Position transducer

Details Position transducer Position transducer

1999-06-28

2003-10-07

Smith, Zandra (Department: 2877)

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

C250S231180, C250S231160

Reexamination Certificate

active

06630659

ABSTRACT:

TECHNICAL FIELD
The invention relates to position and/or velocity transducers for providing signals in regard of the position or velocity of a movable part, in particular of a rotor of an electric motor or generator.
1. Background
There is an increasing need for stiff and compact servo motor systems. To be stiff, a servo motor system must be able to give a fast response to large and unexpected external torques. A fast response requires a very high resolution and a very low delay in the speed signal.
Analog electrodynamic tachometer generators offer a solution providing high resolution but it is difficult to build them so as to give a low delay. The generator adds a not negligible inertia to the rotor. The torsional stiffness in the mechanic coupling of the tachogenerator to the motor rotor is limited, and this gives a resonance frequency that limits the possible bandwidth of the servo motor control loop. Torsionally very stiff designs require mechanically very compact assemblies of the motor and the tacho generator. However, high resolution tacho generators should not be attached close to the motor as the stray flux from the motor windings might affect the output voltage of the tacho generator in the case where the two units are located too close to each other. Tachos also adds considerable cost and increase the size of the motor system.
Transducers that detect or sense rotary or linear motion by using at least two periodic primary signals having a phase difference in the order of magnitude of 90° such as a resolver or an optical incremental encoder have been used for many decades of years. Resolvers and synchros can be made with very high resolutions but the costs of high resolution units are very high. The time delay from a motor speed change to the corresponding change of the detector output signal is not negligible since the torsional stiffness of mechanical coupling of the resolver to the motor rotor is limited, and this gives a resonance frequency that limits the possible bandwidth of the motor control loop. Torsionally very stiff designs require mechanically very compact assemblies of the motor and the resolver. High resolution resolvers should however not be mounted close to the motor as the stray flux from the motor windings might affect the output voltage of the resolver in the case where the two units are located too close to each other.
Incremental transducers are commonly used to give angular position information. By reading position data repetitively at known time intervals, an approximate value of the velocity can be obtained. The primary signal from optical incremental transducers are normally processed in one of two ways. In the first way the analog primary signals are compared to a reference level thus converting the basic sinus signal to a square wave binary signal that is fed to a counter chain that is readable from a computer.
In the second way, the signal is fed to a “multiplying” network that digitalizes the data based on the assumption that the analog signals have a constant amplitude and are sinusoidal. Such converters can for example convert one period of a sinusoidal input signal to 5 or 100 periods of the square wave output signals.
At low speeds, the limited resolution of a digital incremental encoder gives very high quantification errors in the speed estimate. With standard decoding electronics that gives 4 count pulses per period, even a 5000 line encoder gives only 20000 positions per turn. At a relatively high speed like 15 rpm and a test interval of 200 &mgr;s, the change in position is approximately 15/60*20000*0.0002=1 unit, which due to quantification errors can give either 0, 1 or 2 units as a speed estimate input signal to a control algorithm.
The resolution problem can be reduced by using encoders comprising more lines or by using interpolation circuits that generate for example 100 counts for every period of the basic encoder signal. Hardware interpolation requires a very high signal quality; the amplitude of the primary signals must be constant over one full turn and over time, and the shape of the primary signals must fit the assumptions for which the multiplier circuit has been designed, and they must do so over the full turn and over time. The linearity of these converters depends on the linearity of the primary sinus signals, and much work has been invested in different ways to obtain very linear output signals.
Both high line count encoders and encoders having a signal quality suitable for high factor interpolation put stringent demands on the light source and the mechanical properties of the encoder system. Even a 5000 line encoder normally operates with a 20 to 30 &mgr;m gap between encoder disc and receiver mask pattern. Incremental encoders suitable to give a high speed resolution therefore require encoder discs having separate bearings. Such arrangements add length and cost to the motor system.
2. Prior Art
Position and velocity transducers suitable for rotating parts are for instance disclosed in the patent documents DE-A1 27 11 593, DE-A1 35 27 128, DE-A1 38 13 754, DE-A1 39 14 557, DE-C2 39 01 546, U.S. Pat. Nos. 4,990,767, 4,794,251, 4,580,047, 4,580,046, JP-A 57-169611, JP-A 63-6418.
The German patent document DE-C2 32 39 108 for Dr Johannes Heidenhain GmbH discloses a device having more than two signals of different phases, for example signals having phase differences of 0°, 30°, 60°, 90°, 120° and 150°. Improved two phase signals can be produced by multiplying these six primary signals by factors obtained from a fourier analysis of the shape of the six primary signals and by adding/subtracting these multiplied values to obtain two new signals having a phase difference of 90° to each other.
The European patent application EP-A1 0 541 827, also for Dr Johannes Heidenhain GmbH, discloses several arrangements for obtaining a more sinusoidal shape of the signals generated by an incremental encoder by having a pitch of the encoder mask transparent and opaque lines that is different from the pitch of the transparent and opaque lines of the encoder disc.
The company Max Stegmann GmbH has in 1994 introduced a system named SINCOS. The linearity of the sinus signals is claimed to be improved by using photodiodes in a triangular pattern. The pattern of the photodiodes replaces the encoder mask. To reduce the influence of contaminated encoder discs, of unavoidable deviations from an ideal, flat encoder disc surface and variations of the illumination LED due to temperature and age, the two output phase signals are sampled simultaneously at regular intervals. The sampled data are squared and added; the sum should give a constant value, as k(sin
2
&phgr;+cos
2
&phgr;) should be k regardless of &phgr;. Deviations from the expected value k is interpreted as a fault in the light source and is fed back by adjusting the LED current.
Position detectors that gives a high resolution position information at discrete time intervals, for example once every 100 &mgr;s, are quite sufficient for a servo motor controller system that can use this information for controlling the motor torque to obtain the position/velocity profile required. Motor controllers are often used in more complex systems where other devices are dependent on obtaining precise position information using signal protocols and/or requiring data at times not synchronised with the update time of the position detector. There is therefore often a need for transforming periodically available high resolution data to a data stream compatible with the two phase signals obtained from a classical digital output position encoder. The European patent application EP-A1 0 414 953 discloses such a system which however has a very poor resolution of the two phase output signals.
SUMMARY
It is an object of the invention to provide a position/velocity transducer that has a very high resolution.
It is another object of the invention to provide a position/velocity transducer that has very small dimensions.
Another object of the invention is to provide a position/velocity transducer that

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