EMC compatible design for motor position encoder interface - Part 6
In this series, we have discussed different digital interface options for motor position encoders to date, including EnDat 2.2 Bidirectional/Serial/Synchronous (BiSS) and HIPERFACE DSL. These are the main standards for connecting to an absolute position encoder using a serial digital interface based on RS-485 or RS-422. Further interface standards include Profibus Decentralized Peripherals (DP) and Profibus Input/Output (I/O), as well as Controller Area Network (CAN) or Ethernet-based interfaces. There are also proprietary, driver-specific standards such as Siemens' DRIVE-CLiQ, Fanuc serial interface, Mitsubishi's high-speed serial interface, and many more. In this blog series, we only cover EnDat, BiSS, and HIPERFACE. In this blog post, I will summarize all of these standards and share with the reader an EMC-compatible design that supports any of the standards you use in industrial-driven applications. The contents in Table 1 attempt to summarize the specifications of all of these encoder interface standards, comparing the three standards in terms of physical layer (PHY) and supply voltage (EnDat 2.2, HIPERFACE DSL and BiSS). Figure 1 is an overview of the relationship between maximum clock frequency and cable length. Table 1: EnDat 2.2, BiSS, and HIPERFACE DSL interfaces Figure 1: Relationship between maximum clock frequency and cable length specifications (0 to 100m) Table 2 lists the basic technical specifications for RS-422/RS-485 transceivers, as well as the basic technical specifications for those power supplies that apply to the three standards mentioned in Table 1. Table 3 shows the power supply specifications for the same content. Table 2: Transceiver Requirements Table 3: Encoder Power Requirements A solution for absolute position encoder interfaces The Universal Digital Interface Reference Design to the Absolute Position Encoder (TIDA-00179) implements an electromagnetic compatibility (EMC) compliant universal digital interface; this interface interfaces with absolute position encoders such as EnDat 2.2, BiSS or HIPERFACE DSL. The main building blocks of this TI Designs reference design are: A bidirectional four-wire RS-485 interface and a two-wire bidirectional RS-485 interface powered by RS-485. A multiplexer or demultiplexer that selects an active encoder interface. Encoder power supply with programmable output voltage and overvoltage, overcurrent and short circuit protection. A 3.3V digital interface connected to the host processor to run the corresponding encoder standard protocol. This host processor running the corresponding encoder master protocol does not belong to this design. Figure 2 shows a simplified system block diagram of a general purpose digital interface module. As an industrially driven subsystem, this module is connected to an absolute position encoder with a reference design in the light green box. Figure 2: Industrial drive with universal digital interface to absolute position encoder This design is powered by an industry standard 24V supply and features a wide input voltage range of 15V to 60V. A connector with a 3.3V logic I/O signal can be directly interfaced to a host processor such as a SitaraTM AM437x or C2000TM microcontroller unit (MCU) to run the corresponding encoder master protocol. The C2000 MCU provides the DesignDRIVE development platform and TIDM-SERVODRIVE; TIDM-SERVODRIVE can be adjusted to interface with a host port interface and TIDA-00179. This design allows the host processor to activate the four-wire RS-485 physical interface to connect to an EnDat 2.2 or BiSS encoder, or to activate a two-wire RS-485 interface that supports RS-485 power supply, making it compatible with a HIPERFACE The DSL encoder is connected. This protected encoder power supply features a programmable output voltage of 5.25V or 11V to meet the selected encoder power range and specifications. We chose the voltage, along with the voltage ripple and output current, to ensure compatibility with the overall power specifications for the EnDat 2.2, BiSS, and HIPERFACE DSL encoder standards. The encoder power supply also features short-circuit protection, while the overvoltage threshold matches the selected output voltage with fault feedback. You can connect the absolute position encoder to the reference design via a Sub D-15 connector or a 10-pin header. This connector has pins specifically designed to connect two-wire HIPERFACE DSL encoders, including RS-485 power supplies, as well as shared pins for EnDat 2.2 and BiSS position encoders. This design supports cable lengths up to 100m. For the relevant cable specifications, please refer to the appropriate encoder manufacturer recommendations. This design has been tested for EMC against Electrostatic Discharge (ESD), Electrical Fast Transient (EFT), Surge, and Conducted Radio Frequency (RF) with a test rating that meets the ratings specified in IEC 61800-3. The operation of TIDA-00179 requires a hardware and a software layer. Host control is typically performed by the FPGA; this design uses the AM437x host processor to run all three encoder protocols.
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