Phj Piet Schellekens

Design for Precision: Current Status and Trends

Abstract ‘Design for Precision’ reviews the status quo in Precision Engineering and concludes that todays precision engineers put repeatability at the top of their list. The design rules, patterns or principles, quoted here from various authors, are all time-proven insights, to get reproducible results with ultra precision machines and instruments. Modelling and analysis of different concepts, systems, and components is required to adapt the progressing design or to confirm its adequacy. Expenditure on such analysis is worthwhile to avoid realisation of an inadequate design. However, creativity is more important in keeping the cost down by finding other than locally optimised solutions. World-wide, precision engineers agree on design principles, the challenge is to apply them creatively to obtain a thought-out design. In todays most accurate machines, advanced techniques are applied for compensation of e.g. residual geometric errors, errors caused by machine dynamics, or thermo-mechanically induced errors. Future developments in Precision Engineering require nanometre- or even subnanometre positioning-and measuring accuracy, demanding new design concepts with integrated control and error compensation systems.

Modeling and verifying non-linearities in heterodyne displacement interferometry

The non-linearities in a heterodyne laser interferometer system occurring from the phase measurement system of the interferometer and from non-ideal polarization effects of the optics are modeled into one analytical expression which includes the initial polarization state of the laser source, the rotational alignment of the beam splitter along with different transmission coefficients for polarization states and the rotational misalignment of the receiving polarizer. The model is verified using a Babinet Soleil Compensator allowing a common path for both polarization states and thereby reducing the influence of the refractive index of air. The verification shows an agreement of the model with measurements with a standard deviation of 0.2 nm. With the use of the model it is confirmed that the mean of two polarizer receivers can reduce the effect of non-linearity. However, depending on the accuracy of the polarizer angles, a second-order non-linearity remains. Also the effect of rotational misalignment of the beam splitter can not be reduced in this way.

Design of a High-precision 3D-Coordinate Measuring Machine

Abstract In Precision Engineering components are getting smaller and tolerances become tighter, so demands for accuracy are increasing. To improve the precision of Coordinate Measuring Machines (CMMs) we designed an alternative high precision 3D-CMM with measuring uncertainty beneath 0.1 μm in a measuring volume of 1 dm 3 . The machine design is based on the Abbe and Bryan principle, thus smaller measuring errors are feasible with less effort on software compensation. Application of a light and stiff construction, compensated air bearings and well-positioned linear motors result in high stiffness and favourable dynamic behaviour. A statically determined design, extensive use of aluminium and mechanical thermal length compensation make the machine less sensitive to temperature changes. To prevent mechanical disturbances an active vibration isolation system was designed. This paper focuses on machine design aspects showing analytical- and experimental results and design synthesis.

Assessing geometrical errors of multi-axis machines by three-dimensional length measurements

In this paper a method is presented for assessing geometrical errors of multi-axis machines based on volumetric three-dimensional length measurements. A universal machine error model is proposed since a large variety of machine configurations exists. Such models can be used for software error compensation techniques in order to improve the machine’s positioning behaviour as well as for diagnostic purposes. Length measurements are chosen for the measurement of the positioning errors of a multi-axis machine because these measurements can be executed in a short period of time in a relatively simple way combined with a high accuracy. In order to get comparable results for the geometrical errors as measured with conventional techniques, i.e., laser interferometry, the design of the measurement setup as well as the formulation of the machine error model (including parameter correlation effects) appeared to be of major importance and are subject of this paper.

Measurements of the Refractive Index of Air Using Interference Refractometers

Comparisons have been carried out between interference refractometers built in different countries. Individual measurements of the refractive index of air have been made using air from the same sample volume. Direct comparison of refractometers was realized by coupling the instruments to the same air-inlet system. In order to compare the measured results the refractive index was also calculated from accurate measured values of pressure, temperature, humidity and CO2 content, using Edlens formula. Most of the individual measurements show an agreement within 1 part in 107 while direct comparisons show an agreement within 5 parts in 108. Over a period of five days an increase in CO2 content of 400 ppm per day was measured in a 300 m3 laboratory room. The results show the great importance of correcting calculated results for the CO2 content of the air.

Development of a fast mechanical probe for coordinate measuring machines

To decrease inspection times of workpieces, not only do coordinate measuring machines (CMMs) need to operate faster, but also the mechanical probing process requires attention in this field. This paper shows that impact forces attributable to probing are much higher than generally accepted measurement forces, which can result in workpiece damage. Furthermore, it is shown that probe tip bouncing can slow down the probing process and requires mechanical damping to reduce it. Based upon these findings, a fast mechanical probe system has been developed, equipped with an optical measurement system able to measure six degrees of freedom at high speed. The probe system is suitable for high-speed, single-point measurements as well as scanning purposes. The measurement uncertainty of the prototype is approximately 1 μm for probing speeds up to 70 mm/s.

Compensation for dynamic errors of coordinate measuring machines

Owing to the demand for shorter cycle times of measurement tasks, fast probing at coordinate measuring machines (CMMs) has become more important and therefore the influence of dynamic errors of CMMs will increase. This paper presents an assessment of dynamic errors owing to carriage motion, aimed at error compensation. In the adopted approach the major joint deflections as a result of accelerations are measured with position sensors. Other joint deflections are estimated based on analytical modelling of CMM components. Using a kinematic model of the CMM, the influences of the measured and estimated joint deflections on the probe position are calculated. The dynamic errors can be corrected by software compensation, based on the calculated values. The approach has been applied to an existing CMM, using inductive position sensors for on-line measurement of the major dynamic errors. Experiments show that the compensation method is very successful, enabling fast probing without serious degradation of measurement accuracy.

Calibration of displacement sensors up to 300 µm with nanometre accuracy and direct traceability to a primary standard of length

A new class of sensor has recently appeared: nanometre sensors. These sensors are characterized by nanometre or sub-nanometre resolution and an uncertainty of a few nanometres over a range of at least several micrometres. Instruments such as capacitive or inductive sensors, laser interferometers, holographic scales, and scanning probe microscopes belong to the class of nanometre sensors. Linearity errors and drift in the mechanical and electronic system limit the accuracy of all these sensors. In order to determine these errors in a traceable way, the instrumentation described in this paper was developed. The heart of the system consists of a Fabry-Perot cavity. One mirror of this cavity generates the required displacement. A so-called slave laser is stabilized to the cavity length. The frequency of this slave laser is compared with the frequency of a primary length standard. In this way the displacement is measured with a resolution of a few picometres, a range of 300 µm and an uncertainty of about 1 nm. Experiments confirm the performance of this instrument and show typical deviations of the probe systems investigated.

Accuracy limitations of fast mechanical probing

Although dynamic errors of CMMs are getting more attention now, still little is published about the dynamic disturbances acting on mechanical probe systems. The process of mechanical probing is subject to dynamic influences, even at generally accepted low measurement velocities. It will be shown that due to the nature of the mechanical probing principle, impact forces are much higher than measurement forces and can result in considerable damage of workpiece surfaces. Further it is proved that the relation between the position of the probe tip and the workpiece position during impact cannot be determined unambiguously due to bouncing, irrespective of the probe construction. Consequently measurement accuracy can be lost, depending on the principle of probing. Analysis shows that this bouncing effect cannot be avoided during probing and therefore should be taken into account to improve probe performance. Measurement results obtained with two different probe systems will be presented to illustrate those impact phenomena. Based on these results recommendations for probe system design are given in order to speed-up probe operation without degradation of probe performance.

Design of a kinematic coupling for precision applications

To machine a complex precision product, several tools are needed. These tools are placed on a tool turret. A tool must return several times to its original position. To attain a very high repeatability between the upper part and the base of the tool turret mounted on a precision lathe, it is preferable that the parts of the tool turret are statically determined in their contacts. This is attained by using a kinematic coupling. To attain the required stiffness this coupling is provided with a preload of 1.5 · 103 N. The machining forces are typically less than 1 Newton. A special kinematic coupling, consisting of grooves and balls, was designed, made, and tested. By providing the grooves with self-adjusting surfaces, hysteresis is reduced to less than one-tenth of a micrometer. Maximum stiffness is aimed at by using cemented carbide, a material with a high admissible stress, at the contact points. Experiments show that this kinematic coupling, under a preload of 1.5 · 103 N, has a static stiffness of more than 1 · 108 N/m in every direction and a repeatability better than one-tenth of a micrometer.