GB2133572A - A laser optical projection system for balances and fine weighing instruments - Google Patents

Abstract

A laser operated optical projection for the enhanced readability and precision of balances and similarly constructed weighing instruments where the image is read either on an associated direct reading scale or by an automatic light sensor. The system is capable of operation on weighing instruments of capacities normally between 25 kg and 20 g in laboratory conditions under natural or artificial light and does not create any thermal effect which acting on the beam of the weighing instrument would lead to unqualified error. Light generated from a helium-neon laser (a) is reflected via a collimator (b) and beam subdivider (c) onto a principal reflector (e) mounted on the beam of the weighing instrument (f) and then onto a remotely situated direct reading scale (g), or via a light sensing unit (not shown) where the result is displayed on a digital display unit. When used on such weighing equipment the ultimate precision when expressed in terms of parts per million (at 1 kg) can be expressed as 0.5 ppm (minimum) at the 95% confidence level. <IMAGE>

Description

SPECIFICATION A laser optical projection system for balances and fine weighing instruments This invention relates to an optical projection reading system in connection with balances for fine weighing and other scientific apparatus. It is well known that in balances for fine weighing an enlarged image of a small scale may be projected onto a screen and a vernier scale may be provided in order to subdivide the projected divisions, for example into ten divisions.

The lines of the projected scale are necessarily of finite width which can give rise to errors in reading the vernier, since the thickness of the main scale line is approximately the same as the vernier division. It is also well known that the heat generated from the optical projection system affects the beam of the weighing instrument and leads to unquantifiable error in the determination of mass.

It is the object of the present invention to provide an optical projection arrangement which avoids these difficulties and gives an improved means of measuring small displacements oftheweighing beam under load. A laser operated optical projection system for the enhanced readability and precision of balances and similarly constructed fine weighing instruments where the image is read either on an associated direct reading scale which is remote from the weighing instrument or by an automatic light sensor. The system using helium-neon light is capable of operation on weighing instruments installed in laboratories which are lit by natural or artificial light. The system does not require darkened conditions.

When fitted to weighing instruments normally of capacities between 25 kg and 20 g the ultimate precision when expressed in terms of parts per million (at 1 kg) can be measured in tsrms of an uncertainty expressed at the 95% confidence level of at least 0.5 ppm (minimum).

A single helium-neon laster of 632.8 nano-meters wavelength with a power rating not exceeding 0.7 mW at full power (measured 30 minutes or more after switching on) is installed remote from the weighing instrument. The beam diameter should be less than 0.9 mm at emergence, and the beam divergence should be less than 1.0 mrad. The laser should be enclosed in a housing for safety.

Mounted adjacent to the laser is a collimating lens assembly to focus the beam. The lens power is dependent on the final distance the light must travel before it reaches the measurement scale. The value of the lens can be determined by using the following equations: (a) The beam diameter at a given distance from the laster (i.e. the distance to the measurement scale) can be approximated as: Wz = WO + 2z tan 0/2 where Wz = beam diameter at a distance'z' from the laser WO = beam diameter at the laser aperture z = distance from laser aperture 0 = full angle divergence (b) The spot size of a beam focused through a lens can be approximated as:: Wt af8 where Wf = focused beam diameter f = focal length of lens 0 = full angle divergence in radians Beam reflectors constructed either of mirrors or prisms are mounted on the weighing instrument to deflect the laser beam. Where plane mirrors are used they are of optical grade glass having a surface quality of 40/20. The second surface shall be obtained by fine grinding. The mirror shall be coated with aluminium, SiO overcoated.

Where prisms are used they should be manufactured from Crown BK7 optical glass, Flint F2 optical glass, or dense Flint SF 10 optical glass. The surface quality shall be 80/50 with a surface flatness of 2A/25.4 mm. The angle tolerance shall not exceed +5 minutes of the arc. Prisms shall be uncoated. A 50 mm photographic slide shall have approximately 960 parallel lines reproduced over the slide. It is placed in the path of the beam. Suitable mounting and adjustment facilities should be provided so that the uncollimated beam may pass directlythroughthe centre. The object of the beam subdivider is to subdivide the beam of laser light precisely.

The principal reflector is mounted on the beam of the weighing instrument and shall be made of Crown BK 7 optical glass, Flint F2 optical glass or Dense Flint SF 10 optical glass. Aiternatively hand polished aluminium may be used. The mirrors shall be uncoated. The surface quality shall be 40/20 with a 0.1 wave surface flatness. The mirror shall be mounted directly on the centre of the beam on a parallel which coincides with the centre of the beam exactiy. The reflector is glued into position but must be optically flat in relation to the beam.

A direct reading scale is mounted remotely from the weighing instrument and consists of a graduated scale having 40 principal scale divisions each further subdivided into 5 or 10 supplementary divisions. The length of the scale is directly proportional to the distance from the principal reflectorto the direct reading scale. Where this distance is 5 m then the direct reading scale shall have a length of 1200 mm from the principal scale mark representing 0 to the principal scale mark representing 40. The principal scale mark at the left hand side of the graduated scale shall be designated as representing 0. Each consecutive principal scale mark to the right of this '0' mark shall be designated between 1 and 40 in ascending order. The number of scale marks may be varied to suit other distances. The front of the scale shall be protected by non-reflective glass.

As an alternative the remotely situated direct reading scale may be aligned vertically in which case the spots of light will be arrayed horizontally when the final reflector is positioned so as not to reflect the light forwards, but to reflect the light parallel to the initial beam of incoming light. A modified arrangement where the graduations on the direct reading scale are non-proportional can be achieved if the accuracy is required in a particulrsector of the chart by having the principal reflector with a radius directly equal to the distance between it and the direct reading scale.

Finally, as an alternative method of reading, a strip containing a series of photo-electric cells is mounted directly in front of the final reflector so that the beam is reflected and then hits the photo-electric cell strip.

This is connected to a digital readout device so that full scale defiection of the light beam can be translated into a digital readout representing 0-9999 divisions.

The invention is hereinafter described with reference to the accompanying drawings, in which: Figure 1 represents a laser optical projection system for balances and other fine weighing instruments, where the beam is unladen.

Figure 2 is a similar view where the beam is subjected to load.

Figure 3 represents another arrangement where the remotely mounted direct reading scale has been replaced by an automatic light sensor mounted immediately in front of the final beam reflector.

In all cases light generated from the helium-neon laser (a) shines through a collimator (b) and associated beam subdivider (c) via a reflector (d) onto the principal reflector (e) mounted on the beam of the weighing instrument (f). The light is reflected from the principal reflector via another reflector either on to a direct reading scale (g) approximately 5 m from the principal reflector where it appears as a series of extremely small well defined and vertically arrayed spots of light, or via an automatic light sensor (h in Figure 3) which displays the final reading on a digital readout.

Subsequently the readout may be connected to computer facilities for data analysis. As shown in Figure 2, astheweighing instrument is loaded and set in motion the beam of the instrument moves under load. The principal reflector being directly affixed to the beam of the instrument reflects at a divergent angle the laser light.

This angle of divergence can be read either on the direct reading scale or by the automatic light sensor.

Where it is established that a certain weight will move the beam over a certain distance, the true mass can be established mathematically as a resultant value.

Claims (4)

1. A laser optical projection system for balances and other scientific apparatus comprising a heliumneon laser light beam which having been subdivided is reflected off a mirror or similar reflector mounted on the beam of the weighing instrument and displayed on a remotely mounted direct reading scale where the graduations are horizontally aligned and of equal length and value and where the laser light appears as a series of vertically arrayed precision spots of that light.

2. A laser optical projection system according to claim 1, where the direct reading scale is mounted vertically and the laser light appears as a series of horizontally aligned spots of light

3. A laser optical projection system according to claim 1, where the remotely mounted direct reading scale has divisions which are not of equal value and length.

4. A laser optical projection system according to claim 1, and constructed as shown in Figure 3 where the remotely mounted direct reading scale is replaced by an automatic light sensor, the final result being digitally displayed.