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INSTRUMENTATION

OF THE

R. LOSER, D. MEIER,

NINETIES

T. SCHOLIAN

Kern 63 Co. AG Switzerland

1. Introduction Technological

progress, changed measuring

curacy requirements of measuring outlook

necessarily lead to the new and continued

instruments.

on future

tasks and steadily increasing

The following

instruments,

article

ac-

development

is intended

to provide

an

this topic being considered from two view-

points: 1. New development

of instruments

that will be commercially

available in

the near future. 2. Publication

of some investigation

results which provide information

the technical capabilities

of measuring principles

lead to the development

of new instruments.

From the viewpoint NIVEL

of new instruments,

20 will be briefly

about

and methods that might

only the precision inclination

described, since the Laser Tracking

meter

System also to

be expected in 1990 is presented in a separate publication. A second part describes the technical possibilities

for the further development

of distance meters based on the measuring principle ME 5000. In particular, of a two-color

used in the Mekometer

we will report on the possibilities

for the development

distance meter and a distance meter for short distances (2 m

- 200 m) with accuracies in the area of lo-20 pm.


2. Precision

Inclination

2.1 Design NIVEL

Meter

NIVEL

and Operation 20 is a high-accuracy

sensor for the measurement

of the vertical

in two dimensions.

of a deflection

of the vertical

The liquid’s independently

The magnitude

can be determined

sensor. The inclination-sensitive

relative

20

and the direction

with one setup of the

element is a liquid in a closed container.

surface is perpendicular

to the direction

of the sensor’s orientation.

of the vertical,

The inclination

to the sensor is measured by optical

components

of deflections

means.

of that surface All the optical

needed for that purpose are fastened to the underside of a

flat glass plate. The plate also forms t.he bottom of the container with the transparent

liquid.

By means of the optical

components,

surface of an LED is imaged in a position-sensitive light from the LED is guided from below through liquid,

is totally

reflected

at the liquid’s

photodiode.

of the impinging

surface, and then passes again

light spot relative

was adjusted and calibrated

in the horizontal

The flat glass plate, serving as a component support

The

the flat glass and the

through the liquid and the flat glass. Finally, the photodiode position

the luminous

detects the

to the zero position configuration

which

(see Fig. 1).

and simultaneously

as the

for the actual sensor element, is clamped into a trough-shaped

metal base. The sensor is set up on three hardened and ground circular support

surfaces on the underside

have through

of this base.

holes for M4 screws in the center.

placed the printed

circuit

ally, a CPU card with

The support

On the metal base is

board with the analog amplifiers

a serial interface.

plastic hood. The plastic serves primarily

and, option-

The sensor is covered with a as a thermal insulator

meant to prevent external heat effects from causing nonuniform expansions errors.

in the interior

A temperature

of the NIVEL

which is thermal

20 and consequent measuring

sensor is also installed

-185-

surfaces

to monitor

the sensor’s


temperature.

The direction

of the measurement

a stop edge on the metal base. The measurement

axis X is established

axis Y is perpendicular

to it. A bubble level is used for rough determination The values of inclination perature the NIVEL

A NIVEL

of the horizontal. and the sensor tem-

values. The standard

20 has and RS-232 or RS-485 serial interface,

ing the capability

LED Imaging Liquid Liquid Biaxial

in the X and Y directions

are available as measurement

20 with

of operating

up to 32 NIVEL

analog outputs

is provided

20s in the same network. for special applications.

detector

Fig. 1 Functional

Principle

of the NIVEL

-1C6-

version of

the latter hav-

optics container position

by

20 Inclination

Sensor


2.2 Scope

of Application

and Specifications

Thanks to its wear-free, sturdy and thermally stable construction, the NIVEL 20 is suitable for use even under extreme conditions in industry, research and construction trades. With available accessories it is possible to put together complete measuring systems suitable for many applications such as the setting up and aligning of machines and systems, flatness measurements on tables, monitoring of systems and structures, and many others.

Sensor

Specifications

(Valid

For Both

Measuring range (deflection of the vertical Al.5 4~5.2 Linearity error It(O.005 + 0.5% d.M.W.)l &(l + 0.5% d.M.W.) Resolution 0.001 0.2 Zero-point stability <0.005 <l Operating-temp. range -20 to $50 Storage-temp. range -30 to $60 10 to 95 Relative humidity Dimensions (LxWxH) ca. 90x90x63 Weight ca. 850 9-15 Supply voltage

Measuring

mrad or mm/m arc min mrad or mm/m arc set mrad or mm/m arc set mrad/K arc set/K â&#x20AC;&#x153;C â&#x20AC;&#x153;C % mm g V DC

Interfaces: Analog version Digital version

Sensitivity 1000 RS-232 or Serial interface RS-485 Baud rate2400; 9400; 19200

-187-

Axes):

mV/mrad

baud


2.3 Measurement

results

The graphs of two linearity

error measurements

tration.

were performed

The measurements

prototype

sensor, once for inclinations

once for inclinations without

shall be used for illus-

at room temperature

in the X direction

in the Y” direction

(Fig.

(Fig. 3). The tiltings

on a 2) and

were done

lateral inclinations.

The linearity

error and the absolute inclination

the vertical

and horizontal

-2.:

-1.5

-2

are indicated

in mrad on

axes, respectively.

-I

-.S

a .s B.006cab*1

1.5 2 25. Nolpunp4s nrld

1

Inclination

Inclination

in

m-ad

in X Direction

,.QS : c.

82

,

.

:.a[5

.

._

.

.

.

:

.p l .I

.01

It

: ’

‘;,a05 ca

.

, v

.

: *. .

.

.

.-

.

. .

._.

..___.

. .

.0E

.

.

.

-.

._

..;

‘..‘VYYVI

1,”

a

’ .

. .

+lTIYl’ .. .

. .

., .._....

i

._._....._....__

Ir . .

-.01

. 7 -. . Y

-815 v -.a

.

1

2' in

2.5 m-ad

. 3

-.Bz: -2

.’

-2

-1.5

-1

-.T-

h a.(rGn

Fig. 2 Inclination x

.5

1

CIb‘Y

1.5 t4olgur.p

Inclination

in mrad

in Y Direction

1 Tr. note: Probably stands for “des Mittelwert” (of the maximum value). -188-

(of the mean value) or “des Maximalwert”


3. Tests

With

3.1 General

A Two-Color

Distance

Discussion

of Two-Color

In the two-color

in the atmosphere

The d’ff 1 erent propagation result in two different

mulas (e.g., of Owens /2/) in the atmosphere

same formulas two-color

Distance

Measurement

method a distance is measured simultaneously

and blue light /l/.

and humidity

Meter

f or calculating

as a function

with red

speeds of the light waves

distance values. The basic forthe propagation

of wavelength,

speed of light

temperature,

pressure

are known for distance measurement

with one color. The

are used as the basis for calculating

the distance in the

method.

The reduction

simple if a common propagation

of a two-color

measurement

is very

path is assumed.

We have: D

=

Lred

-

A+h,e

-

‘bed)

with

A = Nred (hue - Nred)

D = reduced distance Lred’

Lblue

are distances measured with red

=

and blue light and calculated with the speed in vacuum

Nred’ Nbiue=

are the indices of refraction refraction

reduced by 1, calculated

by the formula of Owens (n - 1)

Since the refractive density,

index for both colors depends linearly

i.e., on the temperature -189-

on the air

and air pressure, the coefficient

A is


independent

of pressure and temperature

in first approximation.

ever, the water vapor causes another wavelength influence on the factor A is determined mixture

ratio of the atmosphere.

primarily

Differences

How-

dependence,

and the

by the variation

of the

of 1°C or 1 hPa cause a

distance change of about 0.001 ppm, while a change of the water-vapor partial

pressure by 1 hPa results in a distance change of about 0.1 ppm.

Due to the refraction

gradients

beam deviates upward

in the zenith

by about

/3/.

the blue light

10 cm at a distance of 30 km.

effect must be taken into consideration formula

direction,

in an exact two-color

This

distance

Th e neglect of this effect results in an error of <l

mm

(3.10W8) for a distance of 30 km. As is evident from formula the difference factor

between

(1)) th e atmospheric

“red”

A. Since the factor

and “blue”

correction

measurements

and from the

A in our setup (HeNe and argon lasers) is

about 34, the error in the difference measurement reduction

results from

due to the distance

of 0.1 mm is increased to 3.4 mm.

The first setup of a two-color

distance meter was intended to allow basic

tests of such a measuring system, to discover critical points and to create the foundations 3.2 Description In principle, resolution

for estimating

the attainable

of the Two-Color

Distance

accuracies. Meter

distance meters based on the FIZEAU and accuracy.

The objective

tem that can measure the red-blue

principle

offer high

was to develop a measuring

sys-

difference to an accuracy of about

0.05 mm at 15 km. This value corresponds to a distance uncertainty

of

1.5 mm (1 . 10 -7). FIZEAU

System

In the FIZEAU

system (see Fig.

once at transmission

4) a light

wave is modulated

and the second time at reception. -190-

twice:


tightde!ection

Fig. 4. Fizeau System in the Tuned State

As with

the Mekometer

frequency

polarization

The frequency

modulation

is shifted until

point or 0 phase). number

ME 5000, this system works with

the modulation

of the light in the 500 MHz range.

the light detection

is minimal

This means that at this frequency

of modulation

a variable-

wavelengths

there is a whole

in the measurement

crystal - reflector - modulation

(minimal

path between

crystal.

We have: 2 . D = Ic . modulation

wavelength

with modulation

wavelength

= c/f

K = number of mod.wavelengths f = modulation df = frequency The variation wavelength

of the modulation can occur only within

width. -191-

in 2 *D = f/df

frequency difference between 2 minima

frequency

and thus of the modulation

the limits

of the modulation

band-


Determination

of the Distance

To determine

a distance it suffices to measure the frequency

imal point (fine measurement) between two minimal especially

and to measure the frequency

points (coarse measurement).

(> 10 km), a somewhat

of a mindifference

At large distances

greater effort in the measurement

this frequency difference is necessary to avoid coarse measurement In our case, the determination

was made via “quasi”-

measured pairs of minima at a maximally simultaneous modulator

means alternating

band limits.

simultaneously “Quasi”-

at the lower and upper

After the measurement

the lower and upper band limits,

errors.

large frequency spacing.

measurements

of

of 5 adjacent minima

the “spacing”

at

of these groups is per-

formed by seeking out and measuring other minima at doubled frequency spacings until the middle of the band is reached. Thus, for a distance of 5 km another 12 measuring

points are obtained

until the overlap in the

middle is reached. Thus, a distance measurement “red” and 22 “blue” frequency measurements modulator

range. The individual

frequency

of 5 km consists of 22

distributed measurement

over the entire consists of an

averaging of 4 frequency values weighted with the corresponding

O-phase

deviation.

sequence

All measurements

of the two-color

distance meter are controlled

complete measurement System

and the complete measurement

as light

A

takes about 10 minutes.

Design

In a first test the setup shown in Fig. complete

by a laptop computer.

FIZEAU

5 was tested.

systems which have common

sources (HeNe 7 mW,

argon 5 mW).

complete systems lies in the continuous of the distance with two colors.

-192-

optics and two lasers The advantage

and simultaneous

The disadvantage

less stable difference of the mechanical-geometrical

It involves two

of two

measurement

lies in the possibly addition

constants.


â&#x20AC;&#x153;

9

cl

4.I;2 13-

.

*

1

7

1

6I

11

s+

1,

,\â&#x20AC;&#x2DC;I\ --===L---D sti 5

14

3

-TFig. 5 Block Diagram

(1) (2) (3) (4) (5) (6) (7)

of the Distance Meter with Two Modulators

(8) (9) (10) (11) (12) (13) (14)

Red laser (He-Ne) Blue laser (argon) Transmit/receive optics Reflector Red-blue splitter Modulator Pol. beam splitter

3.3 Results With

and Knowledge

this two-color

electronic

X/4 plate Light detector Power amplifier Synthesizer Lock-in detector Controller Laptop computer

Gained

distance meter built from two Mekometer

assemblies, a series of test measurements

distances between 200 m and 10 km on different erable inaccuracies

were found,

-193-

ME 5000

was performed days.

at

Some consid-

and the average errors reached values


on the order of 3-4 mm.

These results were basically

pected range, and the actual primary system and discovering tailed working

the critical

goal of investigating

crucial information

points was largely reached.

In the following

A de-

we report only on the

attainable

accuracy, the master oscillator

MHz quartz crystal) was replaced by a Hewlett-Packard it possible to perform

(10

synthesizer, thus

the phase-O tuning

in extremely

small

steps.

The maximum

measured distance resolution

was about 0.02 ppm at 5

km, i.e., 0.1 mm. When a “red” and a “blue” simultaneously,

atmospherically

order of magnitude By integrating resolution

determined

minimum short-term

could be found with an uncertainty

over a measurement

were measured drifts

of equal

of about 0.1 mm.

time of 5-10 minutes,

a maximum

in the red-blue difference of 0.05 mm can be envisaged.

means that the basic accuracy of a two-color mm, if an extrapolation

atmospheric

conditions

distance meter is about 1

in the measurement

unfortunately

is also again dependent on

parameters.

For distances over 10 km, the signal quality curacy can be obtained array of 3 x 3 reflectors

good, problems

necessary for optimal

only with correspondingly with

a diameter

right from the size standpoint.

of the previously

of large distances, calm

are necessary to achieve this order of magnitude.

Thus, the two-color instrument the atmospheric

This

factor of A = 22 can be assumed (see Equation

(1) ). However, especially

ficiently

the overall

for the user.

To estimate the maximum

frequency

the ex-

paper was produced with a number of points to be noted

in developing such an instrument.

making

within

However,

beams.

-194-

An

of 60 mm each seems about if the alignment

may arise for the red-blue

discussed divergence

large reflectors.

ac-

is not suf-

difference

because

between the red and blue light


A better solution rameter

/4/.

is the Cassegrain configuration

Besides treating

it also yields no additional 4. Precision

Distance

The Mekometer distance

Meter

known from the Ter-

the red and blue light waves identically,

polarization for Short

ME 5000 was originally

meter for the measurement

components.

Distances designed and developed as an exact

range of from 20 m to 8000 rn.

first experiences with the ME 5000 already showed that the potential in this instrument using a computer

far exceeds what is guaranteed

The

hidden

by its specifications.

By

to control the ME 5000â&#x20AC;&#x2122;s measurement

sequence, a certain

expansion of its measuring range and possible applications

could be achieved.

In the meantime, a functional principle.

the Development

Department

of KERN had begun to build

sample to be used to estimate the possibilities

of this measuring

For this reason, the results of a series of test measurements

available which serve as the basis for the development

are

of a precision distance

meter for short ranges. 4.1 Possible

Specifications

The objectives

as Basis

in building

oriented primarily

the mentioned

measurement

sample were

of an instrument

Only after the merger of WILD

KERN did the aspect of maximum

and measuring

functional

toward the characteristics

general geodesy market.

import ant again.

For Development

attainable

for the

LEITZ

with

accuracy become more

For this reason, with respect to instrument

size

time and also with respect to accuracy and shortest distance,

usual improvements

realistic

requirements

made in a developmental

a basis for development.

The most important

as well as a few key words indicating

that

far exceed the

step can be imposed as specification

how the technical

features solution

is

possible are listed below. The FIZEAU

principle

distance-determination

already

described

in Section 3.2 and the

solution applied in the Mekometer -195-

ME 5000


serve as the basis. Shortest

Measurement

The raising of the modulation GHz the modified programmability permit

Measuring

frequency

construction

and the special

drift for the zero-point

measurement

detection

of distances < 2 m.

< 5 Seconds:

Time

The time needed for the measurement speed of the synthesizer, surement algorithm pletely

from ca. 500 MHz to 1.3

of the modulator

of the frequency

unambiguous

< 2 m:

Distance

of a distance depends on the

the time for the signal integration,

and the computation

new synthesizer

circuit

in the range of 1 millisecond ingly short integration

the mea-

speed of the CPU. A com-

design with transient

buildup

times

and the test results with correspond-

times make the requirement

for a measuring

time of < 5 seconds seem realistic. 0.001

mm

Resolution

The special coupling only the relatively

Time

of the synthesizer

large frequency

speed, but also an extremely which corresponds

circuit

System:

makes possible not

steps that are necessary for high

small step on the order of 0.1 ppm,

to a distance

m. Under the assumption and taking

of the Measuring

resolution

of ca. 0.001 mm at 5

of an averaging of several measurements

into consideration

the influence of temperature

(0.2â&#x20AC;&#x2122; C

corresponds to 0.001 mm at 5 m), this value is surely sufficient. Measuring

Accuracy

~0.01

mm

The tests showed that the temperature X/4 plate with a semiconductor concerning

a better

optical

reflected light.

-196-

+ 0.1 ppm: compensation

laser works better.

isolation

by means of a Ideas also exist

of the sensor beam from the


The coupling with an exact angle-measuring

system,

the use of small reflectors and the instrument

size of 120 x 120

70 mm are other specifica-

x

tion features. 4.2 Results

of Test Measurements

A series of test measurements tional sample. Although

was performed

the controller

met only the simplest requirements,

with the available func-

software of this instrument informative

setup

results were obtained.

At ranges between 2 m and 200 m a large number of measurements performed

in which individual

wide variety of settings.

instrument

parameters

was

were tested at a

Figures 6 and 7 show two representative

results.

5. Summary The new NIVEL

20 and Laser Tracking System instruments

the field of instrument to a continuous The descriptions ter principle Another

improvement

technological

of instruments

of the possibilities

progress necessarily leads

and measuring methods.

for further

development

of the Mekome-

provide evidence that much more could be realized technically.

prerequisite

that corresponding Unfortunately,

development

document that in

for the beginning

of a new instrument

development

is

results can be expected on the economic balance sheet.

at this time there is no funding

presented here for the development

plan for the two possibilities

of distance meters, and so for the time

being there are no plans to implement

-197-

these projects.


I

Fig. 6 Continuous

Distance:

Measurement

5m

For 10 Minutes

30080 Measurements

23pm t

Fig. 7 Continuous

Measurement -19s

For S Hours


Selected literature: 1. Owens, J.C.:

Laser Applications

Ross M. Academic

in Metrology

Press, NY/London

2. Owens, J.C . : Optical

refractive

and Geodesy.

Published

by

(1971)

index of air. Applied

Optics, vol. 6, No. 1,

pp. 51-59 (1967) 3. Hfibner, DGK,

W. : On the use of dispersion Series C, No. 310, Munich

4. Huggett,

for electromagnetic

measurement.

(1985)

G.R. and Slater, L.E.: Recent advances in multiwavelength

measurement.

Proc.

Refr., Netherlands

Int.

Symp.

on EDM

Geod.

Comm.

Published

and the Influence by Richardus

distance

of Atmosph.

P, Wageningen,

pp. 141-152 (1977) 5. Meier, D. and Loser, R. : The M ek ometer ME 5000 - A new precision distance meter.

AVN, no. 5, May 1986

6. Meier, D. and Loser, R. : Experiments genieurvermessung

88, published

Verlag, Bonn Authors:

with a two-color

by Schnadelbach/Ebner,

R. Loser, D. D. Meier, Th.

AG Schachenallee CH-5001

AARAU

-199-

distance meter.

In-

Vol. 1, Diimmler

Scholian KERN

& Co.


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