Thomas Mauer
If you have been following my multiprotocol Industrial Ethernet blog series, you already know that I am a big fan of the programmable real-time unit and industrial communication subsystem (PRU-ICSS), a programmable peripheral inside Sitara™ processors. In this post, I want to talk about another application where PRU-ICSS helps integrate your chosen digital position encoder master interface into the Sitara processor.
First, let’s look at a system-level diagram that includes position encoders – see Figure 1. Applications like servo drives often have motors with integrated digital position feedback encoders. The position encoder is a type of sensor that precisely measures the angle (in degrees) of a motor shaft or the distance (in micrometers) of a linear axis.
In such systems, the position encoder reads the angle of the motor shaft and provides this information through a digital position encoder protocol to a position encoder master. The field-oriented control (FOC) algorithm on the application processor processes the angle further, along with the motor-phase current measurement, in order to calculate new values for the pulse-width modulation (PWM) peripheral.
The distance between the position encoder and the position encoder master can be as long as 100m. Digital position encoder protocols, operating through an RS-485-based interface, establish a robust communication channel.
Once again – similar to industrial Ethernet – different encoder vendors have developed many position encoder protocol standards that you can use to transfer the position data. The challenge for single-chip drive developers is to select and integrate the right digital encoder protocol standard based on encoder and motor vendors. The three most popular position encoder protocol standards are EnDat 2.2, BiSS C and High-Performance Interface Digital Servo Link (HIPERFACE) DSL.
Let’s look at the history and characteristics of these three position encoder protocol standards.
Each digital position encoder protocol standard traditionally requires a specific hardware implementation in terms of a field-programmable gate array (FPGA), programmable logic device (PLD) or application-specific integrated circuit (ASIC). However, you can eliminate those external device components when integrating the position encoder master protocol into the Sitara processor application by using the PRU-ICSS peripheral. The AM437x processor has two PRU-ICSS peripherals. You can use one for digital encoder master protocols like EnDAT 2.2, HIPERFACE DSL and BiSS C, and the second to run an industrial Ethernet protocol.
The multiprotocol position encoder master interface has been released as Multiprotocol Digital Position Encoder Master Interface Reference Design with AM437x processor on PRU-ICSS (TIDEP0057), where the ARM with PRU-ICSS peripheral detects the type of position encoder attached. See Figure 2.
A second reference design, Single Chip Drive for Industrial Communications and Motor Control (TIDEP0025), integrates the system approach of a single-chip drive, as shown in Figure 1. In this reference design the position encoder master interface uses EnDat 2.2, running on one PRU-ICSS of AM437x, to communicate with the position encoder while the other PRU-ICSS is used for EtherCAT slave communication support.
The PRU-ICSS is very versatile peripheral that can integrate complex functionality that was previously provided in external ASICs and FPGAs. As you learned in this blog post, the PRU-ICSS in the Sitara processor family integrates the position encoder master interface for many position encoder protocols and therefore eliminates those external components. You will save board space, component cost and establish a high-performance throughput system when integrating the position encoder master interface with PRU-ICSS into the Sitara processor.
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