Precision length measuring instrument

An improved height measuring instrument includes a hollow profile support member measuring a scale mounted to the support member, and a relatively flexible guide member which is adjustably mounted to the support member by means of a plurality of threaded fasteners which can be used to push or pull the guide member to correct measurement errors. A measurement carriage is guided along the guide member. This carriage bears an intermediate member onto which is mounted both a sensor and a reading unit arranged to scan the scale. A spring parallelogram including a pair of opposed, parallel plate springs connects the intermediate member to the carriage to provide a substantially constant measuring force. An apparatus is disclosed for altering the spring constant of the parallelogram to compensate for the proper weight of the intermediate member, the reading unit, and the sensor. This apparatus includes two knife edges mounted on the intermediate member and a cylinder, spring mounted on the carriage and biased into contact with the knife edges.

BACKGROUND OF THE INVENTION 
This invention relates to an improvement in precision length measurement 
instruments of the type which include a scale, a reading unit moveable 
along the scale, and a sensor, coupled to the reading unit, for making 
contact with a surface to be measured. 
A large number of references are concerned with length measurement 
instruments and their errors, specifically with the reduction of these 
errors. Several specific problems of this area of technology are treated 
in the descriptive introduction to West German DE-OS No. 16 23 337. Thus, 
it is desirable that measurement instruments retain the comparator 
principle, because in an instrument which employs the comparative Abbe 
principle errors of the first instance are avoided. In some measurement 
instruments, however, this principle cannot be employed for various 
technical reasons and thus other arrangements have been developed which 
employ other measures (such as the Eppenstein principle and the object of 
the application of DE-OS No. 16 23 337) to eliminate as much as possible 
the so-called comparison errors. 
Furthermore, measurement instruments are known which include spring mounted 
measurement sensors. The deviation of the measurement sensor from its zero 
position determines the actual measurement, and the measurement value is 
given a figure that corresponds to the measurement sensor deviation. Such 
an instrument is the object of West German DE-PS No. 23 56 030 and is 
described in the descriptive introduction of that patent. 
Such instruments, however, often require relatively complex structures 
which are not easily operated. Therefore, capital expenditures for this 
type of instrument tend to be relatively high and such instruments 
generally require specially qualified personnel to operate them. 
For example, in height measurement instruments the Abbe principle generally 
cannot be used. Because geometrical considerations often dictate a 
comparator distance of 50 to 100 mm, one must, for example, maintain a 
guide precision of .+-.2 arc seconds in order to have a measurement 
precision of .+-.1 micron. In order to keep manufacturing costs at a 
practical level the use of light metal profiles seems indicated but their 
straightness, however, remains substantially under the above mentioned 
value. 
The requirement for high measurement precision also means that the 
measurement carriage must be moved with little friction. In this way 
measurement forces can be maintained constant within narrow limits in 
order to avoid deformations of the measurement sensor caused by variations 
in measuring forces. Roller bearings, generally used for low friction 
guides, are often not sufficiently precise as a result of the inevitable 
construction tolerances and the gradual deposition of foreign material on 
the guide surfaces. 
SUMMARY OF THE INVENTION 
The present invention is directed to a structurally simple and therefore 
relatively low cost length measurement instrument that provides high 
measurement precision and which is simple to operate. 
According to this invention an instrument is provided as defined in claim 
1. The dependent claims set forth characteristics that advantageously 
expand the invention. 
According to a first feature of the invention, a measuring instrument is 
provided with a measurement sensor and a reading unit mounted on an 
intermediate member. This intermediate member is in turn mounted to move 
in a substantially friction free manner with respect to a measurement 
carriage which travels along the measurement direction. The mounting means 
for the intermediate member preferably includes a spring parallelogram to 
provide a substantially constant measuring force. 
According to a second feature of the invention a measuring instrument is 
provided with a support member (such as a hollow profile); a guide member 
(such as a cylindrical rod) mounted in the support member; and means for 
precisely adjusting the position of the guide member with respect to the 
support member. By proper adjustment, the guide member can be positioned 
to compensate for errors due to the scale, deformations in the support 
member or the guide member, or other causes. 
According to a third feature of the invention, a spring parallelogram is 
provided with means for altering its spring constant. Here, the 
parallelogram comprises two opposed members connected by means of a pair 
of opposed, substantially parallel spring members. The means for altering 
the spring constant includes a pair of knife edges mounted on the first 
opposed member, and a cylinder mounted on the second opposed member and 
biased into contact with the two knife edges. 
The invention, together with further objects and attendant advantages, will 
be best understood by reference to the following detailed description 
taken in connection with the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
Referring now to the drawings, FIG. 1 shows a base plate 1 on which has 
been installed a hollow profile 2 at right angles to the base plate 1. 
This hollow profile 2 is preferably a light metal extrusion and is very 
resistant to bending forces. A comparatively flexible rod 3, which 
preferably consists of a commercially available polished, round steel bar, 
is installed in the hollow profile 2 by means of a number of pull and/or 
pressure screws 4, 5. A linear strip 3a of the surface of the rod 3 is 
aligned by means of the pull and/or pressure screws 4, 5 and serves as the 
primary guide surface for a moveable measurement carriage 6. The 
measurement carriage 6 is provided with two glide shoes 7 which slide 
along the guide surface 3a of the rod 3. The glide shoes 7 are preferably 
made out of self-lubricating, wear-resistant material such as a synthetic 
material, sintered bronze, or the like. Through the sliding action of the 
shoes 7 on the guide surface 3a, a self-cleaning action is provided. The 
measurement carriage 6 is also provided with support rollers 8, 9, 10 
which support the measurement carriage 6 on side guide surfaces 11, 12, 27 
which are not adjustable. The two glide shoes 7 glide on the guide surface 
3a of the rod 3 and provide a very high guide precision in the plane in 
which the greater comparator distance a.sub.1 is absolutely necessary. In 
spite of the relatively lower guide precision of the roller bearings 8, 9, 
10, only a negligibly small error occurs in the second plane, because 
comparator distance a.sub.2 is kept small or is indeed reduced to zero. 
An intermediate member 13 is connected to the measurement carriage 6 by 
means of a pair of plate springs 14 which provide a friction-free 
connection. The plate springs 14 are substantially parallel such that a 
parallelogram is defined by the ends of springs 14. These springs 14 flex 
to allow the intermediate member 13 to translate with respect to the 
carriage 6 along the measurement direction in a low friction manner. The 
springs 14 also substantially prevent rotation between the intermediate 
member 13 and the carriage 6. Mounted to this intermediate member 13 is a 
scanner arm 15 on whose free end is located a measurement sensor 16. A 
narrow blade section 17 is formed on the intermediate member 13, and a 
scanning unit 18 of a photoelectrical measurement unit is mounted on the 
free end of this blade section 17. This scanning unit 18 scans an 
incremental division formed on a scale 19 which is protectively mounted 
within the hollow profile 2 in a known manner by means of a resilient 
intermediate layer 20. The scanning unit 18 generates measurement signals 
which are processed in a known manner by an electronic evaluation and 
display unit 21 and are indicated or printed out as a measurement value. 
The measurement carriage 6 also includes a drive mechanism which includes a 
hand wheel 22 and a pinion gear 22a. The pinion gear 22a meshes with a 
rack 22b mounted on the profile 2. The pinion gear 22a is positioned such 
that the distance between the attack point of the drive force and the 
plane in which the friction affected guide is located is small. In this 
way an almost shock-free movement of the measurement carriage 6 is made 
possible. 
The handwheel 22 is used for positioning the measurement carriage in order 
to measure a workpiece or to calibrate the instrument, as in setting the 
zero point of the measurement instrument. To facilitate individual 
measurements, both the scanner arm 15 and the measurement sensor 16 can be 
adjusted as necessary by means of known elements. In that the scanner arm 
15 is mounted both to slide and to rotate, the measurement instrument can 
be readily adapted to different measuring tasks and different measuring 
geometries. In general, when the arm 15 is positioned for minimum length 
the greatest precision is attained, because the comparator distance is 
then as small as possible. When a longer arm extension with a somewhat 
lower precision is chosen, difficult to reach surfaces of a workpiece can 
be measured. 
The markings 6a, 6b and 13a that are visible in FIG. 2 serve for the 
approximate indication of the attainment of a measuring position. The 
intermediate member 13, which is mounted to move in a friction-free manner 
on the plate springs 14, carries an index 13a. The measurement carriage 6 
also carries two measurement indices 6a and 6b, used as index points in 
the scanning of a workpiece located beneath and above the sensor, 
respectively. The force of the plate springs 14 maintains the intermediate 
member 13 in a neutral position so that the index 13a assumes its rest 
position somewhat in the middle, between the two indices 6a and 6b. During 
the measurement of a workpiece the measurement carriage 6 must be moved by 
the drive 22 so far that the index 13a approximately coincides with the 
appropriate one of the two indices 6a, 6b. Approximate coincidence between 
index 13a with either index 6a or index 6b is sufficient to provide 
precise measurement results. An exact coincidence is not required because 
the measurement sensor 16 is pressed against the workpiece with a 
substantially constant force within the area determined by the 
characteristic curve of the springs 14, and therefore the position of the 
sensor 16 is substantially independent of minor differences in the 
position of the carriage 6. Similarly, the intermediate member 13--which 
carries the reading unit 18 on the blade 17--maintains a substantially 
constant position within the area that is determined by the characteristic 
curve of springs 14, as presented in FIGS. 5 and 6. Accordingly, the 
position of the reading unit 18 with regards to the scale 19 is 
substantially independent of minor differences in the position of the 
carriage 6, and the measurement result is therefore unequivocal and 
constant. 
The measurement precision obtainable with this embodiment of the suspension 
(springs 14) is substantially independent of the precision with which the 
measurement carriage 6 is adjusted with regard to the markings 6a or 6b. 
In addition, stiffening elements 14a are provided on each of the plate 
springs 14 which increase the rigidity of the parallelogram of the springs 
14. See FIG. 2. 
Preferably the spring force of the plate springs 14, the rigidity of the 
spring parallelogram, and the wheel half-diameter of the wheel 23 (FIG. 
4), are chosen to obtain the characteristic lines shown in FIGS. 5 and 6. 
A mechanism is shown in FIG. 4 which can be used to compensate for the 
proper weight of the moveable components 13, 15, 16, 17, 18. This 
mechanism includes a wheel 23 which is spring loaded by lever 23a which is 
pivotably mounted at pivot 23b mounted on the carriage 6. The lever 23a is 
spring biased away from the carriage 6 by a spring 23c so that the 
periphery of the wheel 23 is urged into contact with two knife edges 24, 
25. These knife edges 24, 25 are mounted on the intermediate member 13 
assymetrically with respect to the wheel 23, as shown, such that the lower 
knife edge 25 contacts the wheel 23 nearer the center line of the wheel 23 
than does the upper knife edge 24. In alternate embodiments, the knife 
edges may be adjustably mounted on the intermediate member 13. 
The intermediate member 13 of FIG. 4 is mounted to the carriage 6 by means 
of a friction free suspension which includes two plate springs 14. This 
arrangement ensures that the measurement sensor 16 is pressed against the 
workpiece with approximately the same measuring force whether an upper or 
a lower surface of a workpiece is being measured. To a large degree this 
measuring force is independent of the precise position of the intermediate 
member 13 with respect to the measurement carriage 6. That is, measuring 
force is substantially independent of the deviation of the plate spring 
parallelogram. 
A further feature which contributes to the high measurement precision of 
the embodiment of FIGS. 1 and 2, as well as to its ease of operation, is 
the exact weight balance of the components mounted on the moveable 
measurement carriage. Both the measurement carriage 6 as well as the 
intermediate member 13, with the scanner arm 15 and the measurement sensor 
16, have been dimensioned with regards to their own weight in such a 
manner that even a small amount of friction of the glide shoes 7 on the 
guide surface 3a results in a jamming of the measurement carriage 6 in any 
position if the friction at the adjustment of the measurement carriage 6 
is found to be excessive. In order to obtain this positive jamming effect, 
two of the guide points of the carriage 6 are provided with glide 
shoes--these are sufficient without anything further for a high precision 
guiding. 
FIG. 3 shows a cross-section of a measurement instrument which provides a 
somewhat reduced degree of precision as compared to the embodiment of 
FIGS. 1 and 2. In this embodiment a measurement carriage 6' is located 
within the inside of a hollow profile 2'. Here, as before, a round rod 3' 
serves as guide. Pull and/or pressure screws 4', 5' fulfill the same 
purpose as the elements 4 and 5 in FIG. 1. In addition to equivalent 
elements such as the intermediate member 13', plate springs 14', 
stiffeners 14a', scanner arm 15', measurement sensor 16', blade 17', 
reading unit 18', scale 19' and resilient bonding layer 20', the 
measurement carriage 6' includes a counterweight 26'. The counterweight 
26' moves in opposition to the carriage 6'. The weight 26' and the 
carriage 6' are connected by cables (not shown) connected therebetween. 
These cables extend away from the carriage 6', over pulley arrangements 
(not shown) near the top and bottom of the profile 2', to the 
counterweight 26'. Both the counterweight 26' and the carriage 6' are free 
to move along the length of the profile 2', and the weight of the 
counterweight 26' is chosen such that the carriage 6' will remain at rest 
at any desired point along the length of the profile 2'. In this 
embodiment the carriage 6' is guided by precision roller bearings 7' which 
do not provide the automatic jamming function of the slide shoes 7. In 
addition, in this embodiment, a motor is provided as the drive for the 
measurement carriage adjustment. 
An additional feature of both embodiments, which represents an advance in 
the art, is the adjustable mounting of the guide surface 3a, 3a' in the 
profile 2, 2'. In the past, high precision measurement has been long 
thought to require the highest possible precision in the guidance of 
moveable measurement elements. High guidance precision may drive up the 
costs of a measurement instrument into a range that often can not be 
justified for workplace measurement instruments. 
In providing high precision guidance, however, the errors of the actual 
measurement scale have often been only marginally considered. In 
measurement instruments in accordance with the present invention, 
practically the sole source of error is the comparator error. These errors 
cannot be completely eliminated through improved guiding precision. 
The present invention includes novel adjustment means for the guide surface 
3a, 3a' which make it possible to correct most or all measurement errors, 
be they guidance errors, system errors of the measurement scale, or 
specific comparator errors. This is done by calibrating the measurement 
instrument by precise measurement of known distances. Overall system 
errors are thereby determined as a function of measurement position, and 
these errors are then corrected by appropriately shaping the guide surface 
3a, 3a' by means of the pull and/or pressure screws 4, 5, 4', 5'. These 
screws 4, 5, 4', 5' are designed in such a manner that the round rod 3, 3' 
and with it the guide surface 3a, 3a' can be deformed by means of the 
screws 4, 5, 4', 5' in such a manner that the errors of the measurement 
instrument are corrected. 
The bottom plate 1, 1' of the described measurement instrument is 
preferably supported by three adjustable feet which are covered with 
abrasive resistant material and include a filler material such as high 
pressure-E-module reinforced synthetic material. These feet are mounted to 
the plate 1, 1' by means of adjustment screws which pass through ball 
joints. These ball joints adjust to the mounting surface while providing 
rigid support to the instrument. 
In summary, the present invention provides a measurement sensor 16 coupled 
to an intermediate member 13 which moves in a friction-free manner with 
respect to a measurement carriage 6. This measurement carriage 6 is guided 
with sufficient precision by a guide surface 3a so that the comparator 
errors remain acceptably small. The guide of the measurement carriage 6 
includes two simple profiles 2 and 3 that are braced with regards to each 
other and can thus be adjusted. The measurement carriage 6 must only be 
approximately placed in the measurement position, and then the measurement 
sensor 16 is pressed with a substantially constant measuring force against 
the surface to be measured. The measurement value is then read directly. 
Of course, it should be understood that various changes and modifications 
to the preferred embodiments described herein will be apparent to those 
skilled in the art. Such changes and modifications can be made without 
departing from the spirit and scope of the present invention, and without 
diminishing attendant advantages. It is, therefore, intended that such 
changes and modifications be covered by the following claims.