Probing Systems

S Austin Moses

Coordinate Measuring MachineA coordinate measuring Machine is a device for measuring the

Physical geometrical Characteristics of an object.

Coordinate-measuring machines

include three main components:

A. The main structure which include

three axes of motion

B. Probing system

C. Data collection and reduction

system - typically includes a machine

controller, desktop computer and

application software2

Definition Of ProbeIt is the sensory part of a CMM responsible for sensing different

parameters required for the measurement. laser scanning probes or non-contact probes.

After placing a workpiece on the

machine table, a probe is used to

measure different points on it by

mapping the x, y, z coordinates.

The probe operates either manually via

an operator or automatically via a

control system.

These points are then uploaded to a

computer interface where they can be

analyzed using modeling software (e.g.

CAD) and regression algorithms for

further development. 3

CMM Operation and Controls – Four Main Categories

Manual drive CMM – human operator physically moves the probe and records x-y-z- data

Manual drive and computer-assisted data processing – can perform calculations to assess part features

Motor-driven CMM with computer-assisted data processing – uses joystick to actuate electric motors to drive probe

Direct computer control (DCC) – operates like a CNC machine tool and requires part program

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SensorsThe devices used to explore the area to acquire coordinates are the

"sensors".

Two types of sensors:

“Tactile” sensors.

“Noncontact” sensors or "optoelectronic" sensors

Tactile sensors distinguish:

“Rigid” sensors.

“Point to point” sensors.

“Continuous” sensors

In non contact sensors ,Two main principles are used:

Triangulation 3D

Time of Flight 5

SensorsPoint to Point Tactile Sensors

The point to point type of measurement determines the position of points by bringing in physical contact the tip of the sensors with the part and then retracting.

The point to point measurement represents a fast ideal solution to define Dimension and Position.

Probing (point measurement) sequences

Legend:

1......: Starting point.

2......: Point probing.

3......: Disengagement

after point probing

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SensorsContinuous Tactile Sensors

In the point to point measurement the sensor gets in contact with the part to be measured.

In continuous measurement the sensor remains in contact with the part follows its profile and measuring points according to pre-determined laws in a single measuring path.

Very accurate and relatively larger than point to point, the continuous sensors, can supply very complete information on the form of the measured feature.

Probing sequence in continuous measurement 7

Deviations from a Nominal Surface

Surface measurement can be displayed as the profile of the surface as indicated by the stylus trace or the average of the surface deviations during the trace

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CMM Mechanical StructureArticulated Arm

Kind of architecture: anthropomorphic robot.

Controls in articulated arms is manual.

Portable in physically moving the machine where the part to be measured is located.

Dimensional metrology operations on the welding fixtures for car body and relevant parts assembling.

Portability and easy handling.

“Articulated Arm” Architecture CMM “9

Articulation Or Fixed Sensors?Articulating heads are a standard feature on the majority of

computer-controlled CMMs

Heads are the most cost-effective way to measure complex parts

Flexibility - a single, simple stylus can access features in many

orientations

Indexing and continuously variable solutions

Speed - indexing is faster than stylus changing (done during CMM

moves)

Dynamic response - simple, light styli make for a lower suspended

mass

Costs

simple styli with low replacement costs

small, low cost stylus change racks10

Articulating Heads - Disadvantages

Space - a head reduces available Z travel by a small amount - can be an issue on very small CMMs

Slide 11

PH10MQ in-quill

version of PH10

indexing head

reduces Z travel

requirements

Fixed Sensors - Benefits

Compact - reduced Z dimension makes minimal intrusion into the measuring volume - ideal for small CMMs

Simplicity - fixed passive sensors are less complex for lower system costs

Stylus length - fixed sensors can be

larger than those fitted on articulating

heads, making it possible to carry longer

styli Articulating head

Fixed sensor

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Fixed Sensors - Disadvantages

Programming complexity - complex stylus clusters mean more attention must be paid to collision avoidance

DANGER!

Possible collisions with:

Component fixturing

Stylus change rack

Other styli in rack

Machine structure

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Scanning

Ideal for controlling the form or profile of known features that form functional fits with other parts

Data capture speeds of up to 500 points per second

Incurs wear on the stylus

Scanning allows you to:

Determine the feature position

Accurately measure the feature size

Identify errors in the form or shape of the feature

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Digitising

Ideal for capturing large amounts of data about an unknown surface

Uses many of the same techniques as scanning

Deflection vector of the probe is used to determine the motion vector in which the machine moves next

Digitised surface data can be:

Exported to CAD for reverse engineering

Used to generate a machining program for re-manufacture

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Ideal Applications

Measurement of size, position and

form of precision geometric

features

Measurement of profiles of

complex surfaces

High speed data capture - up to

500 points per second

Large volume of data gives an

understanding of form

Higher costs than basic touch-

trigger systems

Inspection of 3D prismatic parts and

known surfaces

Size and position process control

applications where form variation is

not significant

Slower data capture rate

Less information about the surface

Lower costs than scanning systems

Scanning Touch-trigger

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Why Change Sensors?Not all parts can be measured with one sensor:

Scanning probe

– ideal for features with functional fits

where form is important

–digitising contoured surfaces

Optical probes

– ideal for pliable surfaces

– inspection of printed circuit

boards

Touch-trigger probe

– ideal for discrete point

inspection, for size and

position control

–compact for easy access to

deep features

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Advantages Of Using CMMs Over Manual Inspection

Reduced inspection cycle time – translates to higher throughput rate

Especially with DCC, approximately 90% reduction in certain tasks

Flexibility – CMMs are general-purpose machines

Reduced operator errors in measurement and setup

Greater inherent accuracy and precision

Avoidance of multiple setups – in general all measurements of a given part can be made in one setup

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Inspection Probes On Machine Tools

Mounted on toolholders

Stored in the tool drum

Handled by the automatic tool-changer the same way cutting tools are handled

Inserted into the machine tool spindle by the automatic tool-changer

When mounted in the spindle the machine tool is controlled very much like a CMM

Sensors in the probe determine when contact is made with part surface so that required data processing is performed to interpret the sensor signal

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References

• Renishaw PLC, Metrology Division, New Mills, Wotton-under-Edge, Gloucestershire, UK, Technical Literature, available at <http://www. renishaw.co.in/en/> (accessed on May 15th, 2014).

• Mikell P. Groover, Inspection technologies. In: Automation, Production Systems, and Computer-Integrated Manufacturing,Third Edition, Lehigh University, Pennsylvania, USA.