Patent Application
20090021243
The invention relates to an arrangement for inductive speed detection in accordance with the preamble to patent claim 1. Such arrangements are known for instance from Offenlegungsschrift DE 10 2005 029 764 A1. In a first embodiment thereof the sensor element is embodied as a Hall sensor element. Hall sensor elements are very suitable for detecting variations in a magnetic field and are therefore frequently used for speed detection. Hall sensor elements are inexpensive to acquire and can also be used at temperatures of up to about 160° C.
Hall sensors are disadvantageous in that they are sensitive to dirt particles. In the worst case soiling can lead to the sensors failing entirely.
In accordance with DE 10 2005 029 764 A1, the sensor element can alternatively be embodied as a magneto-resistive (MR) sensor element. The sensor is inductive in another alternative embodiment. In principle inductive sensor elements are also well suited for detecting variations in a magnetic field.
In the known inductive speed detectors, wound coils are used that, in addition to high material expense, are also associated with the definite disadvantage that they are on average several centimeters deep.
The following table lists a few publications that are representative of Hall sensors and inductive sensors that have been used in the past:
Known from the pamphlet “Digital Inductive Position, Speed, and Direction Sensor PO120”, pages 1 through 3, is a digital inductive sensor for linear position determination, angle measurement, and speed measurement having tooth-shaped pulse wheels. The measurement coils are flat and resistant in oil, water, dust, and in magnetic fields. According to the transformer principle, the sensor chip includes a transmitter coil and two receiver coils. The receiver coils are offset in the direction of movement of the pulse wheel (of the target). The possible distance between target and sensor chip is also limited in this arrangement.
The object of the invention is to overcome the problems of the prior art, i.e. to provide an arrangement for inductive speed detection that is not sensitive with respect to soiling by particles, that enables shallow installation depth, and that permits a great distance to the generator.
The tacho generator should also withstand high temperatures and high pressures such as e.g. in a transmission.
In accordance with claim 1, the basic solution is comprised in the use of two planar coils for measuring speed. The two coils are arranged such that they are spatially offset in the direction of rotation. Detection by means of the coils occurs via a moved component, a so-called pulse wheel. The prerequisite for this pulse wheel is that it can periodically influence the inductive sensor. The structure of the pulse wheel can either resemble that of a toothed wheel (see
If, in a special case, the issue does not involve detecting the direction of rotation, a single planar coil can also be adequate for determining speed, as provided in coordinate claim 12. All of the advantages that accrue from the oscillation frequency evaluation of the associated oscillator also apply in this case.
A large effective coil surface area is obtained using the special arrangement of the coils 1 and 2 in
The coil 1 (and the coil 2, if any) are preferably printed on a printed circuit board, but can also be stamped or can comprise curved wire, etc.
In order to prevent the two coils 1 and 2 from influencing one another, they are switched through a multiplexer such that at all times current is flowing through only one coil and the other coil is highly resistive during this period. In the inventive speed sensor the evaluation of frequency changes strongly contributes to supporting detection, even given great distances. Detection under these conditions represents a substantial advantage. With known arrangements in the past it has only been possible to attain a distance of no more than 4 to 5 mm between sensor and pulse wheel. With the inventive arrangement it is even possible to work at a distance of up to 10 mm.
Thanks to the great distance, the pulse generator (pulse wheel) can be exchanged as desired within an application. In the prior art there was the problem that when using smaller generator wheels (pulse wheels) the sensor did not provide any more usable results due to the greater distance. Combining planar coils and frequency evaluation counteracts this disadvantage.
Rotational direction detection is also provided in a particularly simple manner when the advancing of a tooth or lattice element (at least the passing of a pulse-originated segment of the wheel circumference) is first perceived as a pulse on one flat coil and then on the other flat coil. In
In the second solution in accordance with claims 12 through 21, this detection of the direction of rotation is not provided with nothing further, but can be added in a manner alternative to the first solution, specifically if the pulse wheel is coded in the sense of claim 13. Coding means that the individual teeth of the pulse wheel can be differentiated. Then use for detecting the direction of rotation in accordance with claim 22 is possible.
The following additional technical features apply for both solutions:
At room temperature (20 degrees Celsius), let the base frequency be 20.0 MHz. The pulse temporarily increases this oscillation frequency to 20.2 MHz; i.e., the pulse causes a short-term 0.2 MHz change in frequency (which can be detected easily and can be associated with one tooth).
Although the same experiment at a transmission temperature of 150 degrees Celsius results in an increased base frequency of 24.5 MHz, it provides a constant 0.2 MHz change in frequency to 24.7 MHz now. Thus, using the invention it is possible to associate a constant change in frequency (i.e. a temperature-stable and distance-compensatable change in frequency) to each pulse-initiating element. Such pulses can be easily and reliably counted in the evaluation logic element.
When the pulse wheel is coded, the frequency-increasing pulses in the same sense are constant (that is, temperature-stable and distance-compensatable); they differ as well only from tooth to tooth in step shape or in their temporal change. The evaluation circuit can then also localize each individual tooth successively, i.e. can detect the rotational direction and/or the rough angular position from this individualization.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2007 033 745.2 | Jul 2007 | DE | national |
