BACKGROUND TO THE INVENTION During the building of a road it is necessary for the surface to be graded to the correct level. This work is carried out by a grader which is an earth moving machine with an angled scraper blade underneath it.

The surveyor plants posts on both sides of the line of the intended road.

The posts are spaced at intervals along the road. The spacing between posts can be, for example, twenty metres. On each post the surveyor puts a clearly visible surveyor's mark.

A team of three work with the grader driver. The team has a line which is stretched across the road from one pole to another and held on the marks on the poles.

The third member of the team uses a so-called staff which itself has a level mark on it.

The staff is placed against the line with its lower end on the surface to be graded. The position of the level mark on the staff with respect to the line indicates how far the existing level is off the required level. This information is signaled to the grader driver by the third worker who uses his thumb and forefinger to indicate to the grader driver what the difference in levels is, and whether the existing level is too high or too low.

The grader driver then knows to add material to, or scrape material from, the road surface to achieve the correct level.

The present invention seeks to provide a more accurate means of establishing levels during construction.

BRIEF DESCRIPTION OF THE INVENTION According to one aspect of the present invention there is provided a device for use in establishing levels during construction, the device being in the form of a staff having a plurality of sensors located in an array which extends along the length of the staff, the sensors being such that when the staff is placed against a level line, the sensors detect the position of the line and provide a level indicating signal.

The sensors can be normally open mechanical switches with spring means for holding them open, the pressure exerted by a level line on a switch means closing the switch means temporarily against the action of the spring means, In an alternative form each sensor is constituted by a pair of electrical contacts for use with a metallic level line which in use bridges a pair of contacts when the staff is placed against the line.

In a further embodiment the sensors are coils and the device includes means for supplying current to the coils so that the coils oscillate, the coils detecting the proximity thereto of a metal element by being blocked so that oscillation is inhibited.

The present invention also provides, in combination, a device as defined above and a receiving unit for positioning in the cab of the grader, the staff including means for transmitting a signal to said unit which displays in the cab an indication of the difference between required level and existing level as determined by the staff and the level line. In a specific form each sensor is constituted by a coil, there being means for supplying current to the coils so that the coils oscillate, means for detecting the change in the characteristics of a coil's oscillation consequent upon a metal element being brought into its proximity and means for transmitting a signal indicative of the coil whose oscillation characteristics have changed.

The present invention also provides, in combination, a device as defined in the preceding paragraph and a receiving unit for positioning in the cab of a grader, the receiving unit including means for receiving an incoming signal from said device, means for processing said incoming signal and generating an output signal for illuminating that one of an array of level difference indicating lights which corresponds to the coil whose oscillation characteristics have changed.

According to a further aspect of the present invention there is provided a method of establishing levels during construction work which comprises extending a line across a road being constructed from a surveyor's mark on a surveyor's pole on one side of the road to a surveyor's mark on a surveyor's pole on the other side of the road, placing a staff including a vertically extending array of sensors and a transmitter against the line so that the sensors detect the position of the line and provide a level indicating signal, and transmitting the level indicating signal from said transmitter of the staff to a receiving unit in the cab of a grader so that the receiving unit displays to the grader driver an indication of the difference between required level and existing level, BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:- Figure 1 is a front elevation of a pole forming part of a staff for use in establishing levels during construction work; Figure 2 is a rear elevation of the pole ; Figure 3 is a section, to a larger scale, on the line 111-Ill of Figure 1; Figure 4 is an elevation of a casing which forms the second component of the staff and which contains the means for detecting the level of a line ; Figure 5 is an elevation of the casing of Figure 4 taken from the other side; Figure 6 is a top plan view of the casing of Figures 4 and 5 and also shows the pole of Figures 1 to 3; Figure 7 is a view taken in the direction of arrow A in Figure 6 and shows the casing mounted on the pole to form a staff; Figure 8 is a pictorial view showing the front face of a signal receiving unit; Figure 9 is a pictorial view showing the rear face of the signal receiving unit; Figure 10 is a block diagram of the electronic circuitry within the casing of Figures 4 to 6 which detects line level and transmits a signal indicative of line level ; Figure 11 illustrates the layout of a plurality of sensing coils ; Figure 12 is a block diagram of the electronic circuitry of the signal receiving unit; and Figure 13 diagrammatically illustrates the establishment of a level.

DETAILED DESCRIPTION OF THE DRAWINGS Referring firstly to Figure 1 to 3, the pole 10 illustrated is in the form of an aluminium extrusion. The extrusion has a web 12, two flanges 14 which are at right angles to the web 12 and two lips 16 which are at right angles to the flanges 14. The lips 16 define an entrance opening 18 which runs the full length of the pole, An elongate slit 20 is cut in the web 12 and extends over the upper part of the length of the extrusion. The ends of the extrusion are closed by caps 22 of synthetic plastics material which are push fits in the ends of the extrusion.

The casing 24 (Figures 4 to 6) is of strong synthetic plastics material and comprises a base 26 and a two part cover 28 which is secured by screws 30 to the base 26. The split line between the cover 28 and the base is at 32 (Figure 6) and the split line between the two parts 28.1,28.2 of the cover is at 34 (Figure 5). The edge of the cover part 28.1 overlaps the edge of the cover part 28.2. The electronics are concealed by the cover part 28.1 and the cover part 28.2 closes off a battery compartment. The cover part 28.1 is machined so as to provide two recesses 36.1 running along the length thereof and a pair of parallel slots 36.2 (Figure 6) leading-off the recesses 36.1.

Two bosses 38 are secured to the cover part 28.1, each boss having a tapped bore 40 in it. Two knobs 42 each with a threaded shaft are screwed into the bores 40.

An on-off button 44 protrudes from one side wall of the casing.

The casing 24 is fitted to the pole 10 (see Figure 6) by removing one of the end caps 22 and sliding the lips 16 into the slits 36.2. The bosses 38 enter the channel and the casing 24 is slid along until the bores 40 are aligned with the slit 20.

This is shown in Figures 6 and 7. The shafts of the knobs 42 are then inserted through the slit 20 into the tapped bores 40 and tightened thereby to secure the casing to the pole at the desired position and form a staff.

The signal receiving unit 46 of Figures 9 and 10 comprises a vertically elongate casing 48 which has, on its front face, an array of LEDs designated L1, L2 and L3. The LEDs are of different colours. For example LED L1 can be white, LEDs L2 can be red and LEDs L3 can be green.

On the rear of the casing there is an array of bright LEDs L4. A number of LEDs L4 are provided as the fact that they are"on"must be visible from up to twenty metres away. A single light in, for example, bright sunlight is not necessarily visible.

A radio signal receiving aerial 50 in the form of a wire protrudes from the casing 48. A power lead is shown at 52.

Turning now to Figure 10, this illustrates the circuitry contained within the casing 24. An array of coils 56 (see also Figure 11) is provided on a pc board, the coils 56 extending in two vertical arrays when the casing is upright. Each coil has two windings and the arrangement is such that, when power is supplied, the two windings go into natural oscillation. The coils are powered up in succession. The normal oscillation occurring in the coils is inhibited when a metal object comes into the field of the coils. The fact that a coil 56 has stopped oscillating is detected by a microprocessor 58 whilst that core is in the powered up condition. The microprocessor 58 includes a memory for storing data generated in the processor of the microprocessor.

The coils 56 are arranged in two columns. The coils in one column are offset vertically with respect to the coils in the other column.

A transmitter 60 is connected to the processor 58. The transmitter 60 can be, for example, a radio frequency transmitter or an infrared transmitter.

The buzzer shown at 62 is activated upon the microprocessor 58 receiving, and causing the transmitter 60 to transmit, a signal indicating that the field of one of the coils 56 has detected the level line. Once the operator hears this signal, he knows that he can move on to take the next reading. The buzzer can also be activated in the event that the battery power source designated 64 is low. A power management device is shown at 66 in Figure 10. The on-off button 44 is also shown in Figure 10.

The device 66 includes a timer which shuts off power to the microprocessor 58 in the event that no level indicating signal is generated and transmitted within a predetermined period of time. The device 66 also disconnects the power source 64 from the microprocessor 58 if it detects that the battery level is low. If desired a low battery status can be used to generate a signal which is transmitted to the receiving unit 46 to indicate to the grader driver that the staff being used needs its batteries replaced or recharged.

Within the casing 48 of the receiving unit 46 there is a microprocessor which is designated 68. A receiver 70 compatible with the transmitter 60 is connected to the microprocessor and to a power supply unit 72. Signals received by the receiver 70 are fed to the microprocessor along line 74.

A driver for the LEDs L1, L2 and L3 is shown as a block 76 and is connected to the power supply unit 62 by a power line 78 and by a data transmission line 80 to the microprocessor 68. A driver for the lights L4 is shown as a block 82, the block 82 being connected to the block 76 by a line 84.

The receiver can be supplied with power from the grader's electrical system via a power supply regulator 86 which is connected by lines 88 and 90 to the microprocessor 68 and to the unit 72. The regulator 86 provides power suitable for use by the microprocessor 68 and the unit 72 provides power at a level sufficient to illuminate LEDs.

If reference is now made to Figure 13, this shows two surveyor's poles P.

Each pole P has a surveyor's mark M thereon. Reference letter L indicates a line which is stretched by two site workers from the mark M of the one pole P to the mark M of the other pole P. A metal sleeve S is threaded onto the line L. The line L is of a material such as Nylon or gut which can be pulled taut without stretching. Part of the line L and the metal sleeve S are shown in Figure 4.

In use, the staff is placed against the line L so that the sleeve S extends across the casing as shown in Figure 13. The position of the sleeve is detected by one of the. coils 56. A signal is encoded by the microprocessor 58 which is specific to the coil which has stopped oscillating. This signal is transmitted by the transmitter 60 to the receiving unit 46 as a signal indicative of a specific one of the coils 56. In the receiving unit 46 the signal is decoded by the microprocessor and used to illuminate the LED 1 or the one of the LEDs 2 or LEDs 3 of the block 76. The LED which is illuminated corresponds to the field coil 56 which has detected the sleeve S. The grader driver can immediately see whether the level of the road beneath the line is correct, too high or too low. He is also given an indication of the distance by which the level is incorrect.

The LEDs 4 of the block 82 illuminate at the same time as the appropriate LED1, 2 or 3 and this shows the person holding the staff that a signal has been received. In the absence of a further input signal from the receiver 70 the microprocessor 68 can maintain an LED L1, L2, L3"on"for a predetermined time before switching it off. This gives the grader driver time to see it even if he is not looking at the moment the LED is illuminated.

In the preferred form each coil is 20mm above or below the next coil in the same column. The coils in the adjacent column are also 20mm apart but, because of the staggered relationship between coils, are only 10mm above or below the coils in the next column.

If a single coil stops oscillating, the exact position of the line is determined. If the oscillations of two coils, one in each column, are inhibited, then the microprocessor 58 interprets this as meaning that the line is between say 10mm and 20mm and gives a reading of 15mm. It is also possible to cause the LED which pertains to a coil which as been completely blocked to be illuminated constantly and for the LED which pertains to a coil that has been partly blocked to flash. This gives the grader driver a more accurate indication of where the line is.

If the signal from one microprocessor/transmitter combination is distinguishable by the circuitry from a signal transmitted by another microprocessor/ transmitter combination, several staffs can be used on the same site without interfering with one another.

The use of coils within a casing has the advantage that there are no protruding parts which can be broken-off. However, the sensor can be normally open mechanical switches with spring means for holding them open, the pressure exerted by a level line on a switch means closing the switch means temporarily against the action of the spring means. In an alternative form each sensor is constituted by a pair of electrical contacts for use with a metallic level line which in use bridges a pair of contacts when the staff is placed against the line.