The present invention relates to a method of monitoring relative angles and to monitoring apparatus. The invention is particularly, although not exclusively concerned with inclinometers and accelerometers.

It is frequently required to weigh road vehicles in order to provide an indication of the load. When those vehicles are on an incline, the readings received from load sensors will not be accurate. However, if an indication of the inclination of the vehicle can be made then the readings recorded by the load sensors can be extrapolated in order to give an accurate indication of the weight of the vehicle on the incline.

According to one aspect of the present invention, a method of measuring the relative angle between two light polarising filters comprises monitoring the proportion of light being transmitted through those filters.

Accordingly to another aspect of the present invention, a method of monitoring the relative angle between two light polarising filters, in which light falls on one side of the filters comprises monitoring the light received on the other side of the filters through one of the filters only, and adjusting the intensity of the light at the one side of the filters, if required, in order to maintain a datum, and monitoring the light which has passed through both filters on the other side of the filters in order to monitor the relative angle between the filters.

The intensity of the light on one side of the filters, if required, may be adjusted by adjusting the power supplied to a light source.

The light which is received on the one side of the filters through one of the filters only may be monitored by a sensor, and the output of the sensor may be compared to a pre-determined value.

The method may include damping the movement of one of the filters relative to the other, and that damping may be achieved magnetically.

The method may comprise arranging for the light passing through the filters to increase when the relative angle between the filters changes in one direction from a pre-determined position and arranging for the light passing through the filters to decrease when the relative angle between the filters changes in the other relative direction between the filters from the pre-determined position.

One of the filters may be pivotally mounted relative to the other and the method may comprise monitoring light passing through the pivotally mounted filter at a region beneath the centre of gravity of the pivotally mounted filter.

The method may comprise displaying or recording an indication of the relative angle between the filters.

The method may comprise monitoring the rate of change of the relative angle between the filters.

The method may comprise moving one of the filters relative to the other.

According to a further aspect of the present invention monitoring apparatus includes two light polarising filters, the relative angle of which is arranged to change, and first light monitoring means located on one side of the filters and arranged to receive light which has passed through one of the filters only, and means for adjusting the light passing through the filters, and means for making that adjustment proportional to the light received by the first light monitoring means, and second light monitoring means located on the one side and arranged to receive light which has passed through both filters.

The apparatus may include a light source arranged to direct light at the filters, and a means for adjusting the light passing through the filters may comprise means for varying the power supply to the light source.

Damping means may be provided arranged to damp movement of one of the filters relative to the other, and the damping means may comprise magnetic damping means.

One of the filters may be pivotally mounted relative to the other and the region of the filter through which light is arranged to pass to the second monitoring means may be located beneath the centre of gravity of that filter.

Angle change means may be provided which are arranged to provide an indication of the rate of change of the relative angle between the two filters.

The apparatus may comprise an accelerometer or inclinometer.

The present invention also includes monitoring apparatus as herein referred to when used in a method of monitoring a relative angle.

The present invention may be carried into practice in various ways, but several embodiments will now be described by way of example and with reference to the accompanying drawings, in which :-

Figure 1 is a schematic side sectional view of an inclinometer 10;

Figure 2 is a schematic front view of a twin window photo diode 12; and

Figure 3 is a schematic front view of an alternative form of a pendulum 14 for use with the inclinometer 10 of Figure 1.

The inclinometer 10 shown in Figure 1 includes a light source 16 arranged to project light through a first fixed light polarising filter 18 towards a second movable light polarising filter 20. Light passing through both filters is received by a lower window 22 of the diode 12, and light is also received by the upper window 24 of that diode which has only passed through the first filter 18, there being a space 26 just above the second filter 20 on the pendulum 14. The pendulum 14 is mounted on a horizontal pivot 28.

In use, the inclinometer is attached to an article such as a vehicle, the inclination of which is to be

measured, and the pendulum 14 hangs downwardly under gravity. The filters 18,20, light source 16, diode 12 and control circuit 30 are located within a casing 36 provided with seals 38 which engage with an end wall 38 on which the circuit 30 is mounted to seal the components with an enclosure. The angle of inclination of the body is determined by assessing the amount of light received by the lower window 22 in accordance with the following formula :

α COS'' Φ

where L = initial light source

L ₂ = light detected which has passed through both filters φ = relative angle of filters to each other.

The lines making up the polarising filters are arranged such that, if the body moves from being level to an inclination in one sense, then there is an increase in the light received at the lower window, whereas, if the body moves from the level to an inclination to the opposite sense, there is a decrease in the light received at that lower window. In this way, the sense of the inclination can be determined by the inclinometer.

In order to get an accurate reading, it is important to ensure that the level of light received at the photo diode 12 is always proportioned in the correct angle. In this respect, for a given angle of inclination, it may be that the signal acknowledged by the lower window of the diode would vary independence upon variations of temperature or environmental conditions or independence upon the light emitted from the source 16 such as may occur during aging of the source or as a result of

variations of the power being supplied to the source. Such variations are at least partially compensated for by use of the upper window 24 which receives light from the same source as the other window 22 and operates under the same environmental conditions.

A control circuit 30 is provided which monitors the amount of light which the upper window of the diode acknowledges as arriving at that location from the window 18. The circuit 30 compares the acknowledged value with a required datum value and, if there is any discrepancy in these values, a servo loop in the circuit 30 adjusts the power being supplied to the lamp to adjust the intensity of the lamp until the acknowledged value at the upper window corresponds with the datum value. When these values correspond, the reading at the lower window can be taken and the angle of inclination can be accurately determined with the acknowledged value of the light received at the lower window corresponding exactly with the angle of inclination. When operating under these conditions, the value of L may not necessarily be the true value and may correspond to a constant.

The monitoring and comparison of light received at the upper window may be performed regularly or periodically or immediately before a reading of inclination is taken or any combination thereof. The reading of inclination may be made continually or repeatedly or on demand. Readings of selected or repeated or extreme inclinations may be provided for.

In an alternative embodiment (not shown) , the first filter 18 may be mounted directly over one of the windows of the diode in order that the division of the upper and

lower windows which receive light from one or both filters can be accurately determined.

The pendulum 14 is generally of an arc shape having an opening therein which is partially occupied by the filter 20. The lower extent of the pendulum is located between two opposed magnets 32 which act as a damper in a Hall effect on the motion of the pendulum in order that changes in inclination can be read quickly without the pendulum swinging about unduly.

It will be appreciated that the apparatus can also be used to provide an indication of the rate of change of the relative angles of the filters whereby the apparatus operates as an accelerometer. The control circuit 30 can include means for periodically, repeatedly or continually monitoring the acceleration or deceleration of a vehicle, for instance, and may include means for recording extreme acceleration or deceleration. The use of magnetic damping enables friction free bearings (not shown) to be used for the suspension of the pendulum to enable the apparatus to be extremely accurate and fast.

In an alternative embodiment (not shown) the apparatus can be used to provide an indication of the relative angle between two points. To this end one of the filters may be caused to move or point relative to the other, such as by a mechanism, for instance a rack mechanism.

Figure 3 shows an alternative form of pendulum 14. The centre of gravity 34 of the pendulum is located just below the pivot 28 such that, when the device is subjected to acceleration and deceleration forces in a transverse

direction to the pivotal axis, undue rocking or swinging of the pendulum does not occur.

An inclinometer is an instrument used to detect changing disposition of a body, usually relative to a horizontal datum, but not always so.

To be of benefit the inclinometer should detect angular changes of large or small scale.

In the present invention there is provided a movable member and a fixed member, to which are attached polarised light filters.

By mounting the movable member onto a pendulum, or, attaching it to a body, (in order to .cause positive displacement) and mounting the fixed member onto another body, the angle of rotation of the two members is measure by monitoring the change in light transmitted through the filters and substituting the values in the formula L ₂ αCOS ² φL ₁ .

In order to facilitate readings when used with the mounting member as a pendulum it is sometimes desirable to use aluminium for the pendulum and damp out oscillations by suspending the pendulum between magnets.

In order to produce very accurate measurements of change of angle it is sometimes desirable to monitor the light source before and after transmission through the filters to eliminate variations in the light emitted from the source.

In order to transmit a signal to an indicator or other signal processing unit, and avoid corruption of the

signal, it is sometimes desirable to amplify or otherwise modify the signal to eliminate the effects of external influences such as transients or eddy currents. This can be carried out within the inclinometer.