Sensors 102: Signal conditioning
In many control applications, users don’t need to think about what goes on inside of a device. A signal is converted inside of a device to whatever their control system needs. Most industrial systems have standards listed on certain voltage or current signals: 24 Vdc or 120 Vac for digital; 0 to 10 Vdc or 4 to 20 mA for analog.
In most cases, the actual detecting element inside of the sensor is a low voltage device that requires somewhere from 1.5 to 5 Vdc. For commonly used, mass-produced devices, all of the conversion happens inside of the sensor. Users simply have to provide one of the standard industrial voltages mentioned previously and connect it to the input.
With larger systems, signals can’t be distributed at such low levels. Devices are often far away from the control enclosure, though communications-based distributed input/output (I/O) blocks have all but eliminated the necessity of using 120 Vac signals. Still, it is certainly not realistic to transmit transistor-transistor logic (TTL) signals from a sensor to a control cabinet.
What happens when users have to convert the signal? There are a variety of commercially available devices that take in a signal of one type and convert or amplify it into a signal of a different type. Probably the simplest method of doing this with a digital signal is to use a relay. Relays are available with 5 Vdc, 12 Vdc, 24 Vdc, 120 Vac, and even higher coils. Even with such a simple device, care must be taken not to corrupt the signal by introducing noise into the system. It wouldn’t do to tie-wrap your 5 Vdc wires to the motor cables or 480v supply.
Analog converters are available to convert 4 to 20 mA to 0 to 10 V and vice-versa, mV/V signals to standard analog, and various other special purpose devices. One of the problems that can occur is that shown in Figure 1. Not only do users have to consider the aspects of the converter itself—where to put it, how to wire it, its cost, etc.—but also how to provide power to it and the detection device.
Figure 2 provides an example of the devices needed to determine tank levels using load cells, which can bring up some interesting decisions on where to put things and how to interface them. Everything needs to fit in the space available to the users, which means some of the signal processing devices will not be mounted the way they would be in the field.
The signal conditioner itself, which takes the mV signal and amplifies it to a 0 to 10 Vdc signal, uses a 12 Vdc power supply. Again, since the output from the summing board is a mV level signal, it needs to be close to the signal conditioner.
When planning an application, consider these aspects:
- Cost of the components and the enclosures you may need to put things in
- The distance a low voltage signal can travel without being degraded or corrupted
- Power supplies/voltages that may not be available in the control system
- The cost of design/labor.
There is also the issue of how to calibrate all of this once everything is put together. Every time users pass through another conversion or device, another variable is introduced into the system.
Frank Lamb is the founder of Automation Consulting LLC and is a member of the Control Engineering Editorial Advisory Board. This article originally appeared on the Automation Primer blog. Automation Primer is a CFE Media content partner. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, email@example.com.
See additional stories from Automation Primer about ladder logic linked below.
Larger systems may require users to convert the signal from a sensor using a relay.
There are tools available for users who have to convert a digital or analog signal on their own.
When planning a signal conditioning application, users should consider the costs of the components and the field conditions.
What other challenges might users face when trying to convert a signal from a sensor?
See additional stories from Frank Lamb linked below.