Basics of analog & digital sensor signals

For industrial process control, there are a variety of ways that sensors and transmitters communicate measurement information. This article provides a general primer to help understand each signal and where it may be commonly used for measurement and control.

What are Analog Signals?

Analog signals are continuous electrical signals that represent the measurement variable. The analog signal could be based on either voltage or current. The signal is scaled based on the range of the measured variable. A possible analogy for an analog signal could be something like a dimmer control for a light bulb. As the dimmer control is moved the intensity of the light changes.

In control system terminology analog signals may be abbreviated as follows:

AI – Analog input

AO – Analog output

Common Analog Output Types

4-20mA – The most common analog output signal is the 4-20mA current output. This is a universal signal that could be related to temperature, pressure, or analytical measurements like pH or conductivity. In most cases a 24 volt DC supply powers the current output signal.

The transmitter converts the measured variable into the 4-20mA current output where 4mA = 0% and 20mA = 100%. There are multiple wiring schematic for transmitter wiring:

2-wire (current and voltage on the same +/- wires. Normally referred to as “loop powered”)
3-wire (a common 24VDC power supply is used for both the transmitter/sensor and the 4-20mA current signal)
4-wire (separate 24VDC power supplies are used to power the transmitter and the 4-20mA current

0-10 VDC (also 1-5 VDC) - DC voltage outputs are also commonly used as a universal signals for process control. For example, 0-10 VDC is prevalent in building control (HVAC) applications thus may have some cross-over into industrial control. When in doubt, check the specifications of your device to learn about its voltage output range. Also note that mA current signals may be converted to voltage by adding resistors in parallel with the circuit.

Millivolt - sensors such as pH probes or thermocouples may generate a millivolt signal in relation to the measurement. For example, a glass membrane pH sensor will generate a signal of + 414mV to – 414mV corresponding to the 0 to 14 pH scale. Often the millivolt signal is too small to be transmitted long distances so it is converted to 4 – 20mA within the transmitter.

Nanoamp – Electrochemical sensors such as polarographic dissolved oxygen products generate a nanoamp current signal in relation to the density of oxygen molecules within the process. As is the case with millivolt outputs, the nanoamp signal is typically converted to 4-20mA within a transmitter so it can be more easily used by existing control systems.

Advantages & Disadvantages of Analog Signals

The 4-20mA signal has been in use since the 1950’s thus is universally accepted by most control systems. The current is strong enough that it is relatively unaffected by common noise sources such as EMI (electromagnetic interference) and RFI (Radio Frequency Interference). When used in a 24VDC loop powered configuration a 4-20mA signal may be permitted in a hazardous area provided that the electronics has been designed and tested for low voltage potentials (intrinsic safety testing).

With the advent of digital communication, the limits of 4-20mA signals are becoming apparent. The signal only transmits a current proportional to the measurement thus additional information such as diagnostics are not available.

What are Discrete Output Signals?

A discrete output is a simple on-off signal. Often discrete signals are tied to an alarm point in a process. For example, if a process pH measurement got out of range, a discrete output could activate an acid pump to lower the pH to an acceptable value. During normal operation the switch is open and the pump is not activated. Only when the pH measurement gets too high then the switch closes to power the pump.

If we continue with our lightbulb analogy, a discrete output would function more like a traditional light-switch which turns on-off as needed. The voltage needed to energize the light would come from an external source.

Discrete Signal Types

There are two common types of devices used for discrete signals.

Mechanical Relays – This product functions just like the light switch above. A metallic contact is made to complete the electrical circuit. A mechanical relay is normally actuated by an electromagnet to close the contact. In general, mechanical relays can handle higher amperage loads but may be a little slower to actuate due to physical nature of how the connection is made.

Solid State Outputs – These devices use electrical devices to complete the circuit. Unlike a mechanical relay there are no moving parts. Solid state outputs may use a transistor (DC) or triac (AC) to generate the switching mechanism. Solid state outputs are generally used in low amperage high cycle rate applications.

Moving to Digital Communication

While analog and discrete control signals are still the predominant means to communicate process measurements, the trend is largely shifting to digital communication. Digital communication provides a method to transmit more information about the measurement then simply the process variable. If we again return to our lightbulb example, lets consider what information might be valuable beyond just the actual brightness of the light.

4-20mA Analog Signal

Brightness of light

Digital Signal

Brightness of light
Temperature of light
Operating hours
Number of on/off cycles
Serial number of bulb
Date of manufacturer

In the case of the lightbulb, this additional information can be used to make decisions on the status of the lightbulb. For instance, lower brightness may signal dust on the lightbulb (cleaning required). Long operating hours or declining brightness may lead to preventative replacement of the lightbulb before failure.

Another valuable aspect of digital signals is that multiple measurements could be made on the same connection path. This can be used to reduce the cost of wiring and simplify design and installation.

Image showing digital connection on the same connection path

Types of digital communication

As digital signals have evolved different protocols have been developed. Each protocol should be thought of as its own “language”. The communication may be different however the end result of transmitting measurement information still remains the same.

Modbus

Modbus protocol dates to the development of the Programmable Logic Controller (PLC) launched by Modicon (now Schneider Electric) in the late 1970’s. Modbus is referred to as a serial protocol where data is transmitted back and forth on a single channel one bit at a time in a client – server format. Two wires are used for data transmission while separate two wires provide power for the transmitter or sensor. Modbus relies on registers which are read / write variables that must be mapped on each device for proper communication.

While Modbus is an older protocol it has many advantages that have contributed to the longevity of the language. One big advantage of Modbus is its open source status. It can be used by any developer with no royalty of licensing fees (see Modbus.org for more information). Over time the protocol has evolved where it can be used over TCP/IP (Ethernet) connections for the added functionality of polling by multiple clients.

Hamilton Arc sensors use Modbus for digital communication. Each product has a Programmers Manual for register information used to communicate with the control system.

HART

HART is an acronym for Highway Addressable Remote Transducer. The protocol was developed by Emerson in the mid-1980’s. HART protocol offers digital communication on the same 2-wire, 24VDC loop power traditionally used by many field transmitters for 4-20mA signals. The digital signal is a superimposed frequency that does not interfere with the current signal. This design has led to widespread adoption by users of traditional analog-based control systems. Over time, HART has become the most common digital protocol for process measurement applications.

HART required a Device Description (DD) file for each device. This file is uploaded to the control system to fully recognize all variables present in the device. The protocol has a fixed 1200 bits/second (baud) rate thus digital communication is best used for configuration or calibration but may be too slow for rapid process control applications.

Hamilton offers HART on our VisiFerm mA and VisiTrace mA optical dissolved oxygen sensors.

Profibus / Profinet

Profibus is a digital protocol developed by a German government backed consortium and launched in 1989. Shortly after launch, Profibus was adopted by Siemens thus became synonymous with their PLC products and gained a loyal following in Europe. Profibus digital protocol has since evolved in the following versions:

Profibus DP – This protocol follows the RS 485 standard (also used for Modbus) where power and signal wiring are separated into two pairs. “DP” is short for Distributed Peripherals. Instead of running individual cables back to the PLC in the control room, all transmitter connections are managed at Input-Output (IO) termination points which use a single serial link back to the controller. Profibus DP is considered a high speed protocol with data transmission rates up to 12 Mbits/second.

Profibus PA – Profibus PA is targeted at “Process Automation” applications. The communication signal resides on the 2-wire, 24VDC loop wiring also used to power the field device. The design is similar to HART protocol; however the 4-20mA signal does not exist. Profibus PA supports intrinsic safety requirements thus is suitable for hazardous area applications. Data transmission rates are 31.25 Kbits/second. Individual devices are linked to the control system through a segment coupler which provides power for each device as well as converts PA to DP for high speed data transmission back to the control system.

Profinet – Profinet moves the protocol from the RS 485 standard to Ethernet. This advancement allows for much higher data transmission rates (100 Mbits/second or higher). While Profibus DP / PA use a master-slave arrangement Profibus allows for more flexibility so that multiple controllers can access device data in a duplex nature similar to most computer networks.

Profibus has some similarities and advantages over HART. A GSD file is used to upload the device variables into the controller. Profibus Data transmission rates are much higher than HART (31.25 Kbits/second on PA vs. 1.2 Kbits/second with HART). The additional speed of Profibus is more acceptable for actual process control applications. Hamilton has several converters that allow Arc sensor Modbus signals to be connected to Profibus DP and Profinet systems.

OPC UA

OPC is a standard for digital data transfer using Ethernet. It began life in 1996 as an attempt to standardize traditional "sensor protocol" such as Modbus and Profibus into a format that could be easily read by computers. Early OPC versions such as OPC DA relied upon Microsoft Windows. These versions required extensive programming of sensor specific data. This was referred to as creating a tag database. Variable such as measurement range, decimal points, and unit of measure had to be loaded into the OPC server for things to work properly.

The current standard is called OPC UA. It works with all platforms (Windows, Android, Apple, Linux). OPC UA automatically recognizes sensor specific tag data thus eliminates much of the painful programming incurred with prior OPC versions.

Hamilton offers an Arc OPC Converter that can power and transmit the measurement signals of up to four Arc sensors.

Hamilton Connectivity

As a manufacturer, Hamilton understands the requirement to offers products that cover the full spectrum of output signals. The infographic below provides a summary of our measurement parameters and which control signals can be offered.