TEMPERATURE-COMPENSATED SENSOR The present invention relates to a measuring device for accurate measurement of a physical parameter wherein the physical parameter is converted to an electrical signal and the magnitude of the electrical signal is relative to the magnitude of the parameter and wherein the device includes a means for compensating for signals generated by temperature changes. In particular, it relates to a force sensor or load cell for accurate measurement of a force having a means for compensating for sensor span changes induced by temperature changes and a method for production of the load cell.

Within the context of this specification the word"comprises"is taken to mean"includes, among other things". It is not intended to be construed as"consists of only".

A wide range of sensors are available to convert a physical parameter to an electrical signal. Parameters that can be measured in this way include pressure, displacement, torque and acceleration. An example of one of these sensors is a load cell.

A load cell produces an electrical output signal which changes in magnitude depending on the magnitude of a force or weight applied. Load cells can be used as sensors for weighing the contents of vessels, bins, hoppers or other similar applications. Since a load cell measures the resultant force vector, usually arranged to be in a single direction, eg vertical forces acting on a container, in order to achieve an accurate measurement it is important that all restraints, (ie forces other than a force in the direction to be measured) eg forces other than the weight of material being weighed, are kept to a minimum and are elastic and repeatable. Small elastic and repeatable restraints can be compensated by field calibration of an associated instrumentation package.

Typically, a load cell includes a spring element, which may be in one of a number of forms including a hollow or solid column, cantilever, diaphragm, or shear beam, to which

gauges are bonded to measure a strain generated. The spring element extension changes with respect to a force applied to the load cell and the resistance of a gauge is related to the extension. The gauges are typically connected in a 4-arm Wheatstone bridge configuration.

Compensation components are generally fitted in one or more arms of the bridge to compensate for zero output and zero drift with temperature. Additional compensation components are generally fitted in the excitation connections to the bridge circuit to compensate for output span changes with temperature.

Generally, a complete assembly is housed within a protective case and sealed to exclude the external environment, but capable of allowing deformation of the spring element to occur when a force is applied. In some cases restraining diaphragms minimize the effects of side loading.

The structure of a vessel, bin, hopper or platform to be weighed is a factor which must be carefully considered in the arrangement of load cells. The supporting structure must also to be considered since it must carry the full weight of the vessel and its contents via point loading of a load cell. In the light of this, care must be taken to ensure that the supporting structure presents a load acting in line with the axis of a load cell and that there is a minimum of side loading.

Furthermore, a number of conditions must be taken into account in order to achieve accurate measurements. Reasons for poor performance or unreliability of a load cell fall into three main categories: (a) problems presented by a non-axial load; (b) side forces which affect the load cell; and (c) free vertical movement of the load being impaired.

These could be caused by non-axial loading, side loading, shock, stay rods for holding a load in a horizontal plane, pipework attached to the load. In addition, environmental

considerations must be taken into account including high temperatures, temperature changes, moisture, wind, vibration, and electrical considerations.

The known sensors suffer from the problem that their conversion of a physical parameter to an electrical signal is inherently sensitive to temperature changes. The output span for a conventional load cell increases with increasing temperature because the predominant component that causes the span change is a change in Youngs modulus with respect to temperature. This reduces with rising temperature producing an increase in output span.

For example, the span of a load cell spring element changes with respect to temperature in addition to a force applied. This is capable of causing a significant measurement error particularly if the sensor is required to operate over a wide temperature range.

Until now, the problem has been addressed by compensating for errors generated by temperature induced span changes by use of nickel or Balco foil resistors in series with the input sensor excitation connections. However, it has now been found that this method suffers from the problem that it is limited in accuracy and provides a source of error due to the fact that resistance of the foil resistors changes non-linearly with respect to temperature. This problem is exacerbated in view of the fact that the extent of non- linearity of resistance with respect to temperature changes increases at temperatures above 100°C. Therefore, the greater the variation in temperature, the greater the potential error.

Furthermore, the known foil resistors suffer from the problem that they are not suitable for use at high temperatures above about 200°C. Indeed, the known sensors provide errors due to temperature induced span changes at temperatures lower than 200°C. In addition, at high temperatures the backing material of the known sensors deforms. This adversely affects the accuracy of the sensor.

In addition, the known sensors have been found to suffer from the problem that their performance is degraded when span adjustment resistors are added to the excitation input of a bridge circuit.

In addition, it has been found that the known sensors can only be compensated for temperature induced errors over a small temperature range of about 50°C. In the light of this, the known devices are not suitable for use where a greater variation in temperature could occur.

Furthermore, the degree of non-linearity of temperature induced errors is increased with high temperature. In the light of this, the known sensors are not suitable for use in conditions where there is a high temperature of more than about 150°C.

Furthermore, in view of the fact that until now it has been necessary to select particular spesialised sensors for particular uses ie-it has been necessary to match the compensation required to the intended use of a sensor. This has resulted in an increased cost.

Therefore, a need exists for a new device and method for facilitating elimination of errors in measurements caused by changes in temperature.

The present invention addresses the problems set out above.

Remarkably, it has now been found that it is possible to compensate for sensor changes induced by changes in temperature using a thermometer element in a temperature sensitive potential divider circuit at the output of the sensor. Surprisingly, this can be used to address the problems presented by changes in temperature.

Remarkably, an embodiment of the present invention can be used at temperatures of more than 200°C, preferably even more than 250°C, and a surprisingly high level of accuracy can be maintained. This is due to the fact that the components of an embodiment of the invention do not degrade at temperatures below 500°C.

Furthermore, in contrast to the known sensors, compensation for temperature induced span changes is not limited to changes of temperature within only a small range of a mere 50°C. Instead, compensation can now be achieved over a greater temperature range of at least 50°C, more preferably at least 100°C, even more preferably at least 200°C.

In addition, even at high temperatures of about 250°C an embodiment of the invention provides accurate linear compensation for temperature induced changes in span. The degree of compensation does not become increasingly non-linear with increased temperature. Clearly, this is critical for optimum performance.

In contrast to the known sensors an embodiment of the invention can be used in a wide variety of conditions. In the light of this, it is not necessary to select a particular load cell to have the required compensation properties for particular conditions. Therefore, an embodiment of the invention can be mass produced for a wide range of conditions and the cost can be reduced accordingly.

It will be apparent that the invention can be applied to any voltage output sensor that has an output that changes with respect to temperature in addition to the desired output changes caused by changes in a physical parameter being measured.

Consequently, in a first aspect the present invention provides a measuring device having a sensor which converts a physical parameter to an electrical signal for accurate measurement of a physical parameter and a means for compensating for sensor changes

induced by temperature changes wherein the measuring device comprises a temperature sensitive potential divider at the output of the sensor.

Remarkably, an embodiment of a measuring device according to the invention provides accurate linear compensation for changes in temperature over a temperature range of about-50°C to about 500°C.

Preferably, an embodiment of a measuring device according to the invention comprises a load cell or a force sensor. Preferably the device is suitable for measurment of a force applied to the sensor.

Preferably, an embodiment of a measuring device according to the present invention comprises a potential divider having a temperature coefficient which is the inverse of the signal temperature coefficient provided by a sensor span change induced by a change in temperature. This provides the advantage of compensating for any change in the span of the sensor due to a change of temperature. Therefore, only a change in force applied to the sensor is measured.

Preferably, an embodiment of a measuring device according to the present invention comprises a temperature sensitive potential divider which has at least one platinum resistance thermometer element. This provides the advantage that accurate linear compensation for temperature induced sensor span changes can be achieved.

In addition, this provides the advantage that very small sizes of platinum resistance thermometer elements are available allowing the invention to be applied to small load cells. This provides a relative reduction in weight and provides greater flexibility with regard to positioning of a load cell.

Preferably, an embodiment of a measuring device according to the invention comprises a body having a sensor which detects a physical parameter and a temperature sensitive potential divider with a low temperature coefficient resistor being positioned external to the body. This embodiment is suitable for use at temperatures up to at least about 250°C, more preferably at least about 500°C. This provides the advantage that the potential divider is separated from the body of the measuring device, which is subject to changes of temperature, and ensures that compensation for temperature induced span changes is accurate.

Preferably, an alternative embodiment of a measuring device according to the invention comprises a body having a sensor which detects a physical parameter and a temperature sensitive potential divider positioned inside the body. This embodiment is suitable for use at temperatures up to at least about 100°C, more preferably at least about 150°G. This provides the advantage that the potential divider is protected by the body of the measuring device, however it is subject to changes of temperature experienced by the body.

In a second aspect the invention provides a method for producing a measuring device according to a first aspect of the invention which comprises the steps of placing a temperature sensitive potential divider at the output of a sensor.

Additional features and advantages of the present invention are described in, and will be apparent from, the description of the presently preferred embodiments which are set out below with reference to the drawings in which: Figure 1 shows the internal electrical circuit of a typical load cell; Figure 2 shows the circuit arrangement for a single ended voltage output; and Figure 3 shows the circuit arrangement for a differential voltage output.

For the purposes of clarity and a concise description features are described herein as part of the same or separate embodiments, however it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described.

As seen in Figure 1, a measuring device in the form of a load cell comprises a pair of input terminals which provide sensor excitation connections.

Resistors are provided in series with the input terminals to provide output rationalisation.

Nickel or Balco foil resistors are additionally provided in series with the input terminals to provide temperature sensitivity compensation.

A four arm Wheatstone bridge is provided. Input from the input terminals is provided between first and second arms of the Wheatstone bridge and between third and fourth arms of the Wheatstone bridge. Each arm of the Wheatstone bridge comprises a strain gauge resistor and the first and second arms comprise a resistor for calibrating balance and a resistor for calibrating temperature compensation respectively.

First and second output terminals are provided connected to the Wheatstone bridge between the first and third arms of the Wheatstone bridge and between the second and fourth arms of the Wheatstone bridge respectively.

As shown in figure 2, an embodiment of a load cell according to the invention provides single ended voltage output. In addition to the typical features of a load cell, a platinum resistance thermometer element is provided in series with a first output terminal and a low temperature coefficient resistor is provided across first and second terminals in a potential divider circuit. The low temperature coefficient resistor is connected in parallel with the output terminals and it is connected adjacent the first terminal between the platinum resistance thermometer element and the first output terminal.

As shown in figure 3, an alternative embodiment of a load cell according to the invention provides a differential voltage output. In addition to the typical features of a load cell, a first platinum resistance thermometer element is provided in series with a first output

terminal and a second platinum resistance thermometer element is provided in series with a second output terminal. A low temperature coefficient resistor is provided across the first and second terminals in a potential divider circuit. The low temperature coefficient resistor is connected in parallel with the output terminals. It is connected adjacent the first terminal between the platinum resistance thermometer element and the first output terminal and adjacent the second terminal between the platinum resistance thermometer element and the second output terminal.

Alternative sensors according to the invention where the output span decreases with rising temperature rather than increasing the embodiments shown in Figures 2 and 3 are modified. The embodiment shown in Figure 2 is modified by interchanging the positions of the platinum resistance thermometer element and the low temperature coefficient resistor so that: RI is a low temperature coefficient resistor; and R2 is a platinum resistance thermometer element.

The embodiment shown in Figure 3 is modified by interchanging the positions of the platinum resistance thermometer elements and the low temperature coefficient resistor so that: R3 and R4 are low temperature coefficient resistors; and R5 is a platinum resistance thermometer element.

The invention will now be described in more detail with reference to the following examples. Each example shows an embodiment of the invention having a strain gauge bridge measuring circuit. The terminating resistors required for compensation are

calculated by the application of standard potential divider theory. The circuit shown in figure 3 applies to each example. In each case the output is given for the full range force applied to the sensor. The forces were applied using accurate weights and the sensor outputs were measured using an accurate dc ratio meter. Calibration of all the measuring equipment was traceable to National standards.

Example 1 A sensor having a force sensing element comprising a combination of a diaphragm and four bending beams with a 2000Q bridge circuit of eight strain gauges. The compensation circuit was wired as figure 3 with platinum resistance thermometer elements R3 and R5 both having nominal values of 500Q at 0°C.

Tests carried out on an uncompensated loadcell without resistor R4 up to 200°C gave the following results: Average span drift = 0.018% of output/°C Worst span drift error = 3.05% of output Room temperature output = 0. 25615mV/V Calculated value for the terminating resistor R4 = 19032Q Estimated compensated room temperature output = 1.93999mV/V Tests carried out with a terminating resistor of 19032Q up to 200°C gave the following results: Average span drift = 0.0024% of output/°C Worst span drift error = 0.41 % of output Room temperature output = 1.93968mV/V

Example 2 A sensor having a shear beam force sensing element with a 350Q bridge circuit of four strain gauges. The compensation circuit was wired as figure 3 with platinum resistance thermometer elements R3 and R5 both having nominal values of 100Q at 0°C.

Tests carried out on an uncompensated loadcell without resistor R4 up to 200°C gave the following results: Average span drift = 0.018% of output/°C Worst span drift error = 3.75% of output Room temperature output = 1.70689mV/V Calculated value for the terminating resistor R4-4012Q Tests carried out with a terminating resistor of 4012Q up to 240°C gave the following results: Average span drift = 0. 0016% of output/°C Worst span drift error = 0.8% of output Room temperature output = 1.49293mV/V Example 3 A sensor having a cylindrical force sensing element with a 700Q bridge circuit of eight straain gauges. The compensation circuit was wired as figure 3 with platinum resistance thermometer elements R3 and R5 both having nominal values of 1000 at 0°C.

Tests carried out on an uncompensated loadcell without resistor R4 up to 150°C gave the following results: Average span drift = 0.036% of output/°C Worst span drift error = 4.53% of output Room temperature output = 1.2732mV/V Calculated value for the terminating resistor R4 = 1820Q Tests carried out with a terminating resistor of 1820Q up to 150°C gave the following results: Average span drift = 0.006% of output/°C Worst span drift error = 0.68% of output Room temperature output = 0.83689mV/V It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications are covered by the appended claims.