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Fylde Electronic Laboratories Ltd: Capacitance Methods for Displacement Measurement Applications.

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Fig 1.

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Introduction

The FE-419-CDT and the FE-420-CD enable the measurement of displacement by use of a capacitance technique. The method relies on a special configuration of a charge amplifier circuit to enable the measurement using concentric guarded probes.

These amplifiers are designed to operate together with the FE-411-OSC in a capacitance measurement system. In multi-channel systems, one oscillator will energise as many as 16 channels of amplifiers. (Fig. 1 shows a single channel system.)

Capacitance probes, specifically designed to connect via 100% guarded cable, enable the system to generate voltage signals proportional to the distance between the probe and an earthed conductive target.

Capacitance probes generally carry a guard ring enabling a linear relationship to be maintained between distance and voltage output of the system. Smaller probes can be realized by sacrificing guard area when a linear relationship of distance to output is not essential or when calibration of the system allows.

The amplifiers output a linear voltage against distance characteristic. In the case of the FE-419-CDT, a voltage against capacitance output may also be made available.

Fylde offer two types of amplifier ; each will operate with the FE-411-OSC. The technique for each amplifier is marginally different and each is suited to particular transducer ranges and cables.

FE-419-CDT

The FE-419-CDT is better suited to applications where the capacitance returned from the probe is likely to be greater than 0.2 pF. Larger transducers generate larger capacitances and so the FE-419-CDT will operate more optimally with probes greater than 2 mm diameter. Operation with 2 mm probes is still possible, but with this probe the noise in the measurement will usually be 10 dB worse than when using the FE-420-CDT. On the positive side, the FE-419-CDT is able to operate with cable lengths of up to 10 m even in installations employing triaxial cables. The technique is robust and adaptable and can be applied on probes from 2 mm up to many centimeters in diameter.

A ruggedised version of the FE-419-CDT exists ; the FE-486-OCAM is a 4 channel amplifier in an IP65 sealed enclosure for operation in harsh environments.

FE-420-CD

This amplifier is at its best with small transducers. A capacitance of 1 pF should be considered as the upper limit and this rules out 8 mm transducers at closer than 0.5 mm range. Noise will often be lower for a given bandwidth when using this amplifier and it should be selected for 2 mm transducers unless input cable length ( max 4 m) precludes it. The FE-420-CD is able to operate with special versions of the FE-411-OSC with a frequency of 32 kHz. ; the resulting measurement bandwidth is 12 kHz.

Capacitance Probes

These are available from Fylde in 3 sizes ; each is generally known by the size of its inner electrode. The diameter of the inner electrode limits the transducer's operating range. In general, the probes will give a linear output at a range equal to the diameter of the inner electrode :

  2 mm diameter probe Linear range 2 mm Absolute range 4 mm (typical)
  4 mm diameter probe  Linear range 4 mm  Absolute range 8 mm (typical)
  8 mm diameter probe  Linear range 8 mm  Absolute range 16 mm (typical)

Transducer, Cable and Connector

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4 mm Probe and microdot connector

The input cable must be 100% screened carbon filled low noise cable. The outer screen is not an earthed screen and is only connected to the transducer guard ring to maintain the total screening between amplifier and transducer. A microdot connector is used from the front panel of the amplifier. In certain circumstances the user may wish to design his own transducer arrangement; Fylde are able to help in the design of the customer's own transducers. Please contact the factory for advice.

Linearity

The linearity of the measurement depends on factors involving the transducer design, the operating range and the target geometry. In general, a linearity of ±1% of full scale is achievable under typical operating conditions; thus an amplifier and probe configured for a range of 2 mm full scale will be linear to ±20 µm at up to 2 mm distance.

Dynamic Considerations

One of the attractions of the capacitance technique is that it offers good bandwidth in the measurement. A typical bandwidth setting for the measurement is 1 kHz (rise time 300 µs) but bandwidths up to 12 kHz are possible (FE-420-CD only).

 

 

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