DIT-5200 Overview

TECHNOLOGY OF DIFFERENTIAL BRIDGE SYSTEMS

DIFFERENTIAL MEASUREMENTS

In an eddy current differential system, the two coils in the inductive bridge are housed in two separate sensors. Rather than one active coil and one reference coil, both sensors contain active coils as in figure 2. These two sensors are usually placed on opposite sides of a target or opposite sides of a target pivot point, as in figure 1.

THEORY OF OPERATION

As the target moves closer to one sensor, it moves away from the other. Therefore as the impedance in one leg of the inductive bridge increases, the other decreases. This push-pull effect amplifies the linear output-per-displacement and eliminates the need for summation amplifiers that add noise and drift. As a result, differential systems provide greater resolution and thermal stability than single-ended systems.

[Differential target configurations]

Figure 1: Differential target configurations.

[Differential system block diagram]

Figure 2: Differential system block diagram.

CALCULATING EFFECTIVE RESOLUTION IN YOUR APPLICATION

Equivalent RMS input noise: A figure of merit used to quantify the noise contributed by a system component. It incorporates into a single value, several factors that influence a noise specification such as signal-to-noise ratio, noise floor, and system bandwidth. Given a measuring system's sensitivity/scale factor and the level of "white" noise in the system, equivalent RMS input noise can be expressed using actual measurement units.

Effective resolution: An application dependent value determined by multiplying the equivalent RMS input noise specification by the square root of the measurement bandwidth.

Example: A 15N sensor monitoring a reciprocating target moving ±10 mils (FR) filtered externally to a 15KHz bandwidth.

1. Calculate a value for equivalent RMS input noise. From the equivalent RMS input noise table, use the value of equivalent RMS input noise for a 15N sensor calibrated over a ±10 mil range. Multiply this by the full range of the calibration. Divide by 100. The noise value is a percent of full range.

(0.00007% X 0.020 inches) / 100 = 1.4 x 10^-8 inches or 0.014 µinches

2. Calculate effective resolution. From step 1, take the equivalent RMS input noise and multiply by the square root of the measurement bandwidth in Hz.

0.014 µinches per root 15000 = 1.714 µinches

3. Approximate peak-to-peak resolution. From step 2, take the effective resolution and multiply by 6.6.

1.714 µinches X 6.6 = 11.312 µinches

EQUIVALENT RMS INPUT NOISE

Range ± mils	Range ± mm	Sensor	% Full Range at Full Range	% Full Range at Null
10	0.25	15N	0.00007%	0.00002%
10	0.25	20N	0.00012%	0.00004%
20	0.51	15N	0.00004%	0.00002%
20	0.51	20N	0.00007%	0.00002%
35	0.89	15N	0.00004%	0.00002%
50	1.27	20N	0.00004%	0.00002%
75	1.91	20N	0.00002%	0.00002%