SNLA420A September 2022 – January 2024 DP83TC812R-Q1 , DP83TG720S-Q1
As designers look for new ways to meet faster cycle time and higher throughput to meet big data requirements, and have these wider-bandwidth systems work at the highest efficiencies with minimal downtime, designers can also wish to minimize impact on the upgrade by reusing existing cabling infrastructure. Most also employ advanced features like smarter diagnostics, higher safety specifications and faster and better real-time characteristics for motor control.
Robotic system architectures must meet communication interface requirements like bandwidth margin. As bandwidth requirements continue to increase, designers are incorporating both Ethernet and optical designs that are faster than legacy interfaces like RS-485 and Controller Area Network (CAN). However, when moving to Ethernet, consider how to minimize latency in the real-time performance of the Ethernet protocol, either through industrial Ethernet protocols such as Ethernet/IP, EtherCAT, Profinet, and so forth, or by implementing a proprietary communication protocol.
This article discusses the benefits of SPE (Base-T1) for robotics applications as well as key challenges when designing with compact, efficient, robust and low-noise communication interfaces for robotic systems. These challenges need to be understood to implement single twisted-pair Ethernet in the robot systems and still achieve the needed performance to operate the robot efficiently.
Typically, two key design parameters are data rate and cable size or length. These two parameters are related which means that the cable length for some communication interfaces define the data rate which can be achieved. A second parameter is the physical amount of cables and connector pins which are needed to implement the interface.
Table 1-1 lists the standard data rates and cable lengths of PHY types typically used in robotic systems.
| Communication Interface | Data Rate | Cable Length | Twisted-Pair Wires |
|---|---|---|---|
| 4mA to 20mA I/O | 0.01Mbps(1) | 3000m | 1 |
| HART | 00012Mbps | 3000m | 1 |
| CAN | 1Mbps | 40m | 1 |
| CAN-Flexible Data Rate (FD) or CAN-Signal Improvement Capability | 10Mbps | 10m | 1 |
| RS-485 | 20Mbps | 40m | 1 |
| 100Base-TX | 100Mbps | 100m | 2 |
| 1000Base-TX | 1,000Mbps | 100m | 4 |
| 1000Base-SX | 1,000Mbps | 1,000m | Multimode fiber |
| 1000Base-LX | 1,000Mbps | 5,000m | Single-mode fiber |
| Low-voltage differential signaling | 360Mbps | 10m | 1 |
| 100Base-T1 | 100Mbps | 50m (UTP) 100m (STP) | 1 |
| 1000Base-T1 | 1,000Mbps | 15m (UTP) 40m (STP) | 1 |
| 10Base-T1L | 10Mbps | 1 | |
| 10Base-T1S | 10Mbps | 1 |
When considering the data rate and cable lengths required for a particular robotic system, also consider the cable aging (highly affected by the movement of the robot), cost, diameter, and weight of the cables used in the system. The cable tree of the manipulator In a robotic arm is typically replaced every 2 to 3 years due to cable aging. This is performed as preventive maintenance, without testing the cable function. With this in mind, by reducing the number of wires (which can age) and by introducing smart diagnostic features in the PHY (to understand the ongoing quality of the cable), there are fewer points of failure and cable and connector health indicate a need to change cabling, rather than arbitrarily doing so every couple of years, needed or not. Another benefit is that the mechanical construction of the arm gets smaller and more cost effective due to less space being needed to route smaller cables.
There are specifications beyond data rate and cable selection which impact the performance of a robotic system and so these specifications must be understood. The following list shows some system elements which influence the system performance of robots and these points must be considered in the design of a system.