Linear measuring devices

A measuring instrument comprises a portable case (45) housing a drum (11) on which is wound a flexible mesuring line or tape (10) which runs over a rotary capstan (15) provided with teeth (16) engaged by a sensor (24) acting as a pulse generator to an electronic counter (27). In addition there is a direction sensor (21) arranged to provide a count-up or count-down signal and a further direct measuring wheel (30) as teeth (32) acting as a pulse generator to the same counter (27).

This invention relates to linear measuring devices as used for example for 
measuring dimensions of rooms, buildings, or other objects, or for 
measuring the distance travelled along a surface. The term `linear` is 
intended also to include measurement of distances along non-straight paths 
or over non-flat surfaces. 
So-called "tape measures" are well-known, consisting of a length of tape 
along which are marked the units of measurement, (e.g. centimeters or 
inches), the tape being wound on a rotary reel provided with rewind 
facilities. Such tape measures have certain limitations, for example only 
a limited length of tape can be stored on a reel, the tape cannot easily 
be read in the dark, or by the blind, and it may become dirty or be 
damaged. 
Wheel measuring devices have also been proposed in which the wheel is run 
along the surface to be measured and the motion of the wheel operates a 
counter or display which represents units of measurement. These wheel 
measuring devices, are only applicable to certain specific uses and are 
subject to certain operating difficulties. 
It is an object of the invention accordingly to provide an improved 
measuring device which will overcome some of the problems experienced with 
existing instruments. 
Broadly stated the invention consists in a linear measuring device 
comprising a holder, a movable measuring element, a sensor for detecting 
movements of the measuring element, an electrical pulse generator coupled 
to the sensor, and electronic digital counter or calculator circuit 
connected to the pulse generator, and an output circuit connected to a 
visible or audible or recording output unit. 
According to a preferred feature of the invention the sensing means 
includes a pulse generator sensitive to the movements of the element, and 
an electronic counter or calculator circuit connected to the pulse 
generator. In some forms of the invention the sensing means includes a 
rotary element engaging the measuring element so as to be turned by linear 
movement thereof, and arranged to generate pulses accordingly. Also the 
measuring element is elongate and has physical, mechanical, magnetic or 
optical linear measuring points, and the sensing means includes a detector 
capable of sensing such measuring points. 
According to another preferred feature of the invention the measuring 
element is movable in one direction for measuring purposes, and includes 
means for shifting the measuring element in the opposite direction into a 
starting position. 
In some forms of the invention the measuring element may be a rigid 
extendable member, this being particularly useful for measuring small 
dimensions. In other forms of the invention the measuring element is a 
long thin flexible line, tape or the like, wound in a coil or drum. 
Conveniently, it may be in the form of a fine thread. This is the most 
economical form and permits the maximum length to be contained on a drum 
of given dimensions. 
Preferably the device also includes a spring, or motor-driven or manual 
rewind mechanism for the coil or drum. 
Alternatively or in addition to the measuring element referred to, it may 
be in the form of a direct rotary measuring device comprising a rotor with 
at least part of its periphery exposed. This allows the device to measure 
uneven or non-linear surfaces or dimensions and it may, for example, be 
used to measure the internal circumference of a drum. 
For a number of purposes such as use by the blind it is of advantage that 
the device should include an audible signalling device to provide audible 
information on the movement of the measuring element. Furthermore, the 
device may also include means for converting the display from imperial to 
metric or other units. 
It is also of advantage to include means for connection to an external 
remote signal generator 
The device may also be incorporated with a height-measuring system 
operating on the "vertical sextant angle" principle. Thus the device may 
include a vertical angle measuring device and a calculator programmed to 
combine the tangent of the measured angle with the sensed linear 
measurement to provide a measure of the vertical height of an object. To 
establish the vertical datum there may be a bubble level, pendulum or the 
like. The device preferably includes means for totalling the movement of 
the measuring element in one direction, and for subtracting measured 
movements in the opposite direction and it may also include means for 
setting and/or adjusting the datum or zero of the sensed measurement. 
In a particular preferred form of the invention the device includes means 
for attaching the holder to a tool such as an electric power drill, saw or 
moulder, with the measuring element engaging the workpiece so as to 
indicate the depth of `cut` made by the tool.

Referring first to FIG. 1 the measuring apparatus in this example comprises 
a flexible line 10 wound on a drum 11 provided with an automatic rewind 
spring 12 and having a clip or other fitting 13 at its free end. The line 
may be formed of any suitable flexible non-extensible, rotproof, material 
such as nylon, steel, or synthetic plastics. The line in this example is 
preferably a circular section monofilament and it is wound around a 
friction pulley or capstan 15 having a shallow external groove with which 
the line makes a good frictional grip. This capstan is formed with a 
number of closely spaced teeth or notches 16 at intervals corresponding to 
the smallest unit of measurement required for the measuring line. For 
example, if the smallest required unit of measurement is 0.5 m.m. the 
teeth will be spaced at the appropriate angle measured from the rotary 
axis 17 to give a circumferential measurement of 0.5 mm at the periphery 
of the groove between adjacent tooth positions. 
The apparatus also includes a first detector comprising a feeler 23 
positioned to engage the teeth 16 as the capstan rotates and to influence 
a sensor 24 which may, for example, be a piezo electric crystal or a 
microswitch, the output from the sensor being fed on line 25 to the 
digital counter circuit to be described. The apparatus also includes a 
further directional sensor comprising another finger 20 also arranged to 
engage the notches 16 and connected to a sensor 21 which may take a 
variety of forms but produces an output signal on line 22 which provides 
an indication of either forward or reverse movement of the capstan 15. 
This directional sensor may, for example, be an electronic filter circuit 
coupled with a piezo electric crystal designed to detect the shape of the 
pulses generated by the crystal in response to movements of the feeler 20. 
Alternatively the two sensor 21, 24 may be coupled with circuitry designed 
to produce a directional signal in response to the phase difference or 
phase change between the two feelers. 
The electronic digital counter includes a "de-bouncing unit" 26, including 
filter circuitry to eliminate unwanted or unintensional pulses, a binary 
coded decimal counter 27, and a binary coded decimal de-coder 28 linked 
with a seven segment LCD display 29 arranged to display the measured 
increments in decimal form, in this case in meters, centimeters and 
millimeters. 
When the line is rewound on to the drum 11 by the spring 12 the sensor 21 
detects the reverse rotation of the capstan wheel 15 and provides a pulse 
on line 22 to the counter 27 to indicate that the pulses are to be 
subtracted thereby providing a countdown towards zero. At any instant the 
resultant measurement from the counter 27 may be held on the display 29 or 
in an electronic memory by pressing a button 35, which prevents further 
pulses reaching the counter 27 and holds the last reading on the display 
29. When switched off the device is automatically reset to zero. 
The apparatus also includes a direct wheel measuring facility comprising an 
exposed rotor wheel 30 mounted to rotate on an axis 31 and having a rotary 
pulse generator including a toothed rotor 32 engaged by a feeler 33 
attached to a piezo electric or other electric pulse generator 34. The 
output from this pulse generator is fed to the de-bouncing unit 26 and to 
the other components of the counter already described. In addition there 
is a further feeler 37 connected to a sensor 38 providing an electric 
signal signifying forward or reverse movement of the wheel 30. This 
component of the instrument operates in somewhat the same manner as the 
flexible line measuring device, but the measuring pulses are generated by 
running the wheel 30 along a surface whose measurements are required. 
In addition this instrument includes an auxiliary input 40 in the form of 
an electrical socket intended to co-operate with an auxiliary jack 
attached to an external pulse generator such as a pedometer, or manual or 
foot operated switch. These pulses will be added and displayed in the unit 
29 as previously described. 
In the instrument illustrated in FIG. 2 and FIG. 2A the case 45 has a 
centimeter scale 46 along one edge with the flexible measuring line 10 
issuing through a hole at the centre, the line having a flat clip or plate 
48 at its free end with a slot 49 which allows the clip to be temporarily 
held at the end of the scale when required, or alternatively to allow the 
line to be temporarily anchored round any suitable fixed object such as a 
hook or pole. In this example the external direct measurement wheel 30 is 
located at one corner of the case, the LCD display unit 29 is located 
centrally, the auxiliary input 40 on the opposite edge of the case and, in 
addition, there is a switch or button 41 controlling a metric to imperial 
conversion circuit, and a mode selector switch 42. 
In the alternative embodiment illustrated in FIG. 3 like parts are 
indicated by the same reference numerals. This unit includes an additional 
audible unit indicated generally at 50, which includes a loudspeaker 51 
connected to a tone generator 52 combined with counter and register store. 
In addition there is an audio on/off switch 53 and an external audio line 
54 operated by switch 55. 
In the example illustrated in FIG. 4 again the main components are 
indicated by the same reference numerals as in FIG. 1 and in this case the 
rewind drum 11 is connected to a "Tensator" (RTM) spring 60 whose opposite 
ends are wound around the rewind drum 11 and the spring housing drum 61. 
This provides a substantially constant rewind force over the whole length 
of the measuring line. The other components in this illustrated example 
are similar to those of FIG. 1. 
In the further example illustrated in FIG. 5 the direct measuring wheel 30 
of the FIG. 1 example is replaced by a detachable measuring wheel 63 of 
larger diameter mounted on a fitting or bracket 64, which is designed to 
clip on to the body or housing 65 of the instrument and is provided with 
electrical contacts 66, 67 to engage corresponding contacts 68, 69 on the 
body of the housing so that the impulses generated by the wheel are fed to 
the counter and display unit in the same manner as previously described. 
FIG. 6 illustrates a modified form of clip 70 at the end of the measuring 
line 10 intended to facilitate fastening of the line around a fixed pole 
or hook 71. 
A further example illustrated in FIG. 7 contains components similar to 
those of the FIG. 1 example and indicated by the same reference numerals. 
In addition, this example includes a sextant and bubble level unit 75 
linked to the first calculator unit 76. The mercury "bubble gauge" 77 has 
an output 78 providing an indication when the instrument is held level. 
The sight tube 79 is coupled to an angle meter 80 providing a pulsed 
output on line 81 which is a measure of tan .theta. where .theta. is the 
angle of elevation of the sight tube. When combined with the measurement 
signal D generated by the pulse generator 21 from the measuring line 10, 
the calculator 76 provides an output equivalent to D tan .theta., i.e. the 
height of the object being observed. 
In the FIG. 7 example 87 is an external pulse generator such as a foot 
switch, connected via jack 40 to the calculator, 82 is the metric/imperial 
conversion switch, 83 is the zero or reset datum switch, 84 is a manual 
on-off switch for the tone generator/synthesiser 52, and 85 is an external 
printer connected to the counter circuit through the jack 86. The 
calculator 76 includes a decimal keyboard 88 by which programmed 
mathematical functions or further numerals may be introduced. 
It will be appreciated that the line or tape 10 may in use accummulate dirt 
or in other ways suffer an increase or change in thickness, and this may 
affect the accuracy of the measurements made by the rotary capstan 15 and 
sensor device 16, 24. To eliminate or compensate for this the device 
preferably includes a cleaning element 18 in the form of a felt or fibre 
wiper and an electrical thickness sensor or gauge 19 which continually 
senses the dimensions of the element 10, and applies a corresponding 
correction as a negative percentage factor to the counter 27, the 
correction being proportional to the sensed change in dimensions. 
Alternatively or in addition the gauge 19 may be connected to an 
electrical warning device, such as a flashing light or audible alarm, to 
indicate that the element 10 needs to be cleaned, and that the 
measurements are suspect. 
In the embodiment of FIG. 3 the audible unit 50 with the loudspeaker 51 may 
take various different forms. For example, the tone generator may be 
linked to the counter circuit to produce a series of tones or "bleeps" 
equivalent in number to the digits which would be displayed visibly, for 
the assistance of the blind, or in the dark for example. Alternatively or 
in addition to the tone generator there may be a speech synthesiser also 
linked to the counter/calculator unit and arranged to generate speech 
sounds representing the digits in the display unit. The calculator 
preferably includes an electronic memory which may, for example, be of the 
magnetic or diode type and the calculator preferably includes at least the 
basic mathematical functions of a portable hand-held instrument, i.e., +, 
-, .times.and .div.. Thus, for example, it is possible to take a first 
measurement a which will be held in the memory to be followed by a second 
linear measurement b also held in the memory, so that the operator can 
selectively either add a+b, or multiply a.times.b to produce an area. 
Likewise, the volume of a room or tank having side a, b, c, can be 
obtained by three successive measurements a, b, c, combined by a double 
multiplying fuction. Likewise, simple mathematical calculations can be 
performed with simple inbuilt or programable functions; for example, the 
radius of a cylinder R can be obtained from a measurement Q of its 
circumference using the programmed function R=Q/2.pi. 
Conveniently the calculator unit is coupled to the counter so that the 
measured values can be entered directly in the calculator. Alternatively, 
however, the calculator may be a separate unit in the same holder having 
manual binary decimal input keys. The operator then enters the measured 
values into the calculator as in a normal independant calculator unit. The 
sextant-bubble level unit 75 may have a fixed sextant angly .theta. if 
preferred, in which case the calculator may have a pre-programed tan 
.theta. value to obtain the height h of the observed object directly from 
the formula h=d tan .theta., where d is the measured distance from the 
object. 
The output from the counter may, if required, be connected to an internal 
or external printer to provide a permanent record of the measured or 
calculated values. The instrument may also include mechanical and/or 
electronic "hold" or locking devices to retain the instantaneous position 
of the capstan or measuring wheel or to maintain the sensed measurement in 
the electronic memory. The direct measuring wheel may, if required, be 
provided with an adjustable zero/datum index to facilitate direct 
measurements and to afford an adjustable set zero. The control buttons on 
the casing of the instrument may be recessed and they may be combined with 
an electronic bleeper circuit to provide an audible indication when any 
button is actuated.