Temperature Effects on LVDT Outputs Macro Sensors

Ambient Temperature Variations

Variation of ambient temperature, if great enough, may affect Linear Variable Differential Transformer (LVDT) characteristics. The output signal from the LVDT can be affected, in various degrees, by temperature variation in one or both of two ways:

1. The sensitivity, or the change in output voltage for the unit core displacement, may increase or decrease although the position of the core for zero or null input (the usual starting or reference position for measuring or calibrating LVDT output), may remain unchanged. This is equivalent to an error produced by multiplying the calibrated output signal value by a scale factor that is slightly larger or smaller than unity.

2. The actual core position range producing the calibrated output signal range may shift in one direction or the other by a distance that is the same when measured at any position within the range. This is equivalent to an error produced by adding or subtracting a small constant to or from the calibrated output signal value, an effect commonly referred to as zero shift error, or scale shift error. This kind of error does not affect sensitivity )volts out per unit core travel).

A rise in ambient temperature and resultant rise in LVDT temperature results in an increase in the resistance of the copper wire used in the primary and secondary coils. The most direct consequence of this resistance increase, particularly at the lower frequencies, is an in crease in primary impedance, with an associated reduction in primary current, which, in turn, affects output and sensitivity levels. This resistance change results in errors of type #1.

Resistance variations in the secondary circuit are not an important factor if a very high impedance load is used, but will factor in the transfer of power into a low impedance load. For example, neglecting the effects of temperature change upon primary impedance and it's resulting sensitivity variation, a change in secondary resistance of + 50% would result in a voltage output decrease of approximately 1%, if the load impedance to secondary resistance ratio were 50 to 1, and 20% if this ratio were 1 to 1.

Changes in temperature cause materials to expand or contract in accordance with the thermal coefficient of expansion for that material. When mounting and LVDT the use of materials having widely differing thermal coefficients of expansion, may cause errors of type#2.

Figure 1

For example, if the Core extension rod in figure 1 were of brass, which has a thermal coefficient of expansion 0.000019 and is 10 inches long, the mounting surface is made of steel, having a coefficient of .0000105 and Dm equals 12 inches. Using a 30 degree C change in temperature, the rod would increase 0.0057 inches in length and the LVDT would move 0.00378 inches, resulting in a null shift of 0.00192 inches.

Methods for Compensating for Change of Temperature

There are several reliable methods of compensating for changes in ambient temperature, which will be described.

1. LVDTs using manganin wire windings instead of copper. The resistance change with temperature variation is thus reduced almost to zero. This method is advantageous since no additional circuitry or space is required. Disadvantages are: the temperature variation must be quantified and a special LVDT ordered. Manganin wire windings have a higher resistance resulting in reduced sensitivity compared to an LVDT built in the same configuration wound with copper windings.

2. Stabilization of input current. Through the use of a constant current power source, primary current is held constant regardless of resistance changes; thus holding output and voltage sensitivity constant. When a constant current power source is not available, primary current may be stabilized by use of an external resistance in series with the primary. This series resistor should have an appropriate negative temperature coefficient.

3. Thermistor insertion. Thermistors may be inserted in series with the primary or in some cases, the secondary circuit, to compensate for the effect of temperature change on sensitivity. Since essentially this entails adding a resistance, which deceases with the rise in temperature, a thermistor must be chosen with correct negative temperature and coefficient values, dependant on the resistance and impedance of the LVDT being used.

4. Mechanical arrangement and choice of materials. The problem of scale shift errors may be reduced by careful selection of materials used for mounting the LVDT. The material used for the core extension rod