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Electronic Distance Meters inside

Trimble’s Geospatial Instruments

Christian Graesser, Martin Koehler, Danderyd, Sweden

1st Workshop – Metrology for Long Distance Surveying @IPQ Portugal – 11 /2014

Trimble Danderyd, Sweden

Overview

Christian Graesser Metrology for Long Distance Surveying @IPQ Portugal – 11 /2014 3

Three different distance meter technologies

developed and produced at Trimble

Calibration traceability

TOF Pulse accumulation digitizer (Trimble Danderyd, Sweden)

Long Range DR, fast

High accuracy

Synchronous

Laser class 1

Technology 1

S6

DR-plus S8 VX

DR-plus

RTS Construction SPS Machine control

Phase Shift (Trimble Jena, Germany)

Medium range DR

Visible laser

Highest accuracy

Technology 2

S8 HP S3

Focus30/35 Focus8 Christian Graesser Metrology for Long Distance Surveying @IPQ Portugal – 11 /2014 5

TOF Single pulse digitizer (Trimble Paris, France)

Medium Range

Super fast

High Accuracy

Clean data

No Post processing

Technology 3

Scanner TX8


Technology 1

TOF Pulse accumulation digitizer

The “Time of Flight” principle

Pulsed laser versus ambient light

Accumulation of signal

Digitization of the signal

S6

DR-plus

S8 VX

DR-plus Christian Graesser Metrology for Long Distance Surveying @IPQ Portugal – 11 /2014 7

Technology 1

TOF Pulse accumulation digitizer

The „Time of Flight“ Principle

Time difference between transmitting and receiving the pulse

Distance proportional to the “travel”- time t of the laser light

ct

D 2

Transmitter

Receiver

Difficult to see red laser pointer dot under sun light

(1mW)

Our pulsed laser is in the peak

10000x stronger than a red laser

pointer => better visible for the

receiver electronics

Laser still class 1 because in average

the laser power is small.

(only 4 ns on and 40000 ns off)

Pulsed laser


Setup

IR pulsed laser transmitter

905 nm

Laser class 1

25KHz

pulse rate


Laser

Transmitter

Receiver

APD

t

Fiber Optics Target

Mirror

Setup

Reference Pulse Measure Pulse


Length of a Pulse

1.2 m at FWHM

Pulse length: 1.2 m = 4 ns


Determining a Laser Pulse

Trigger

SHI SHI SHI SHI

SHI = Sample & Hold Integrate

Analogue signal accumulation

To ADC


Double Sample & Hold

Channel

Processor

Trig Channel A

Trig Channel B

SH1 SH2 SH3

SH1 SH2 SH3 SH4

SH4

APD APD 25 KHz 1 KHz

8 channel

A/D-

converter

1 KHz

20 Hz

Distance Christian Graesser Metrology for Long Distance Surveying @IPQ Portugal – 11 /2014 14

The Sample & Hold Circuit

Reference pulse is measured permanently

alternating with channel A and B

Measure pulse with the circuit of last reference

pulse measurement

Reference pulse

Measure pulse

A B A B A B A B

t A B

Advantages

Better accuracy

Laser has no warming up time

No break by internal reference measurement

Continuous measurement possible Christian Graesser Metrology for Long Distance Surveying @IPQ Portugal – 11 /2014 15

Digitizing

T = 200 ps

Polynomial

f=f(t)

psnTn 200

Reference pulse

Measure pulse

Reference

Polynomial

f = f(t) dt2

mdtTnc

D 2

dt1

dt3

dt4


Key Performance

Range Prism 5300 m

Range Grey Card 1000 m

ISO 17123-4 1mm 2 ppm

Synch 20 Hz

Pulse accumulation digitizer

Technology 2

Phase Shift Distance Meter

Setup

The Trimble concept of phase shift EDM


Transmitter

Visible red laser

coaxial

Receiver

coaxial

Phase Shift Distance Meter


Phase Shift Technology

Red semiconductor laser with 660 nm

7 modulation frequencies around 400 MHz

They contribute all to the fine measurement

All derived from one TCXO (15 MHz)

400 MHz corresponds to fine scale of 0.375 m

Ambiguity solved with difference frequencies of

fine scales

Unambiguity range: 7.3 km

According to ISO 17123-4 0.8mm + 1ppm


Technology 3

TOF

Single pulse

digitizer


Trimble TX8

Key Specs Value

Technology Time-of-flight, waveform digitization

Laser wavelength 1550nm

Laser class Class I

Accuracy < 2mm

Precision (1σ) < 2mm for [18%,90%] reflectivity up

to100m

Range Standard: [0.6m, 120m]

Extended: up to 340m

Pulse rate 1MHz, 3 minutes 138 Mpoints

Scanning speed 1M pts/s for standard range

0.4M pts/s for extended range

Scan grid width 0.37 mrad = 18 mm/50 m


Challenges

Scanning at 1M pts/s with time-of-flight technology

requires to measure distances with single shot

– on a broad variety of materials: from dark coal to shiny pipe

– with different incidence angles


Compensated distance calculation at 1 MHz –

No postprocessing!

1. Search the pulse

2. Calculate distance

3. Apply calibration

=> Distance

=> Luminance

Distance calculation

Uncompromised performance: TX8 can acquire over 80% of

points on surfaces of 18% to 90% reflectivity to within 2mm of accuracy

up to 100m with 1M pts/s speed


Example – Rodin Museum in Paris

• Historical building

• Purpose of scanning: renovation preparation

• 230 stations in total: exterior + interior

• 1 day with 2 operators / scanners

• Data processing and registration during field operation


Overview


Three different distance meter technologies

developed and produced at Trimble

Calibration traceability

Calibration Traceability Trace-back process

Zero constant Measurements at PTB Braunschweig

Scale factor Calibrated frequency counters

Accuracy of measuring tools in production

Corrections Tools Accuracy

Scale factor Frequency counter ≤ 0.01 ppm

Cyclic error Calibration bar ≤ 0.05 mm

Zero constant Nominal distances

and

Interferometer

2 σ = 0.30 mm


Trimble Danderyd Interferometer

31

Instrument on the

Interferometer Stand

Laser interferometer Track


Measurements at the PTB Braunschweig

Interference comparator: 50 m (nominal distances 2σ = 0.1 mm)

Mechanical adaption: calibrated (2σ = 0.1 mm)

Measurement at required sampling points

Forward backwards

each: 5 shots (2σ = 0.1 mm)

Several reference instruments

Trace-Back Procedure


Scale Correction

Calibration of frequency counters

Accredited calibration laboratory

or GPS-time standard

accuracy 1x10-8 (0,01ppm/year)

monitoring frequency counters by DCF77 time

signal, GPS- or Rubidium time standards

Frequency correction in production

TCXO frequency: 2 Hz of 15 MHz ≈ 0,13ppm

Tra

ce

-back p

rocess

C

alib

rati

on


Phase Shift Time of Flight

Frequency continuously f0 = 400 MHz Pulse frequency fP = 25,3 kHz

Photo detector fG ~ 500 MHz fG ~50 kHz

Fixed division ratio f0 / fTCXO = 26,7 fP / fTCXO = 0,0013

Input sensitivity 25 mV 20 mV

Continuous zero crossing measurements

External Scale Calibration

60 10

f

ffppm


1. Measuring reference baselines (PTB) with

reference instruments at predefined sampling points

Forward-backwards, 5-times, final result 2 = 0,2 mm

2. Transfer via reference instrument onto baselines in

production

(Range 2,5 – 50m; accuracy 2 = 0,3mm)

3. Measuring zero constant for each instument

4-6 prism distances, 5-times

4. Verification

Several prism distances, several DR distances

Zero Constant

Tra

ce

-back p

roced

ure

C

alib

rati

on


36

1955

Long Range

Capability:

Geodimeter 2.

45 minutes

measuring time.

Range 50 km

(31 miles)


Thank you

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