Industrial Ethernet Guide - Execution Speed and Determinacy
The following is part of A Comprehensible Guide to Industrial Ethernet by Wilfried Voss.
High-speed and high-precision manufacturing equipment such as, for instance, a robotic arm utilizes electric components like motion controllers, programmable logic controllers (PLC), feedback devices, and more, and it is mandatory for the machine’s performance that these components work together in a synchronized fashion. The sampling of a measurement at non-deterministic or random times will deliver incorrect results.
To stay with the example of controlling a robotic arm, representing a closed-loop control, the motion controller must know at all times where the arm is located. The controller needs to know the precise position data, and if that data is not immediately available, the controller might tell the arm to move when it shouldn’t.
While the speed of the process is critical for real-time solutions, it is not the main factor. Determinism, i.e., the support specific and constant response times, translates directly into reliability, meaning a controller must respond within an absolute bounded response time to events, or safety and productivity may be compromised.
The level of a control system’s determinism is a function of both hardware and software. Naturally, it makes sense to use high-performance (CPU) hardware plus a suitable real-time operating system (RTOS), because deterministic control and real-time operating systems go hand in hand.
However, the advantages of such a combination may be in vain with the wrong choice of network protocol.
As the OSI 7-Layer Model demonstrates, a protocol following the OSI specification will affect the control system’s hardware (refer to Layers 1 and 2) and software (refer to Protocol Layers 3 to 7).
Industrial Ethernet solutions will definitely provide the necessary speed, but when it comes to high-speed and high-precision performance, execution speed is sometimes important while determinacy is always important.
The question is, are Industrial Ethernet protocols, in fact, real-time capable, i.e., do they support the necessary level of determinacy, or are they only fast?
This question plays into the definition of “hard” and “soft” real-time control.
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