Years ago, as PLCs became smarter and more powerful, there was a push to get PLCs to perform basic PID control. Shortly after, some manufacturers developed units that functioned as a PLC but also had the ability to perform eight PID control loops. In the early 1980s, this was pretty exciting stuff.
Since then, modern PLCs have become extremely powerful. Because of advances in technology, many customers want to know whether a PLC or a stand-alone temperature controller is the right choice for a process. At the same time, many processors say they are reluctant to employ PLCs to control a new process, and some say they would never use a PLC for temperature control. Common reasons for these decisions include cost as well as ease of use, adjustment and troubleshooting.
Expense. The PLC cards needed to perform temperature control are relatively expensive. Depending upon the PLC, you may need a thermocouple input card, a temperature control module and possibly a special output card. By contrast, a stand-alone temperature control had all of those components and more for far less money.
Ease of Use. Using a PLC requires special software that can be expensive. Generally, there is an engineer responsible for operating and programming the PLC. If the PLC is to be set up for temperature control, it might be necessary to have an engineer or someone trained in the use of the specialized software configure the PLC.
Additionally, configuring a PLC requires someone to go out in the plant, where the machine is running, and plug into the PLC. As the new module is programmed for temperature control, changes are being made to a running process. Anybody who has worked on PLCs for any length of time will attest that there are dangers in editing a PLC while it is running the machine.
Adding a stand-alone temperature control does not typically require special software or personnel. An engineer simply wires the input and output, and the equipment is ready to go.
Ease of Adjustments. Once a system has been installed, there will likely come a time when the user wants to make changes to the process. These might include changing the setpoint, adjusting the PID parameters or tuning the system.
To adjust the settings of a PLC managing the temperature control, an engineer uses a laptop and plugs into the PLC. To adjust the settings of a stand-alone temperature control, an engineer or operator enters the changes via the keypad. Many temperature controllers also have an autotune setting that allows the controller to tune the system.
Ease of Troubleshooting. When things go wrong — as they tend to do in plants — the temperature control settings are a likely culprit that must be investigated.
To troubleshoot a PLC and the PID parameters, special software is needed along with a computer, cables and training to access the PLC and start troubleshooting.
With a stand-alone temperature control, troubleshooting typically is easier. One option is to simply swap out the old controller with a new one, which takes a few minutes. Once swapped out, the old controller can be taken to a bench and tested. If it needs repair, the operator can send it in for repair and have it back in a few days.
Ways to Achieve Temperature Control
In doing research, I have found a chemical processor who employs a distributed control system (DCS), but throughout the entire plant, temperature control is done with discrete temperature controllers. With this particular setup, all of the temperature controllers have options in them that allow users to send the temperature controller a setpoint remotely. Additionally, all of the temperature controllers have an option to retransmit the process valve back to the DCS system using a 4 to 20 mA output. At the same time, all of the temperature controllers include a local display so the operators can see the temperature and setpoint. Any operator can initiate an autotune by simply pushing a button on the front of his controller. This local display and control is why they continue to use stand-alone temperature controllers rather than a PLC.
Many people use stand-alone temperature controllers to manage process temperatures and a PLC for machine control. Some of the newer models of temperature controllers even look and feel like a small PLC. They offer communications so that they can easily exchange information with the PLC and share data.
Several manufacturers of temperature controllers offer devices with communications, including Ethernet/IP or DeviceNet protocol, that allow the controller to communicate implicitly to the PLC. Based upon the common industrial protocol (CIA), the PLC is able to change data in the controller and get specific data returned in real time from the controller without having to write a single line of ladder logic. The PLC can explicitly write or read data that does not need to occur in real time. Explicit messaging typically is used to initiate an autotune, for instance — an action that is rarely requested unless the process or setpoint has changed significantly. So if a user wants to use a stand-alone temperature controller and also use a PLC for machine control, these two devices can easily communicate with each other without writing lines of complicated code.