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Taking The Mystery Out Of Sensor Outputs

oscilloscope and multimeter stackedEver wondered about sensor outputs? Why are there so many to choose from? Why not just standardize on one output, and be done?

Let’s take a quick look through the most popular types of sensor outputs. Each type has strengths and weaknesses that give it advantages in certain situations. Matching these qualities with your situation will help you maximize your sensors’ performance.

4-20 mA Current Loop

How it works: A current regulator in the sensor turns supplied voltage into a current proportional to the physical measurement driving the regulator (pressure, distance, etc.). The current regulator in the sensor exempts the current loop from Ohm’s Law to a rather useful extent. As long as enough voltage is supplied to the sensor to power the regulator, varying voltage from the supply will not change the current output of the sensor. That is a neat trick.

Why it’s popular: Only two wires are needed between the sensor and the supply/receiver, and return measurements are unaffected by varying source voltage or voltage drop due to circuit length. Also, because the zero signal (4 mA) is non-zero, a signal of 0 mA indicates that the current loop is broken, rather than just a zero measurement. This makes debugging a 4-20 mA circuit easier.

Downsides: The only major downside to 4-20 mA for sensors is that each sensor requires its own cable run to the power supply/receiver. For larger scale control systems, each 4-20 mA loop only sends information one way, either to or from the instrument.

DC Voltage

DC Voltage output sensors come in many output ranges: 0 to 2, 5 or 10 VDC; 1 to 5, 6, or 10 VDC; even 0.5 to 4.5 VDC; and so on. But they all operate on the same principle: a voltage output that is proportional to the physical measurement.

How it works: The physical measurement puts strain or stress across piezoelectric gauges, which turn supplied voltage into a proportional output voltage (sounds kinda familiar…). As with 4-20 mA sensors, the output will operate independent of source fluctuations as long as the supply voltage is sufficient for sensor operation.

Why it’s popular: Since voltage is easier to measure—in fact, current is often measured using a proxy voltage—control systems and signal receivers are cheaper to build for voltage output sensors. VDC sensors generally use less power than current sensors. And, the number of ranges available allows for flexibility in matching sensor output to anticipated input range.

Downsides: Voltage drop and noise sensitivity are two major disadvantages for VDC sensors. Since the signal from the sensor is voltage, voltage drop across the length of the cable run from sensor to receiver directly affects the accuracy of the measurement. Thus, run length must be considered for VDC sensors. Also, since electromagnetic interference intrudes into systems as voltage disturbances, noise directly affects the accuracy of the measurement as well. And, compared to the two wires needed to run 4-20 mA loops, a VDC sensor needs four wires (two for supply voltage, and two for output voltage).

mV/V

Unlike 4-20 mA and VDC output, output signals from mV/V sensors are directly dependent on input signals.

How it works: As with the previous sensors, the physical measurement has a proportional effect on the output signal of a mV/V sensor. However, mV/V sensors do not have the signal isolating circuits of 4-20 mA and VDC sensors. Consequently, the output is affected by both physical measurement and source voltage.

Why it’s popular: mV/V technology is cheap. We’ve already seen that measuring voltage is cheaper than measuring current; add to that the additional cost savings of much simpler electronics in the sensor itself (no output isolation circuitry), and a mV/V sensor, with the same piezoelectric components as a 4-20 mA or VDC sensor, comes at a much lower cost.

Downsides: Unamplified signals have a short range and are susceptible to noise interference. We recommend no more than 30 ft. between mV/V sensors and control equipment.

Communications or Control Standard (Modbus, Fieldbus, Profinet, etc)

There are several (perhaps a plethora) of communications or control standards used in industrial settings. Some are simple enough that they can be implemented as homegrown systems with compatible components. Others require specific devices and controllers from specific manufacturers.

How they work: Each system varies. Basic standards are just that: standards. They guarantee that components built to the standard, by any manufacturer, will communicate with other components. If you can program a computer or other controller to communicate according to the standard, it will be compatible with any components you buy.

More complicated protocols go beyond simple structures for communication between controllers and components. In theory, the more complicated protocols allow for more flexibility and customization in the control system. However, this can also require components capable of interactions with the more complex protocol.

Why they are popular: Enhanced control. Each layer of complexity in a communication or control standard allows for more user control. These enhancements can be system visualizations, remote control, automated control, data collection and storage, even multiple system integration. Using a communication or control network also allows for daisy-chained sensors, rather than individual cable runs to each sensor.

Downsides: Complexity and cost. Whereas a multimeter might help you quickly and accurately diagnose a problem with a sensor on a 4-20 mA loop, a complex system may require a trained technician. And all the extra control requires additional equipment, which means more money. However, for large, complex plants, the added capital investment up front will probably pay dividends in coordinated, centralized control.

So which output is best for you and your sensors? There are a lot of variables to look at. Do you have existing control equipment that already uses one output type? Are you looking to replace several different sensor types with one common system? Are you adding a few sensors, or installing a lot of new sensors? Do you need centralized or remote control? Each situation and system is unique, so one-size-fits-all answers don’t make much intellectual or financial sense.

Still have questions? Our application technicians know the ins and outs of sensor outputs, so whether you’re replacing an old sensor or looking at a whole new system, we can help you find the best solution for your needs, even if it isn’t one that we can supply.


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top image credit: ben dalton via flickr cc cropped

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