What to Consider When Selecting A Capacitance Sensor

What are the important parameters that you need to consider when selecting a capacitance sensor?

The most important parameter is the range that needs to be measured. We typically start out by asking customers how far away they need to measure. Capacitance sensors are quite precise and measure extremely small distances of around a few millimeters. The next important parameter to consider would be what resolution is needed. Typically, we can provide 10,000:1 in terms of resolution and in some cases, better than that.

Other important parameters are whether the gap is clean and what the maximum sensor diameter that you can have in your application. A lot of people have restricted space, so the sensor needs to be small. We can then perform range extension. For example, MTI’s Digital Accumeasure has multiple range extension that allows smaller probes to sense further away than other past traditional amplifiers.

Capacitance-based measurement probes have long been employed as a means of non-contact measurement of electrically conductive materials. Why is this considered better?

Capacitance can be very accurate. By marrying a processor to a capacitance measurement, we can make the measurements very linear and accurate in terms of other sensing technologies.

The target does not have to be conductive as we have ways around that, but in general, we want a conductive target. With the digital capacitance sensors that we have now, the accuracy is getting to the point where physically we cannot improve it any more.  We are starting to run up against the laws of physics.

We have also addressed problems of stability. Early capacitance probes had drifty components in the amplifiers. By using digital means now, we have gotten stability below 200 parts per million, because the components are better at using digital techniques.

Capacitance can be a linear measurement or a non-linear measurement. Most people have highly non-linear measurements, and with the microprocessor, in there we can linearize it. We also have a technique called constant current, were using analog techniques we can obtain a lin

apacitive measuring is considered the most reliable and stable non-contact measuring techniques? How will this benefit applications?

The capacitance of air does not change much if the humidity, temperature or pressure changes. They are very small physical parameters that affect capacitance measurements, making measuring the capacitance of a gap highly stable when considering the basic capacitance equation.

Where applications need long-term stability and high resolution, and you do not want drifting, thus capacitance can be cheaper, better than a laser, and more stable than any current or other types of measurements that are affected by temperature.

Capacitance is not affected by temperature, humidity, and pressure. Why is this perfect for industrial applications?

Based on those parameters, we can operate in extremely high temperatures of up to around 1,200 degrees Celsius. As long as our actual parts don’t melt, we can make measurements there. We have also operated in high pressure and vacuum environments.

As an example, this is useful for measurements that need to be made in space programs where they are trying to simulate space. To obtain these conditions it requires cold, cryogenic temperatures of around four degrees Kelvin at almost absolute vac and around 10 to the minus 8 Torr. We can then build probes that work in those environments.

We can operate in what we call extremely difficult atmospheres, climates, temperatures, and hazardous conditions. We can also build probes with the current so low that they are intrinsically safe to be used in an explosive environment. So most industrial environments are tame co

What are the factors that you need to consider when embedding a capacitance sensor into an industrial application, and can you give us any examples or any ones that you know of?

The critical points are having a clean gap, range < 15 mm , frequency response < 5kHz and resolution greater than 1 /10,000 of range . Furthermore, you need to consider if the target is conductive or can be grounded. When embedding a probe, we are taking our capacitance technology and trying to adapt it to a measurement that somebody is trying to make, for example, pressure.

If we take a metallic diaphragm and place it in front of an embedded capacitance probe, we can measure the deflection very accurately, as the pressure pushes on that diaphragm in a sealed sensor. We can turn a physical parameter, such as pressure, into a very highly stable capacitance reading proportional to pressure.

What benefits can the capacitance probes bring to the industrial applications that other methods cannot?

We have made recent advances with the digital capacitance technology that others do not have. By using a microprocessor to sample the capacitance, we have increased stability. We have been able to make our probes smaller and this has enabled something called range extension with high accuracy. This allows you to take a probe designed to work up to one millimeter and get the sensor to  work up to 10 millimeters by reducing the current and using digital techniques and filtering.

A benefit of capacitance  is where we embedded into a wind turbine with an extremely high magnetic field (two tesla), to monitor the armature gap as the wind turbine rotates. Capacitance is immune to magnetic fields and we use our range extension technology  to be able to see that 10 millimeter gap while having good stability for a period of around 10 years. The digital techniques that we use with the capacitance amplifiers are embedded and do not need pulling out and recalibrating.

Another advantage is the absolute accuracy that we obtain using capacitance. When you calibrate your probe to measure a gap, it is not affected by things like the temperature of the target or the material of the target. This means we can look at either a stator iron core or at a rare earth magnet and measure that absolute distance without any kind of recalibration. Capacitance probes can be miniaturized through lithography techniques. We have printed capacitance probes using advanced 3D printing methods. The circuitry can also be miniaturized. We have even embedded the amplifier circuitry into the probe body.

The stable calibration requirements allow these things to be embedded in other people’s products. We can then measure things such as vibration or seismic activity. Companies who are trying to make these types of measurements and  are using some other sensing technology may want to consider capacitance due to the low cost and high accuracy of it.

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Post time: Jun-18-2019