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August, 2016 The Case for PC-Based Machine Control over PLCs Continued from previous page Depending on the operating environment,
safety can be a major concern for the user. It is vital in cases where human interaction with the machine could be dangerous, and when safety standards in a fab or fac- tory are crucial to machine acceptance. If a PC controller failed, or crashed, which was much more commonplace earlier in the development of the PC, the results could be disastrous: a dam- aged or destroyed machine, line, prod- uct, or even an injured operator. Thus, many machine designers have stuck to the steady reliability of the PLC with its significantly lower potential for failure.
The PC controller performed an
important role in that it was used pri- marily for complex calculations, moni- toring, measuring and factory net- working as well as being a user-inter- face to the PLC. It was usually housed in a more environmentally-controlled location because it couldn’t operate as reliably as a PLC in a harsh factory environment. Over time, the PC evolved to include PLC functionality.
Choosing Between PC and PLC Today, the PC has become more robust, hard-
ened for operation in the rough industrial environ- ments that were once only suitable for the PLC. Similarities and crossover functionality between the PC and PLC (which has not stood still in advancement as the PC controller gained ground) have grown such that the machine designer today faces a difficult choice in choosing between two types of controller that have seen a good deal of functional convergence. The choice can be simpli- fied by analyzing and comparing seven major areas of differentiation: operation, robustness, serviceability, hardware integration, security, safety, and programming.
Operation. PLCs contain an embedded real-time operating system (RTOS) with a dedicated proces- sor that ensures a high degree of control system
quickly and with ease while the system is in oper- ation — “hot-swapped.” PC replacement is not as easy and may require changing out the PCI board in the computer, which can lead to workstation downtime.
Hardware Integration. Both the PLC and PC have the ability to control a multitude of devices using familiar industrial communication networks and more modern Ethernet-based counterparts. PLC has many of these options built-in. The PC is equipped with a more open array of interfaces such as USB, Ethernet, etc., giving the user access to more off-the-shelf devices to handle tasks that a PLC could not usually match. One example is the utilization of
Block diagram of Sensoray’s Model 826.
reliability. Its task is only to handle automation and/or a specific process. PCs, however, can achieve the same level of control reliability with a real-time kernel or real-time operating system. The software running the PC is dedicated to automation and will have a minimal chance of locking up.
Robustness. The standard off-the-shelf PLC has no moving parts and can therefore withstand harsh environments, including shock and vibra- tion, for millions of cycles. PCs contain some mov- ing parts, such as fans or hard disk drives, which does not suit them for vibration, but “hardened” industrial versions offer options including solid state drives, fanless cooling, and in-cabinet mount- ing, making them just as durable as a PLC.
Serviceability. PLCs can generally be replaced
an advanced hi-res imaging system where the images would be stored, analyzed, compared, and archived. The PC is better-suited due to the advanced nature of the task and the
amount of memory required, while a PLC would have limited storage and processing capabilities.
Security. Traditionally, a PLC is less exposed to unauthorized access from the outside, and due to its dedicated OS, very few instances are known of virus attacks on PLCs — though they are not actually immune. PCs are far more susceptible to virus attacks, but strict security measures can be taken to reduce potential threats significantly, and standard software is available to detect and remove viruses.
Safety. PLCs have designated communication channels that slave devices to monitor operation tightly, and also contain integrated circuits to allow for redundancies. Sensoray builds integrated safety into its PC-based platforms, including the Model 626 and 826 PCI board controllers with Ethernet field bus design and built-in electrical
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