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Sensors in production systems such as machine tools or robots may be classifiedinto four categories Figure 2-1.. Three types of sensors may be applied during the operation:those which me

Sensors in production systems such as machine tools or robots may be classifiedinto four categories (Figure 2-1) They are activated either during operation or inthe set-up phase Three types of sensors may be applied during the operation:those which measure kinematic values such as position, velocity, orientation or an-gular velocity, sensors which are applied to control the process in adaptive controlsystems, and sensors which are used to monitor the production systems and toprovide diagnostic functions to assure a high availability of the systems Sensorsfor process control functions are shown in Chapters 3 and 4 and will not be men-tioned here Sensors which are applied in the set-up phase are used after assem-bly to adjust or test the accuracy of the systems or to calibrate the interactingmembers of the kinematic chain.

Sensors for Machine Tools and Robots

H K Tönshoff, Universität Hannover, Hannover, Germany

Fig 2-1 Sensors in production systems

Copyright © 2001 Wiley-VCH Verlag GmbH ISBNs: 3-527-29558-5 (Hardcover); 3-527-60002-7 (Electronic)

sential importance with the introduction of position control loops for numerical trolled machines (Figure 2-2) [1] The applied electric and in a few cases hydraulic orpneumatic drives are not able to be positioned by themselves in a control chain butneed a feedback of a position signal An exception to these control loop-based drivesare the digital controllable stepping motors Their application is limited because ofaccuracy, dynamics, and power Thus every numerical controlled axis of a machine,tool, or robot that means every feed movement needs at least one position sensor.Since the feed movement in a machine tool, eg, the tool movement of a turning ma-chine, determines the accuracy of the machine, properties of the sensors namelytheir resolution, their repeatability, their drift velocity, and others (see Chapter 1.3)are of fundamental importance for the accuracy of a production system.

con-A sensor can be set either directly or indirectly Indirect means that the travel

or the position of a moved machine part such as a slide or a table of a machinetool or the arm of a robot are not directly measured but by means of a move-ment- transforming device (Figure 2-3)

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Fig 2-2 Control loop in numerical controlled machine tools

Fig 2-3 Placement of position sensors

This is normally a transformer from rotational to translatory movements for ear moving slides such as a ball screw (see Figure 2-2), a pinion-rack, a screw-rack, a roll-band, or a wheel-track device Strongly speed-reducing transmissionssuch as harmonic drives, worm drives, and others are used for rotational movingtables or arms Because of the high speed reduction, the demands on the accu-racy of a rotational sensor are much higher if it is placed behind the transformerthan before it Indirect sensing in general has the advantage that there is no needfor cost-effective measuring devices so that simple and reliable seals can be used.

lin-It has the disadvantage that errors of the transmission system are introduced inthe measured quantity These can be, for instance, thermal or elastic deformations

of the ball screw or geometric and kinematical aberrations of the transmissionsystem in robots (Figure 2-4)

Therefore, the direct measuring principle should be used if high accuracy andsmall aberrations are required, eg, for radial positioning in grinding or turningmachines On the other hand, it has to be considered that indirect measurementvery often gives a better chance to follow Abbe’s principle in machine tools (Fig-ure 2-5) This principle demands that the probe, in this case the travel of the ma-

Fig 2-4 Direct and indirect position measurement

Fig 2-5 Abbe’s principle for machine tools

chine component, and the scale for measuring the travel be in alignment, wise errors can occur by non-orthogonality and by tilting effects It is possible toreach the necessary alignment for indirect measuring systems approximatelywhereas the direct measuring system is usually placed parallel to the slide and isthus sensitive against tilting errors.

other-Sensors can be separated in accordance with the kind of signal into analog anddigital position-measuring devices An example of an analog system is the voltagedivider (Figure 2-6) The sensor is applied for limited relative resolutions It can

be based on a resistance wire or a vapor-deposited layer of carbon The resistance

to be 0.1 mV of a 10 V maximal voltage That means a relative resolution of 10–5

or a positional resolution of 1lm limits the maximum travel to 100 mm

The digital sensor determines the position either by counting increments tal-incremental measurement) or by reading coded numbers (digital-absolute mea-surement) (Figure 2-7) The resolution is given by the width of the incremental

(digi-unit The length of measurement l and the incremental widths are connected by

where n is the number of bits necessary to describe the maximum length with a

resolution ofs For the example of s=1 lm and l=250 mm,

This means that 18 bits have to be implemented in the counter of an incrementalsystem or 18 channels have to be incorporated in an absolute system The incre-mental measurement has the disadvantage that the position can only be deter-mined relatively The digital absolute sensor, on the other hand, has the disadvan-tage that the system needs a large number of channels for a long measuringlength and a high resolution and is therefore expensive All three mentionedkinds of measurement, the analog, the absolute-digital and the incremental-digitalprinciple, have their specific advantages Therefore, the idea of combining favor-able abilities is not unreasonable This leads to the cyclic-absolute measuring prin-ciple It takes advantage of the absolute character of the analog system applying itonly on limited measuring lengths and of the incremental sensing principle withits simple structure and robustness It is shown schematically in Figure 2-8.

Fig 2-7 Digital measuring principle

Fig 2-8 Principle of cyclic-absolute position measurement

The resolver is an absolute-cyclic measuring system It is based on the inductiveprinciple and measures angular or rotational movements The resolver is often ap-plied in an indirect measuring system It is basically a transformer consisting ofthree windings (Figure 2-9) The stator has two windings whose active directionsare exactly placed at 908 The rotor carries a third coil, the secondary system.High-frequency voltages are acting at the two stator systems which are 908 electri-cally phase shifted:

U1ˆ ^U0sinxt

U2ˆ ^U0cosxt

They induce corresponding voltages in the secondary coil which is rotated againstthe stator bya:

Ui1 ˆ k ^ U0sinxt cos a

Ui2 ˆ k ^ U0cosxt sin a

where k is the coupling factor of the transformer The two voltages U i1 and U i2

are added to give

Comparing the phase between U1 and U i 1,2, the searched for anglea can be rectly determined A phase comparison can be made by a phase-locked loop In

di-another kind of implementation the rotor system is supplied with UR= ^U sin !t.

The induced voltage in the stator coils is modulated by the angle of rotationa due

to the spatial arrangement:

In this implementation the quotient of the stator- induced voltages is calculatedby

Us1

The searched for anglea is determined by an arctan algorithm This is called theratiometric method

Resolvers are supplied with alternating current of high frequency, hence thespace requirement can be minimized An upper limit is given at 0.4–1 kHz be-cause of the iron within the transmitter The resolution might reach 1.5·10–3 de-grees, but the accuracy of the sensor is mainly determined by the manufacturingaccuracy, which influences the costs substantially The resolver is therefore mostlyapplied for less critical resolutions where it is fairly inexpensive

The resolver principle is also used for linear sensors The inductosyn® sensor

is basically a resolver which is straightened in the plane It is a very common plied cyclic absolute measuring device (Figure 2-10)

ap-Similarly to the resolver, the scale or the reader can be supplied with a quency alternating current (120 kHz) The signal processing methods are thesame If the alternating voltage is applied to the scale the following voltage is in-

high-fre-duced in the reader with coupling constant k:

UR1ˆ k ^ Ussinxt cos2spx

because the reader includes a longitudinal phase shift of s/4 The position x can

therefore be determined, for instance, by the ratiometric method Compared withthe resolver, the resolution can be 1000 times higher The measurement is analogwithin the domain of the division of 2 mm or 0.1 in The signal is repeated cycli-cally The divisions have to be counted or resolvers have to be applied additionally

Fig 2-10 Principle of the inductosyn ® sensor

to provide the coarse measurement Inductosyn®devices are available in modules

of 250–1000 mm They can be serially mounted The assembly has to be very curate to avoid errors at the joints It is usually made by interferometric means

ac-A digital-incremental sensor is shown in Figure 2-11 The information is givenonly relatively The impulses have to be counted A reference point must be given,for instance, by driving to a micro limit switch when starting the measurement.These sensors often work by the optical principle The divisions are applied onglass scales by vapor deposition The sampling can work by direct or transmittedlight The principle is explained by the transmitted light method Fine lines areapplied on the glass scale The transparent and black lines can be equal in width.Divisions of 10lm are used in practice The width is limited because of the wave-length of the applied light A scanning reticle is moved along the scale with thetable whose position or travel is to be measured Using a scale with several divi-sions means an increase in the light energy which is received by the photo detec-tor

The width ratio of the transparent to the non-transparent slots can vary Using

a narrower sampling slot (d  s=2), the light intensity at the receiver is stronger.The receiver gives almost a rectangular signal (Figure 2-12; left)

The properties of this principle are:

· the received light energy is comparatively low;

· the signal is of digital nature;

· it is only appropriate for coarse divisions;

· the information is gained by counting the signal impulses

According to another principle, the non-transparent and the transparent sectionsare equal in width The photo receiver delivers a value-continuous signal (Fig-ure 2-12 right) The properties are:

· the signal is of analog nature and the resolution is determined only by the nal-to-noise ratio;

sig-· the received light energy is comparatively large;

· a further increase in resolution is possible

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Fig 2-11 Digital-incremental sensor

The value of the continuous signal can be used for further improvement of the solution as can be seen from Figure 2-13 Instead of the direct shaping of one im-pulse, several impulses are gained from the triangular signal by a comparisonwith threshold values, a kind of interpolation.

re-The Moiré effect can be used for scanning glass scales (Figure 2-14) re-The scaleand the reader are tilted against each other by a small angle Moiré’s stripes aregenerated using the through-light method These stripes move with much higherspeed than the scale A photo receiver delivers a sine signal of the moving Moiré’sstripes A sine and a cosine signal are received if two photo receivers which aredisplaced by one quarter of the period of Moiré’s stripes are applied Information

on the position can be obtained with high accuracy by an electronic interpolationunit The two signals are amplified by defined factors and added according to thefollowing equation:

Fig 2-12 Influence of sampling slot on photosignal

Fig 2-13 Increase of resolution for incremental sensors

Several phase-shifted signals are achieved by this algorithm, which can generate

a high resolution of the sensor after an impulse shaping operation (Figure 2-15).The measuring step can be 1/20 or 1/200 of the division period, which is an inter-polation factor up to 1 : 100 A resolution of 50lm can be reached by such incre-mental sensors

The resolution is limited without further means by the scale division or the mal which is applied for digital measuring systems The required accuracy forhigh-precision machine tools is <0.5lm It is difficult to produce scales with suchdivisions with the necessary accuracy One measure to overcome this limitation isthe application of interpolation methods One increment can be divided electroni-cally into an integral number of partitions

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Fig 2-14 Scanning with Moiré stripes

Fig 2-15 Magnification of division for incremental sensors

The number of impulses is counted in incremental systems A discrimination

of the moving direction is necessary for this method, otherwise vibrations alreadyexisting between the scale and the reading device can falsely indicate a move-ment Figure 2-16 shows an electronic discriminator Impulses are taken from anincremental scale by a reading device This is, for instance, a scanning reticlewhich consists of a lamp and two photo diodes (a, b) The signals are shiftedagainst each other by s/4 Thus the information of direction can be deduced Awiring diagram is given in Figure 2-16 The signal of diode b is differentiated, in-verted and compared with signal a If the original signal b' is equal to a, the coun-ter goes forward; if b is equal to a it counts backward Only the positive parts ofthe b' or b signals are relevant in the device

The time deviation of travel or distance can be used to measure speeds or cities indirectly Direct measurements of speeds are possible using the inductionprinciple, when moving a magnet against an electric coil This is an electrody-

velo-namic speed sensor (Figure 2-17, left) The induced voltage U is

Fig 2-16 Direction discriminator Source: Herold, Massberg, Stute

Photodiodes

The time deviations of travel or speed may be used to measure accelerations

in-directly The linear acceleration a can also be determined by the reaction force F

of a known mass m according to

The piezoelectric principle is commonly used for this purpose Figure 2-17 (right)shows an accelerometer which contains a piezoelectric element sensitive to shear.Other sensors use the piezo effect for normal stresses, ie, in a cantilever or a rod.Such sensors can be built to small sizes and thus have high limit frequencies up

to 100 kHz The measuring domain may range from 10–2 to 106 m/s2 The sors can be designed in a triaxial manner to determine acceleration components

sen-in the space

2.2

Sensors for Orientation

The orientation of a machine tool or robot component is given by the angle of therelevant direction to a reference plane To determine orientation means measur-ing the inclination angle

The autocollimator is an optical sensor which is used for measuring small angles

of inclination It works like a telescope (Figure 2-18) using the collimator lens twice.From a light source a beam is focused in the plane of an ocular scale The beam ismade parallel in the collimator lens (telescope adapted to infinity) It is reflected by ameasuring mirror which is perpendicular to the plane to be measured If the mirror

is inclined by a small anglea this generates a shift x of the ocular scale image The

measured anglea and the shift x are independent of the distance between the limator lens and mirror because of the parallel light between them The shift x

col-can be transformed to an electrical signal by a linear CCD camera High-precisionautocollimator sensors work with an accuracy of 10–7(0.1lm over 1 m) They areused to measure the straightness of machine tool guideways or planeness of tables

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Fig 2-17 Measurement

of speed and acceleration