CMMs, Measurement, Encoders Probes, Flashcards

Question

What is used for Laser Based Measurement?

Answer 1

* The laser must be incorporated into a system of optical components known as an INTERFEROMETER. * There are many different designs of interferometer, but one of the best known is the MICHELSON interferometer

Answer 2

- A fixed reference reflector - A moving measurement reflector - A measurement of the environment

Answer 3

- A more common approach for optical encoders, uses a phenomenon known as the Moiré Effect. - This phenomenon is observed when two regular patterns (usually linear gratings) are placed in contact, and almost in alignment. - In most applications, there is usually a fixed grating, variously described as either the reference, master or index grating. - This is used to form moiré fringes with a distorted or displaced specimen grating.

Answer 4

- Two gratings in near contact. - Projection of one grating onto another by an optical system. - Projection of one grating onto another via an optical medium (mirror, liquid)

Answer 5

D = d/θ “D” is the separation of the Moiré fringes. “d” is the grating interval. “θ” is the angle between the rulings of the two gratings, and is assumed to be small.

Answer 6

A single detector will indicate the extent of movement, but not the direction of movement. * Ideally a minimum of two detectors are used, spaced preferably one quarter fringe pitch apart, giving rise to output forms in quadrature. * This leads to a lead or lag in electrical phase of the sensor signal, which may be used to determine direction of movement. * Typically four photocells or photodiodes will be used for fringe counting.

Answer 7

* Describes the magnification of angular error over distance. * The largest error component is quite often the Abbé Error. * This error could typically introduce 1 micrometre per 200 mm offset, for each arc-second of angular change.

Answer 8

Only measure using one degree of freedom at a time

Answer 9

CMMs can be prone to a large range of systematic and random uncertainties. The purpose of calibration and the traceability chain is to provide confidence in the CMM output.

Answer 10

One data point does not tell us everything we need: - Measurement value - Reliability of the measurement - Measurement trend

Answer 11

- Mean, Median, Mode... - Statistical methods - Probability based methods - Linear/non-linear regression - Least squares methods

Answer 12

Repeatability is the variation in measurements taken by a single person or instrument on the same item and under the same conditions. * Reproducibility is the variability induced by the operators. It is the variation induced when different operators (or different laboratories) measure the same part. * Reproducibility also takes into account longer timescales and the possibility of external influences such as the environment. * Testing of R&R is an influential tool to help assess the quality of a gauging operation, across a range of different metrology tools and types. * A common approach is to implement a Gauge R&R study, which specifically tests the repeatability and reproducibility of a process.

Answer 13

The Gauge R&R is performed by measuring parts using the established and calibrated measurement system. * The aim is to capture as many sources of measurement variation as possible, so they can all be assessed and addressed. * To assess repeatability errors, the same part is usually measured several times by one operator. * To assess reproducibility errors, multiple operators are needed. The specification for the number of operators will be dependent on the company/laboratory, and may vary between 2 or 3, and 10. * To many companies, a %GRR under 10% is acceptable, 10% - 30% is marginal, and over 30% is unacceptable.

Answer 14

* This philosophy has changed the concept of “acceptable” quality from parts per hundred, to parts per million, or even parts per billion. * In statistical terms, typical levels of performance range between two and four sigma. * Three sigma identifies a defect rate of 66,810 per million components produced, or a defect rate of 6.68%. * Six sigma identifies a defect rate of 3.4 per million components produced, or a defect rate of 0.00034%.

Answer 15

If a measuring instrument is calibrated with respect to a traceable calibration standard (known value) then the experimental error can be deduced. This value may be stated or more likely used as an adjustment for the measurement data: - A micrometer is found to have an error of +20 microns Readings could be: 20.480mm + 0.020mm or: 20.500mm * If the micrometer suffers from backlash in the thread form which is random in its presence, then this may be estimated as an uncertainty: - Readings would be: 12.740mm +/-0.050mm