GB1567419A - Sensors for heliostats - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO SENSORS FOR HELIOSTATS (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company of 190 Strand, London W.C.2. England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a sensor for determining the angular relation between a surface and a beam of electromagnetic energy.

Such a device which is a sensitive indicator of the alignment of a surface to a beam of energy is useful in optical equipment and test benches where accuracy is critical. Another application of such a device is in association with a solar cell array, when its indications as to the angular relation of the beam and the surface may be used to keep the solar cell or cell array properly aligned with the beam.

According to the invention, therefore, there is provided a sensor for determining the angular reaction between a surface and a beam of electromagnetic energy incident thereon, which includes an array of energy-responsive cells located on said plane surface, and a shadow-producing object so located with respect to said cells that when the beam is substantially normal to the surface the beam illuminates said cells substantially equally so that in that condition the outputs of the cells are substantially equal, in which when the beam ceases to be normal to the surface the object casts one or more shadows which partly or wholly obscure one or more of the cells so that the cells output alters, the difference the outputs has produced being dependent on the extent to which the beam is off normal.

According to the invention there is also provided a sensor for controlling the alignment of the plane surface to a beam of sunlight in which the beam is intended to be substantially normal to said plane surface, in which an array of solar cells is so arranged with respect to a shadow-casting object that when the beam is substantially normal to the plane surface all of the solar cells are substantially equally illuminated, so that their outputs are also substantially equal, in which when, due to movement of the sun, the beam causes the solar cells to be unequally illuminated, with the result that their outputs are unequal, and in which a servo system is provided to which the output of said cells are applied, said servo system responding to said outputs to alter the alignment of the plane surface until the beam of sunlight is once again substantially normal to that plane surface.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figs. 1 and 2 show schematically two arrangements embodying the invention, Fig. 3 shows the characteristic of a typical solar cell, Fig. 4 is a diagram explanatory of the operation of the device of Fig. 1, Fig. 5 is an electrical circuit useable with a device as shown in Fig. 1 or Fig. 2.

Fig. 6 shows schematically a further embodiment of the invention Fig. 7 shows the application of a device such as that of Fig. 1, 2 or 3, in conjunction with a solar cell arrangement.

Since the main applications contemplated for the devices embodying the present invention is in association with solar energy devices, the following description assumes that the beam of electro-magnetic energy is sunlight.

However, as.already indicated this invention is not so limited.

In its essence, as applied to sunlight, we arrange an array of solar cells about a shadow-casting object in such a way that when the beam is normal to the surface on which the cells are arranged, the outputs of the cells are, at least within manufacturing tolerances, equal. Then as the incident beam's alignment alters as the sun moves, the shadow cast by the object obscures one or more cells, the degree to which the cells are obscured varying. Thus the relation between the cells' outputs indicates the difference between the alignment of the surface with respect to the beam and what it should be. This difference is used to generate an electrical error signal, which is used to re-align the surface. When used in association with a power-generating solar cell or solar cell array, the latter is re-aligned at the same time as the sensor is re-aligned.

In Fig. 1, we have a plane surface 1 with a centrally-located pillar 2 of square cross-section about which there are disposed four long narrow solar cells 3, 4, 5, 6.

Each of these cells has electrodes on its two surfaces between which electrical potentials are developed when sunlight is incident on them. These solar cells are so chosen as to be (within manufacturing tolerances) electrically identical. Hence when a beam of sunlight is incident on the device and is normal to the plane surface 1, all of the cells produce the same output.

If the beam ceases to be normal to the plane surface, as will inevitably occur due to the sun's diurnal progression across the sky, the pillar 2 casts a shadow which obscures one or more of the solar cells 3, 4, 5, 6. Hence the outputs of cells differ, and these differences can be applied to a servo system which re-aligns the plane surface so that it returns to a condition in which the incident beam is substantially normal to the plane surface 1.

The device of Fig. 2 is in principle the same in operation as that of Fig. 1, but here the solar cells are on the floor of a shallow box whose walls together form a shadow-casting object similar operationally to the pillar 2, Fig. 1.

Fig. 4 is explanatory of the operation of the device shown in Fig. 1, and shows how the system works for two diametrically opposed cells, e.g. 4 and 6 in Fig. 1. In Fig. 4, (a) is a side view, (b) a plan view, and (c) is a side view in which the beam is no longer normal to the surface on which the cells are mounted.

As shown in Fig. 4(a), when the beam, which is collimated, is normal to the surface, none of the cells is shadowed. However, as can be seen from Fig. 4(c), when the beam is tilted, e.g. due to the sun's movement in the polar application, one or more of the cells is shadowed by the pillar, the extent of the shadowing depending, obviously, on the angle of tilt. Thus in Fig. 4(c) the left-hand cell gives more output than the right-hand cell, and the difference between their outputs can be used to produce a difference signal. In the application currently contemplated this difference signal is used, via a servo-system, to return the surface to the alignment shown in Fig. 4(a).

Thus for solar cells, whose characteristics are as shown in Fig. 3, the output of cell 1 when tilted is given by S1=IOKab Cos 0+iso Krhb Sin 0 Amps, where

lo is the input viscosity of the collimated light beam in Wm-2 K is the conversion efficiency of the solar cell in AW-' (assuming that is not affected by angle of incidence) 0 is the angle of tilt.

r is the reflection coefficient of the side of the obstacle.

a, b, h and x are as indicated in Fig. 4.

The first term represents the output current due to the absorption from the beam and the second term that due to reflection from the side of the obstacle.

The output from the cell 2, assuming the same conversion efficiency as for cell 1 is given by S2=I0Kb(a-h tan 0)A, for

Fig. 5 is a suitable circuit for obtaining an output voltage V proportional to the difference S1S2, where each cell's output causes current flow in a resistor R.

These resistors are equal in value, and that value is low enough to keep the voltage across the solar cell in the region where output current is independent of voltage, see Fig. 3, which for silicon solar cells is an output voltage less than 0.1 volts. If the solar cells have slightly different output characteristics this can be allowed for by adjusting the values of the resistors until zero output signal is obtained when the device is aligned. The two terminals shown are connected to a difference detector, e.g. a long-tailed pair, whose output provides an error signal which is usable for realignment.

The value of the voltage V is given by: V=RS1-RS2 =RIOKb(a cos 0+rh sin 8-a cos o+h sin 8) =RO,Kbh(l+r)sin 0 i.e. the output voltage is proportional to the sine of the off-axis angle.

In the arrangements of Figs. 1 and 2, the two arrangements such as that of Fig.

5 are used to allow of re-alignment in both co-ordinates.

Thus either of the arrangements described above gives a simple device for indicating when a surface is aligned perpendicular to a uniform collimated light beam, when the signal output falls to zero, and can be used to indicate how far offaxis the surface is. This, of course, needs a knowledge of the input intensity and conversion efficiency, or a calibration operation.

The sensitivity of the device can be increased by maximising R, K, h and r within the limits indicated. The range over which 0 can be varied for a sinusoidal output voltage is limited to 04+ tan-' a/h if a reflectivity-surfaced obstacle is used, above which reflected light is not collected, and to SA tan-' a/h if r=0, above which no part of the cell 2, Fig. 4(c), is illuminated.

The output voltage may be linearised with respect to 0 by varying the width of the cell, which can be achieved by the use of an obscuring top electrode to the cell, or a cover slip having the same effect. If the width of the cell varies as

where x is the length of the shadow along the cells in Fig. 4c, the output signal V becomes proportional to the product of input light intensity and off-axis angle 0 up to the limit in which the shadow completely covers one cell.

Reverting to Fig. 1, movement in one axis in a collimated beam affects the output of the other axis for two possible reasons. One is that the tilting of a cell reduces its output by a factor dependent on the cosine of the tilt angle. Thus although the zero output/on-axis condition is not affected, sensitivity is, which may limit the usefulness of the device for measurement. The other reason is that when one axis is tilted, movement of the other axis may cause shadows to move off the cells, which depends on the geometry of the obstacle and the recess.

For the device of Fig. 6, the maximum angle of tilt 4 for a perpendicular axis which will not affect shadowing is given by (l-b) =tan1 2h where 1, b and h have the significations shown in Fig. 5.

Although we have described the devices as used with collimated beams, they will also work with a divergent beam, with modified response although the zero signaVon-axis condition still applies.

The basic requirement for the detectors-solar cells in the arrangements described above-is that response varies only with total input power received anywhere on its surface. Examples of devices with this response are solar cells, some thermopile detectors, solar-pneumatic converters, and some pyro-electric devices. Most such devices have upper limits on input power.

The overall size of the device depends on the application. Thus with silicon solar cells, and using the circuit of Fig. 5, the size of the cells can be reduced to the limit when the output impedance is too high to give a useful signal to associated circuitry. Thus to obtain a signal of 100 mV across 10 Kohm, the minimum area of individual solar cells is of the order of 2x l0-8m2 (e.g. 100 ,umx200 yam). Hence the cells can be mounted on transistor headers. An ultimate lower limit of size is reached when diffraction effects become significant.

Fig. 7 shows how a device such as described above can be used with a solar cell array. Here the sunlight is concentrated to fall on the power-generating solar cell 10, and also falls directly on a sensor 11, the light falling therein being almost parallel. The output from the sensor 11 is applied via a control box 12 to a motor 13, which re-aligns the sensor and the solar cell 10. This enables the power consumption of this motor to be reduced by operating the motor only when the sun is out, or by using a fast-running motor drive system so that the system moves by small increments within the limits of accuracy needed.

WHAT WE CLAIM IS: 1. A sensor for determining the angular relation between a plane surface and a beam of electro-magnetic energy incident thereon, which includes an array of energy-responsive cells located on said plane surface, and a shadow-producing object so located with respect to said cells that when the beam is substantially normal to the surface the beam illuminates said cells substantially equally so that in that condition the outputs of the cells are substantially equal, in which when the beam ceases to be normal to the surface the object casts one or more shadows which partly or wholly obscure one or more of the cells so the cells' output alters, the difference between the outputs thus produced being dependent on the extent to which the beam is off normal.

2. A sensor as claimed in claim 1, and in which said object is a pillar at the centre of a cruciform array of said cells.

3. A sensor as claimed in claim 1 and in which the cells consist of a number of cells in cruciform array surrounded by walls which together form the shadowproducing object.

4. A sensor as claimed in claim 1, 2, or 3, and in which the cells are solar cells, the beam being a beam of sunlight.

5. A sensor for controlling the alignment of a plane surface to a beam of sunlight in which the beam is intended to be substantially normal to said plane surface, in which an array of solar cells is so arranged with respect to a shadowcasting object that when the beam is substantially normal to the plane surface all of the solar cells are substantially equally illuminated, so that their outputs are also substantially equal, in which when, due to movement of the sun, the beam ceases to be normal to said plane the shadowing effect of said object causes the solar cells to be unequally illuminated, with the result that their outputs are unequal, affd in which a servo system is provided to which the outputs of said cells are applied, said servo system responding to said outputs to alter the alignment of the plane surface until the beam of sunlight is once again substantially normal to that plane surface.

6. A sensor as claimed in claim 5, in which said solar cells are four in number and are arranged in a cruciform array, and in which the shadow-forming object is a pillar at the centre of the array.

7. A sensor as claimed in claim 5, in which said solar cells are four in number and are arranged in a cruciform array, and in which the shadow-forming object is a set (,f walls surrounding the cruciform array.

8. The combination of an energy-generating solar cell or solar cell array with a sensor as claimed in claim 5, 6, or 7, and in which said sensor and said cell are movable together, so that the response of the sensor maintains the solar cell or solar cell array aligned to the sun.

9. A sensor for determining the angular relation between a surface and a beam of electro-magnetic energy, substantially as described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. solar cells, and using the circuit of Fig. 5, the size of the cells can be reduced to the limit when the output impedance is too high to give a useful signal to associated circuitry. Thus to obtain a signal of 100 mV across 10 Kohm, the minimum area of individual solar cells is of the order of 2x l0-8m2 (e.g. 100 ,umx200 yam). Hence the cells can be mounted on transistor headers. An ultimate lower limit of size is reached when diffraction effects become significant. Fig. 7 shows how a device such as described above can be used with a solar cell array. Here the sunlight is concentrated to fall on the power-generating solar cell 10, and also falls directly on a sensor 11, the light falling therein being almost parallel. The output from the sensor 11 is applied via a control box 12 to a motor 13, which re-aligns the sensor and the solar cell 10. This enables the power consumption of this motor to be reduced by operating the motor only when the sun is out, or by using a fast-running motor drive system so that the system moves by small increments within the limits of accuracy needed. WHAT WE CLAIM IS:

1. A sensor for determining the angular relation between a plane surface and a beam of electro-magnetic energy incident thereon, which includes an array of energy-responsive cells located on said plane surface, and a shadow-producing object so located with respect to said cells that when the beam is substantially normal to the surface the beam illuminates said cells substantially equally so that in that condition the outputs of the cells are substantially equal, in which when the beam ceases to be normal to the surface the object casts one or more shadows which partly or wholly obscure one or more of the cells so the cells' output alters, the difference between the outputs thus produced being dependent on the extent to which the beam is off normal.

2. A sensor as claimed in claim 1, and in which said object is a pillar at the centre of a cruciform array of said cells.

3. A sensor as claimed in claim 1 and in which the cells consist of a number of cells in cruciform array surrounded by walls which together form the shadowproducing object.

4. A sensor as claimed in claim 1, 2, or 3, and in which the cells are solar cells, the beam being a beam of sunlight.

5. A sensor for controlling the alignment of a plane surface to a beam of sunlight in which the beam is intended to be substantially normal to said plane surface, in which an array of solar cells is so arranged with respect to a shadowcasting object that when the beam is substantially normal to the plane surface all of the solar cells are substantially equally illuminated, so that their outputs are also substantially equal, in which when, due to movement of the sun, the beam ceases to be normal to said plane the shadowing effect of said object causes the solar cells to be unequally illuminated, with the result that their outputs are unequal, affd in which a servo system is provided to which the outputs of said cells are applied, said servo system responding to said outputs to alter the alignment of the plane surface until the beam of sunlight is once again substantially normal to that plane surface.

6. A sensor as claimed in claim 5, in which said solar cells are four in number and are arranged in a cruciform array, and in which the shadow-forming object is a pillar at the centre of the array.

7. A sensor as claimed in claim 5, in which said solar cells are four in number and are arranged in a cruciform array, and in which the shadow-forming object is a set (,f walls surrounding the cruciform array.

8. The combination of an energy-generating solar cell or solar cell array with a sensor as claimed in claim 5, 6, or 7, and in which said sensor and said cell are movable together, so that the response of the sensor maintains the solar cell or solar cell array aligned to the sun.

9. A sensor for determining the angular relation between a surface and a beam of electro-magnetic energy, substantially as described with reference to the accompanying drawing.