than encoders with TTL/HTL outputs because you can interpolate their positions within a single line count using the arctangent function of the measured sine and cosine signals. This interpolation can increase the resolution by as many as 16 bits, with a possible total resolution of 25 bits or more. The product of the selected encoder's line count and rotational 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 mixed analog and digital protocols to serial interfaces based on purely digital protocols. The serial communication standard is typically vendor-specific and can utilize RS-485 or RS-422 differential signals for bidirectional data transmission. For example, EnDat2.2 not only transmits absolute position but also allows data to be read from or written to the encoder's memory. Mode commands sent to the EnDat2.2 encoder via a subsequent electronic device (often referred to as the EnDat2.2 master station) allow you to select the type of data to be transmitted—absolute position, number of revolutions, temperature, additional parameters, diagnostic data.
Standards based on purely digital serial protocols, such as EnDat2.2, BiSS®, and HIPERFACEDSL®, compensate for propagation delays and support communication over cable lengths up to 100 meters. Purely digital protocols have a constant clock frequency that does not change with rotational speed. For most protocols, you can select the clock frequency/baud rate to accommodate external factors such as cable length.
Encoders with hybrid analog-digital or purely digital communication interfaces typically have vendor-specific power supply voltage ranges. Table 1 provides an overview of widely used encoder standards.
Table 1: Position Encoder Interface Standards and Power Supply Voltages
When any of these encoders is connected to a frequency converter for closed-loop control via an interface, the position interface module contains the following functional blocks, as shown in Figure 2:
Physical simulation or digital interface.
Compliant with IEC 61800-3 standard for electromagnetic compatibility (EMC).
power supply.
Location decoding and/or signal processing for digital protocol master stations.
Figure 2: Simplified block diagram of the position feedback interface module on an industrial drive/inverter
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 an analog signal chain with dual analog-to-digital converters. You will need to design the physical interface to meet EMC immunity requirements, such as immunity to electrostatic discharge (ESD), electrical fast transient (EFT) bursts, and surges—the relevant standards specified in IEC 61800-3 are as follows:
ESD: The voltage is ±4kV (for direct contact discharge) or ±8kV (for air discharge).
EFT: Voltage ±2kV, frequency 5kHz, via capacitive coupling clamp.
Surge voltage: ±1kV, source impedance 2Ω, coupled through cable shielding.
TTL/HTL encoders require minimal signal processing, needing only a single quadrature pulse counter. Incremental sine/cosine encoders also require this quadrature counter; additionally, signal processing is needed to calculate the arctangent for interpolation. Standards based on digital serial interface protocols require more signal processing and are typically implemented on field-programmable gate arrays (FPGAs), but more recently on innovative processors such as the Sitara™ AM437x, which utilizes programmable real-time unit subsystems and industrial communication subsystems (PRU-ICSS) peripherals.
In the next article in this series, my colleagues and I will explore each encoder interface in greater depth and provide detailed information on how to implement an EMC-compliant industrial interface/BiSS-C master interface for BiSS-C position encoders.
