Capacitive displacement sensors are non-contact by design. That is, they are able to precisely measure the position or displacement of an object without touching it. Because of this the object being measured will not be distorted or damaged and target motions will not be dampened. Additionally, they can measure high frequency motions because no part of the sensor needs to stay in contact with the object, making them ideal for vibration measurements or high-speed production line applications.
The range of a capacitance sensor is dictated by the diameter, or area, of the sensor. The larger the area, the larger the measurement range. Measurement range is typically specified starting when the probe is touching the target. At this point the output from the amplifier is zero volts. When the gap is increased to equal the full scale measurement range of the capacitive system the amplifier output is 10 volts (Vdc). In theory, the probe can operate anywhere between these two extremes, however, it is not recommended to operate below 10% of the gap. With this said, the ideal operating or standoff distance is somewhere between 5Vdc to 7Vdc which will allow the target to move closer to or further away from the probe without going out of range. Figure 3 (below) is a simplified diagram showing range, output voltage and recommended standoff for a typical capacitance sensor.
All of MTII’s Accumeasure measurement sensors have a built-in dc voltage offset. Once engaged, the output voltage can be changed by as much as 10 volts by simply adjusting a potentiometer. This is ideal if your data acquisition or monitoring system requires a -5Vdc to +5Vdc input or if it is desired to take relative measurements from some fixed voltage point.
Capacitance probe operating range
The resolution of a displacement sensor is defined as the smallest amount of distance change that can be reliably measured by a specific system. Capacitance sensors offer extremely high resolution and stability often exceeding that of expensive and complex laser interferometer systems. Because of their ability to detect such small motions, they have been successfully used in many demanding measurement applications including computer disk drive runout, microscope focusing and nano-positioning within highly complex photolithography tools.
The primary factor in determining resolution is the system’s electrical noise. If the distance between the sensor and target is constant, the voltage output will still fluctuate slightly due to the “white” noise of the system. It is assumed that, without external signal processing, one cannot detect a shift in the voltage output of less than the random noise of the instrument. Because of this most resolution values are presented based on the peak-to-peak value of noise and can be represented by the following formula: Resolution = Sensitivity X Noise
Sensitivity is simply the measurement range divided by the voltage output swing of the capacitance amplifier. From the formula, you can see that for a fixed sensitivity the resolution is solely dependent upon the noise of the system. The lower the noise, the better the resolution!
It is important to note that some manufacturers specify resolution based on peak or rms noise, resulting in claims that are 2x and 6x respectively better than peak-to-peak. Although an acceptable method, it is somewhat misleading as most users do not have the ability to decipher voltages changes less than the peak-to-peak noise value.
The amount of noise depends on the system bandwidth. This is because noise is generally randomly distributed over a wide range of frequencies and limiting the bandwidth with filtering will remove some of the unwanted higher frequency fluctuations. All of MTII’s Accumeasure capacitance systems have plug-in low pass filters that allow for easy adjustment in the field.
Amplifier output noise with 20kHz low pass filter
Amplifier output noise with 100Hz low pass filter
The bandwidth, or cutoff frequency, of a system is typically defined as the point where the output is dampened by -3dB. This is approximately equal to an output voltage drop of 30% of the actual value. In other words, if a target is vibrating with an amplitude of 1mm at 5kHz and the bandwidth of the capacitance sensor is 5 kHz the actual sensor output would be 1mm X 70% = 0.70mm. Bandwidth curves are provided with all of MTII’s systems and can be used to correct for this dampening in high frequency applications. MTII’s applications engineers typically select a filter that has a cutoff frequency higher than the application requires to prevent any attenuation of the output. Additionally, this higher setting provides less phase shift in the capacitance sensor output allowing them to be ideally used in closed-loop feedback systems.
Push or Range Extension
Typical capacitive amplifier systems operate over a specific capacitance range, limiting their ability to measure large motions or operate at comfortable standoff distances. To overcome this problem MTII created a proprietary circuit that, with minor component modifications, can be adjusted to change the range and meet a wider variety of customer requirements. For example, a small diameter probe with a 1/2mm measurement range can be “pushed” to have a measurement range of 1mm or even 2mm. This allows MTII’s capacitive probes to be used in applications where space is limited or the target being measured is small. It is important to note, however, that a pushed probe should have a guard width sufficient enough to maintain the performance required, as mentioned above. Additionally, pushing a sensor also amplifies system noise, reducing probe resolution. Noise increase is proportional to push; 2 X Push = 2 X Noise.
The field established between a capacitance probe tip and measured object is typically larger than the diameter of the probe tip. This is because there is an epoxy gap between the tip and guard elements. The field diameter is equal to D plus 1x the epoxy gap. When taking measurements, capacitance probes provide a distance equal to the average surface location within the spot area. They are not capable of accurately detecting the position of features smaller than the size of the spot, however, they can repeatably measure to rough surfaces. Because of this, the probe tip should always be 25% smaller than the smallest feature you are trying to measure. Smaller sensors can distinguish smaller features on an object.
Effective spot size of a capacitive sensor
All MTII Accumeasure capacitance sensors typically have an output of 0-10Vdc over the full scale measurement range (FSR). In an ideal world this output would be perfectly linear and not deviate from a straight line at any point. However, in reality there will be slight deviations from this line which defines the system linearity. Typically, linearity is specified as a percentage of the Full Scale Measurement range. During calibration the output from the amplifier is compared to the output of a highly precise standard and differences are noted. MTII’s Accumeasure capacitance probes offer the highest linearity available today. Most systems exceed +/-0.05% FSR with some achieving +/-0.01% or better.
Accuracy is a function of linearity, resolution, temperature stability and drift, with linearity being the majority contributor. Fortunately, the linear response of MTII’s capacitive sensors is very repeatable. Calibration reports provide data that can be used to correct for the non-linearity of a system with inexpensive computers and correction software.
Stability is a function of a variety of different internal and external factors. For short term or relative measurement applications stability is typically not an issue. However, if high accuracy is required over a long period of time care must be taken when designing fixtures, selecting components and specifying materials of construction.
Temperature is typically the biggest factor that affects stability. Temperature swings can not only cause electronic drift but it can also cause fixture and probe expansion and contraction. For critical applications MTII uses high quality capacitors, resistors and inductors specifically designed for stability to minimize the electronic affect. To minimize the mechanical affect, probes can be manufactured from special low thermal coefficient materials such as Invar and MTII’s Application Engineers can provide fixture design assistance. Thermal correction coefficients can also be provided and used for real-time compensation.
Active capacitance probe systems should never be used in high stability applications because any localized temperature change surrounding the sensor will result in drift.
For low-end proximity sensors calibration is typically not important because linearity of the senor is not critical. Most high performance are, by design, inherently linear to approximately +/-0.2% of the full scale measurement range. Some capacitance manufacturers offer sensors with this performance, however, they are typically not suitable for high precision applications. Improved performance can be obtained by adding adjustable break point linearization within the amplifier circuit.
During calibration the output of the amplifier verses position of a target is recorded. A best fit straight line is generated based on this data. Each recorded point is then compared to the generated straight line and the percent deviation is calculated and plotted. Based upon the results adjustments can be made to improve the deviation to within acceptable limits.