We need better accuracy now, so we're making a move to digital panel meters (DPMs) from our trusty analog panel meters. As we examine possibilities, it also looks like we could substantially reduce the number of different meters we use by using higher-end digital panel meters that offer optional expansion cards with a lot of extra functions, maybe to the point of needing fewer controllers. But it puts us in expertise areas where we haven't been before. Anyone care to share experiences about this possibility?
—From August'13 Control Design
SEE ALSO: Digital vs. Analog Panel Meter Choices
Think It All the Way Through
We build medium-sized ($1-5 million) wet-process machines for electroplating and similar wet-process operations. Our machines use numerous analog readouts and some process loop controls, and in past years we did use analog meters. We had a brief dalliance with DPMs, which lasted only a few years before we moved all of the analog readout functions, including process loop controls for temperature and such, into the PLC/HMI system on the machines.
Is it accuracy or significant digits and easier reading in the display that you need? If it is indeed accuracy, then that pretty much rules out any cheap DPMs. Look carefully at the specs for accuracy and at drift with respect to ambient temperature when making a DPM selection if accuracy is actually important to you. It is hard to beat those old mechanical Weston meters for accuracy over wide temperature range, year after year.
The high-end meters to which the questioner refers will be required if the goal is to reduce the number of different meter models used and if good accuracy is required. These meters are quite expensive and are also sometimes more accurate and more versatile than the $10 meters sold to the hobbyist market. If reducing the number of different models is important to the questioner, then I surmise that they must have quite a few meters to replace.
After about 10 meters, the total installed cost will in many cases be less to install a PLC with analog inputs, signal conditioning if necessary, and a nice HMI screen. In addition to replacing the analog meters, you get the following advantages for free:
- Control logic is already linked to the metered variables, so it is easy to write control programs based on those variables. There are no communications issues between "smart" meters or other analog input devices and the controller if the analog signals are brought right into the controller.
- Sophisticated alarm logic can be created. While some digital panel meters do have a simple alarm function, they lack the ability to mask the alarm at times when it is inappropriate for the alarm to be given, and no way to display whether an operator has acknowledged that alarm.
- Plaintext alarm messages can be generated, and the controller can even contact personnel through IM, e-mail, or cell phones in the event of alarms.
- Help scripts can be provided that are linked to specific alarm conditions.
- Adding datalogging is trivial.
- Calibration can be easier and in many cases can be automated.
- No hardware cost is involved to change the display style (bargraphs, rotating needles and direct numerical readouts all have their place in a good HMI), or to make the display color change prominently in the case of some alarms. Display sizes can be adjusted according to importance.
- Secure remote monitoring of the displayed values done via WiFi, Internet or cellular (and even alteration of setpoints or other process settings if permitted) from within the plant, or from the other side of the world, is possible with the correct choice of hardware and software options.
If those meters display the same process variables used by the control system, then there's an advantage to having the actual numbers come from the same source — only one calibration to do; no concern over the control system and the meter display disagreeing in the least significant digit; and immediate indication if the control system is no longer receiving the signal.
If the questioner's plant has a lot of analog meters, then to the extent that's a reflection of the age of the rest of their control system, it's a good bet they would see good payback from a complete control system upgrade instead of just swapping out the meters. Some outside expertise could be required, but the end result will be satisfactory for much longer than a patchwork approach. In some cases, better control and more user-friendly operator interface yield truly amazing gains in profitability when older systems are replaced.