Digital panel meter design

Panel meters are commonly mounted in industrial control panels to measure various physical quantities like temperature, pressure, voltage, current, power, and speed.

These meters take an electrical signal that is proportional to the physical quantity being measured and display the value either on a mechanical dial (in the case of analog panel meters) or a display (in case of digital panel meters). Panel meters come in various standard sizes as listed in the Table 1 below.

Table 1: Size standard for panel meters

Analog panel meters (Figure 1a below) display the measured quantity on a dial using a needle. These analog meters suffer from various drawbacks like non-linearity and parallax errors. They are also hard to calibrate and have a short life due to aging and mechanical wear and tear. Also, they provide fewer features to meet today’s changing market needs.

To overcome these disadvantages, analog panel meters are being widely replaced by digital panel meters (Figure 1b below). Unlike analog panel meters, digital panel meters (DPM) are linear, more accurate, and easier to read as they are equipped with LED or LCD displays.

They also have memory to store various configuration and user parameters. In addition, they can perform functions beyond making simple measurements. For example, they can monitor system health and generate alarms when a particular input is outside a specified range.

Figure 1: a) An analog voltmeter and b) a digital voltmeter

There are various implementations available for digital panel meters. Each of these implementations offers specific advantages and some disadvantages, as discussed below.

ASIC and ASSP based solutions

Application specific ICs (ASICs) and Application Specific Standard Products (ASSPs)for digital panel meters usually integrate the ADC and the display driver in a single device. Figure 2 below shows the basic block diagram of a digital panel meter implementation using an ASIC.

Figure 2: ASIC-based digital panel meter

In the above block diagram, the signal conditioning circuit varies based on the range and physical quantity being measured. Generally these ASICs support a couple of standard input voltage ranges; for example, from 0 to 199.9 mV or 0 to 1.999V.

For inputs other than those supported, external signal conditioning circuits have to be used to scale the input signal to within the range supported by the device. For example, if an input is 0-19.9mV, the signal conditioning circuit would be an amplifier with a gain of 10. For an AC input, a precision rectifier or an RMS converter would be used. And so on.

The advantage of an ASIC-based approach is ease of design. These designs can be made with discrete components and do not need any microcontroller or software programming. However, with these advantages come certain disadvantages as well:

* ASICs generally require capacitors with low dielectric loss that add significant cost to a design.

* External signal conditioning increases the number of components and cost.

* These devices support a fixed number of displays and counts; for example, a display might support 3 ½ digits with 1999 counts or 4 ½ digits with 19999 counts.

Displaying an input with a maximum range that is less than the full scale requires using only part of the input range, thus affecting the maximum resolution displayed.

* They require manual calibration using multi-turn potentiometers which increases cost and manufacturing time.

* Because different signal conditioning circuits are required for different inputs, separate boards have to be designed for different input ranges. This increases inventory cost.

* Implementing communication protocols like MODBUS require special ASICS which are expensive.