An Introduction to Capacitive Position Sensors

Oct 30 2014

Capacitance sensing has been around for more than five decades. This article enumerates the characteristics of capacitive sensing, the performance characteristics and fixturing of capacitive sensors.

Characteristics of Capacitive Sensors

Capacitive sensors are used for non-contact displacement measurements with high precision rapidly over a modest range.

The key features are:

Capacitive sensing is a non-contact measurement technique. Since the system does not contact with the part where there are no parts distortion, witness marks or probe wear. As there is no sensor cycle time, high volume measurements are possible.
The capacitive sensor has a very high resolution, matched only be the laser interferometer. Typical resolutions range from 0.1 nm (0.004 pin) to 50 nm (2 pin) Operating ranges are normally less than 3 mm (1/8 inch)
Capacitive sensors measure rapidly. They are suited to high volume sorting applications and high bandwidth measurements applications.

Principle of Operation

When two parallel conductive plates are brought close to each other and a charge is arranged on one of the plates, a capacitor is formed. The current is transmitted across the gap between the plates. The amount of current that flows across the gap is determined by the voltage, the area of the plates, the material that separates the plates and the distance between the plates.

Q is defined as : Q = C*V

Where:

Q = Current (amps) C = Capacitance (farads) V = Voltage

Capacitance (C) is given as:

C = K* E0 * A/D

Where

C = Capacitance (farads)

K = The Dielectric Constant of the material between the plates.

A capacitive sensor can be termed as a device that determines the displacement of two parallel plates one of which is a sensor and the other of which is the object being measured by the sensor.

Figure 1. Simple Capacitive Sensor (side view)

Figure 2 shows a typical sensor configuration and Figure 3, sensor side view.

Figure 2. A Typical Sensor Configuration

Figure 3. Sensor Side View - Guard Ring Protects Sensor

Types of Capacitive Sensors

There are two types of capacitive sensors:

Passive
Active

Passive systems have certain benefits. They provide higher flexibility in probe configuration, stability, and lower cost. Their disadvantages are cable length restrictions, lower bandwidth and lower drive frequency, which makes them unsuited for some applications.

Active systems are not as subject to cable length restrictions, They operate at higher frequencies and higher output bandwidths. They are suitable for applications which may involve stray electrical noise on the target such as spindle run out analysis. The disadvantages of active systems include higher costs and less configuration flexibility.

Sensor Size

The larger the sensor size, the resolution tends to be better. The choice of sensor size is based on the size of the target. Sensor size needs to be as large as is practically possible.

As a general rule a doubling of the sensor area will halve the RMS noise. Sensor Size also effects the range. The larger the sensor the larger the practical range. As a general rule for round sensor the diameter of the sensor should be no less than 4 times the total range.

Figure 4. Determining Sensor Size based upon Target Shape

Spatial Resolution

A main disadvantage of larger sensors is reduced spatial resolution. Capacitive sensors take an average reading of the surface under the sensor. Smaller sensors can differentiate smaller features on a component.

Figure 5. Sensor Size determines Spatial Resolution

Range and Standoff

The distance from the face of the probe to the center of the range is termed as standoff. This is an approximate distance. Range is the measuring range of the probe. Typical ranges for a capacitive measurement systems are from +/- 10 um (+/- 0.0004in) to +/- 1000 um (+/- 0.040 in). Ranges in excess of 25mm (1 in) are possible under special circumstances.

Figure 6. Range and Standoff

Resolution

Resolution is the smallest increment of displacement that can be measured at a particular bandwidth. Resolution is defined by MicroSense as an RMS value, since it is measured with a true RMS meter which provides an unambiguous result.

Bandwidth

A key advantage of non-contact capacitive measurement is its ability to measure very rapid motion. Applications such as servo control, spindle analysis, and vibration analysis often require the ability to measure very small very fast motions. Active systems such as the MicroSense 5810 and MicroSense 6810 are developed for highly challenging applications with filter bandwidths to 100kHz. Passive systems such as the MicroSense 4810 and MicroSense 8810 also operate well at bandwidths up to 20kHz. However the system resolution is proportional to the square root of the bandwidth.

Both Butterworth and Bessel filters are offered in MicroSense Active systems and Butterworth filters are offered in MicroSense Passive systems. Filters are chosen in software or by jumpers on the boards.

Linearity

Since the advent of inexpensive computers and analog/digital cards linearity has become less of an issue in the past 10 years. It is possible that a system has built-in non-linearity and this relationship is repeatable within resolution parameters and can be compensated with simple software corrections. MicroSense offers linearity compensation data with all its systems.

Stability

Sensor stability is a function of several factors. These can be classified into external and internal factors. Relative measurement stability is not an issue for several measurements such as spindle analysis, servo control or other short duration relative measurement stability. For measurements where accuracy is needed over long time periods careful selection of sensing systems and external factors are critical.

As a general rule, Passive systems such as the MicroSense 4810 and MicroSense 8810 have better long-term stability characteristics than Active Systems such as the MicroSense 5810 and MicroSense 6810. Typical probes are made from stainless steel and

have a thermal coefficient in the area of 200 ppm/degree C.

Target Characteristics

In the conventional capacitive sensing model, the sensor is driven and the target grounded. For good performance a conductive path from the target to the sensing electronics is required. The target may be capacitively grounded. Capacitances of 1000 pF or higher work well.

Environmental Considerations

The key environmental consideration for a capacitive sensor is that there is a uniform, nonconductive material between the sensor and the target. This material is air in most applications. However, capacitive sensors function very well in vacuum environments, including UHV. Probes need to be clean. Capacitive Sensors do not work well in on

line cutting and grinding environments where there is a steady stream of cutting fluids and grinding waste. Also capacitive sensors can be easily fixed.

About MicroSense

MicroSense, LLC, previously known as ADE Technologies, is comprised of three primary businesses – precision capacitive sensors, vibrating sample magnetometers and magneto-optical Kerr effect (MOKE)tools for state-of-the-art magnetic measurement and wafer measurement systems.

Until the company was sold in November, 2009, we were a subsidiary of KLA-Tencor Corporation, a leading global supplier of semiconductor wafer defect inspection and metrology tools. MicroSense serves a host of industries including semiconductor equipment, data storage, machine tool, solar, automotive and high brightness LED. MicroSense provides customer value and security, through extensive business experience, financial strength, world-wide support, and technical leadership.