In this series, so far we have discussed different digital interface options for motor position encoders, 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 Distributed Peripheral (DP) and Profibus Input/Output (I/O), and 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 more. In this blog series, we only covered EnDat, BiSS, and HIPERFACE. In this post, I will summarize all of these standards and share with the reader an EMC-compatible design that will support any standard you use in industrial drive applications.
The contents of Table 1 attempt to summarize the technical specifications of all these encoder interface standards, comparing these three standards from the physical layer (PHY) and the power 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 specification (0 to 100m)
Table 2 lists the basic specifications for the RS-422/RS-485 transceivers, as well as the basic specifications of 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 interface
The universal digital interface reference design (TIDA-00179) to an absolute position encoder implements an electromagnetic compatibility (EMC) compliant universal digital interface; this interface is connected to an absolute position encoder such as EnDat 2.2, BiSS, or HIPERFACE DSL.
The main building block of this TI Designs reference design is:
A two-way four-wire RS-485 interface and two-wire bidirectional RS-485 interface powered by RS-485.
A multiplexer or demultiplexer that selects the 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 universal digital interface module. As an industrially driven subsystem, this module is connected to an absolute position encoder and its reference design is in the light green box.
Figure 2: Industrial drive with a universal digital interface to an absolute position encoder
This design is powered by an industry-standard 24V power supply and features a wide input voltage range of 15V to 60V. A connector with a 3.3V logic I/O signal allows direct interfacing 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 and the TIDA-00179. This design allows the host processor to activate a four-wire RS-485 physical interface, connect it to an EnDat 2.2 or BiSS encoder, or activate a two-wire RS-485 interface that supports RS-485 power supply with a HIPERFACE. DSL encoders are connected.
The protected encoder power supply features a programmable output voltage of 5.25V or 11V to meet the selected encoder power range and specifications. We have chosen the voltage, together with the voltage ripple and output current, to ensure compatibility with the overall power specification for the EnDat 2.2, BiSS and HIPERFACE DSL encoder standards. The encoder power supply also has a short circuit protection function, and the overvoltage threshold matches the selected output voltage with fault feedback.
You can connect an absolute position encoder to the reference design through a Sub D-15 connector or a 10-pin header. This connector has pins specifically for connecting two-wire HIPERFACE DSL encoders, including RS-485 power supply, and common pins for EnDat 2.2 and BiSS position encoders. This design supports cable lengths up to 100m. For the relevant cable specifications, see the corresponding encoder manufacturer's recommendations.
This design has been tested for EMC against electrostatic discharge (ESD), electrical fast transients (EFT), surges and conducted radio frequency (RF), and the test levels are in accordance with the levels specified in IEC 61800-3.
The operation of TIDA-00179 requires a piece of hardware and a software layer. Host control is usually performed by the FPGA; this design uses the AM437x host processor to run all three of these encoder protocols.
Similar to the combination of TIDA-00179 hardware for EnDat 2.2 (TIDA-00172), BiSS C (TIDA-00175) and HIPERFACE DSL (TIDA-00177), the new software provided with the AM437x will be associated with 3 of them The software is grouped together - TIDEP0050 (EnDat 2.2), TIDEP0022 (BiSS C) and TIDEP0035 (Hiperface DSL), to implement an interface with an encoder that meets any of the three main criteria discussed in this blog post.
The TI Sitara AM437x uses parallel Industrial Ethernet to provide an integrated solution for single-chip drives and modular architectures. It provides a register-compatible implementation for the existing FPGA IP core. Internal and external synchronization with the driver application can be achieved without the host CPU running any of the master protocols.
For more information and to view live demos, visit TI's booth at the SPS trade show in Nuremberg from November 24th to 26th. This demonstration will show you the real-time detection and connection with different position encoder protocols.
Figure 3: Multi-protocol position encoder demonstration on SPS
This post summarizes the Bowen series related to EMC-compatible interface design for motor position encoders.
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