Sensors 102: Signal Conditioning

Today’s post is about what goes on between an actual detection device and the input of a control system.

In many control applications you don’t need to think about what goes on inside of a device. The signal is converted inside of the device to whatever your control system needs. Fortunately most industrial systems have standardized on certain voltage or current signals; 24vdc or 120vac for digital, 0-10vdc or 4-20mA for analog. In most cases the actual detecting element inside of the sensor is a low voltage device that requires somewhere from 1.5-5vdc, but for commonly used, mass produced devices all of the conversion happens inside of the sensor. You simply have to provide one of the standard industrial voltages mentioned previously and connect it to your input.

When I was in engineering school we built small digital circuits on bread boards using LEDs, resistors, transistors, bridge circuits, photodetectors and sometimes IC chips. Mechanical mockups of physical systems were made of cardboard, balsa wood or whatever a poor student could get their hands on. In general, everything was done at the 5-12 volt level and the signal didn’t have to travel very far.

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 I/O blocks have all but eliminated the necessity of using 120vac signals. Still, it is certainly not realistic to transmit TTL (Transistor-Transistor Logic) signals from a sensor to the control cabinet.

But what happens when you have to convert a signal yourself? Well, 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 5vdc, 12vdc, 24vdc, 120vac 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 5vdc wires to your motor cables or 480v supply!

Analog converters are available to convert 4-20mA to 0-10v 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 at the top of this post. Not only do you have to worry about 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.

In last week’s post I described my tabletop factory. In it I talked about using load cells on the process side to determine tank levels. The diagram above shows the layout of the devices that are required to do this; all of the components have been ordered and I have even received some of them. This has brought up some interesting decisions on where to put things and how to interface them. Since everything needs to fit in the space that I have, some of the signal processing devices will not be mounted the way they would be in the field. Though my “factory” is miniaturized, unfortunately the devices aren’t.

The fact that the load cells only have 24″ cables (a little over half a meter) gives you a clue that the mV/V signal isn’t meant to be run very far. My summing boards are also pretty huge, so it isn’t easy to figure out where to put them. Then there’s the fact that I need to run 5vdc to the board also; fortunately my control panel is close to the tanks so I do have a convenient place to put the power supplies.

The signal conditioner itself, which takes the mV signal and amplifies it to a 0-10vdc signal, uses a 12vdc power supply. Again, since the output from the summing board is a mV level signal, it needs to be close to the signal conditioner. In the field these devices might have been located underneath the thing being weighed, but my process tanks are only 12″ high.

In 2012 I described an application where I did this same thing for a batching system at Alcoa. In this case the hopper was MUCH bigger than the devices, and it was easy to locate a small enclosure underneath the hopper itself. Effectively, it was like building your own sensor and bringing the signal from the enclosure as a fully conditioned analog input.

Another option I would have had on this system was to use a pressure transducer at the bottom of the tank itself, similar to what I have described in some of my posts on ABD in Miami. The nice thing about doing this is that the transducer’s output would already be in a usable form such as 0-10vdc or 4-20mA. The problem is both space and the low volume (weight/pressure) of my tanks.

Anyway, some things to think about when planning your application: cost of the components, including 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 your control system, and cost of design/labor to put all of this stuff together. There is also the issue of how you are going to calibrate all of this when you are done putting it all together. Every time you pass through another conversion or device you introduce another variable into the system.

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One of the areas in my new training center is an electronics bench. complete with oscilloscope, signal generator, bread boards and lots of components. The idea is to provide a development/innovation area for those interested in learning how sensors work at the component level or wish to learn basic electronics. Until I write up a formal class on it I don’t expect it will get used much, but it will be available for those taking my other custom classes. Hit me up for more information!

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