Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
1999-03-18
2001-07-03
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S207150, C340S008100
Reexamination Certificate
active
06255812
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention pertains to a system for monitoring the distance between a first and a second magnetically conductive part which are movable relative to one another, in particular, between a knife of a rotating cutterhead and a shearbar of a harvesting machine with at least one permanent magnet and one induction coil for generating an electric signal corresponding to the distance, which is supplied to an evaluation circuit, as well as a forage harvester with such a device.
It is desirable to monitor the distance between cutterhead knives and a shearbar in a forage harvesters, because this distance strongly affects power consumption. Several possibilities for measuring the distance between cutterhead knives and a shearbar are known. For example, in U.S. Pat. No. 3,646,421 capacitive sensors determine the capacitance between the knives and shearbars. In such a system a high voltage is necessary and consequently, the knives and shearbars must be electrically insulation from each other. Also, the capacitance may vary due to conditions other than a change in the distance between knife and shearbar. For example, a change in the nature of the material to be cut, and/or a change in dampness, can lead to capacitance fluctuations.
In EP-A-0,291,216 an acoustic knock sensor is proposed. It records the mechanical contacts of knife and shearbar. In the process, the “knock signals” are converted into digital signals. In order to permit a parallel positioning of the terminal edges of the knife and the shearbar, the two ends of the shearbar can be alternately moved up to the knife.
Other methods are based on a current measurement, as shown, for instance, in EP-A-0,679,330. When a constant supply voltage is applied, a current flows through the shearbar and a motor device which changes the position of the shearbar, a contact between a knife and the shearbar can be detected on the basis of a change in the current amplitude, since the ohmic resistance changes in this case. In case the current amplitude lies above or below prescribed limits within a given time interval, the operation of the motor is interrupted. In keeping with DE-OS-3,345,749, it is also possible to perform the distance measurement on the basis of a voltage breakdown between the shearbar and the knives. For this purpose, a high voltage is necessary.
In DE-A3,535,902, an optoelectronic sensor of distance measurement is proposed.
Additionally, the use of inductive sensors is known. These operate in two known manners. In the first design, incipient eddy currents are measured on the moved object. Inductive fields of very low strength are involved here. In the second, and more frequently employed design, also used in the present invention, induction coils are placed on the shearbar, in which a voltage is induced if existing magnetic fields are interrupted by the mutual motion of knives and the shearbar, as is shown, for instance, in EP-A-0,072,016 and U.S. Pat. No. 4,198,006. The large magnetic fields required for this are generated by one or more permanent magnets which are housed in the shearbar. The magnetic fields generated by the permanent magnet or magnets must be considerably larger than the magnetic fields generated by the intrinsic magnetization of the knives. The signals are dependent on the gap width and the velocity with which the knives pass the shearbar. It is possible with electronic integration devices to obtain a signal dependent on this velocity and to trigger the proximity sensors individually, insofar as several proximity sensors arranged along the shearbar. With an appropriate electronic signal evaluation, it is in principle possible to monitor whether knives are missing or broken off.
A problem with such a known system is that the induction coils are housed in the shearbar and consequently become worn or damaged if the shearbar is damaged. The permanent magnet or magnets are normally recessed in the shearbar and are embedded in plastic in order to avoid magnetic short circuits. This plastic housing becomes an additional wearing part. The embedding of the permanent magnet(s) and sensor coil(s) in the shearbar further reduces the mechanical stability of the shearbar, since a correspondingly large depression must be milconnected in the shearbar. Finally, a deterioration of the quality of the permanent magnets has an unfavorable effect on the sensitivity of the distance measurement.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a system for monitoring cutterhead/shearbar distance wherein the induction coils are less subject to wear and damage.
Another object of the present invention is to provide such a system for monitoring cutterhead/shearbar distance wherein the shearbar has improved mechanical stability.
Another object of the present invention is to provide such a system wherein a deterioration of the quality of the permanent magnets has a reduced unfavorable effect on the sensitivity of the distance measurement.
Another object of the present invention is to provide such a system wherein which is less susceptible than conventional mechanical sensor devices to malfunction due to mechanical influences.
These and other objects are achieved by the present invention wherein a system for monitoring the distance between a shearbar and the knife of a rotating cutterhead includes permanent magnets and induction coils. The magnets are spaced apart from and exterior with respect to the coils. The permanent magnets create a magnetic field which extends into an area through which the cutterhead knives move as the cutterhead rotates. Movement of the cutterhead knives through the magnetic field causes a change in the magnetic flux density in area of the induction coil, and induces a voltage in the coil. This voltage depends on the velocity of the knife and its distance from the shearbar. In case several rows with knives are arranged along the shearbar, a separate signal depending on the mutual distance between the knives and the shearbar is provided via an evaluation circuit for each row and each knife. The separation between permanent magnet(s) and the coil has the advantage that the induction coil and electronic components directly connected to it need not be housed in the area of the cutting edge of the shearbar and are thus no longer subject to wear. For instance, the induction coil can be fastened to the shearbar bed on the side of the shearbar opposite to the cutting surface. In principle, several induction coils can be employed simultaneously, e.g., opposite each row of knives or one induction coil can be used for several magnets and knife rows.
The induction coil generates voltage signals or pulses which are applied to an evaluation circuit. The evaluation circuit includes an amplifier which can also be cast together with the induction coil so that there are no long connection wires and the effect of outside electromagnetic radiation is reduced. The evaluation circuit also separates the amplified signals into a high and a low frequency components and forms a quotient therebetween. In forming the quotient, either component can be divided by the other. The separation into high and low frequency signal components increases the sensitivity of the measurement of changes in the distance between knives and shearbar. While the low frequency signal component is received almost unchanged in case of changes of distance, the high frequency component is inversely proportional to the distance between knives and shearbar. It can therefore provide a precise and, in principle, accurate distance measurement. Furthermore, changes of the stationary magnetic field due to, for instance, a deterioration of the magnetic characteristics of the permanent magnet act by the same factor on both high and low frequency signal components, so that a more precise distance measurement becomes possible due to the quotient formation.
To insure that a sudden magnetization of the knives, for instance, in case of a collision, has no influence on the distance measurement, the intrinsic magnetization of the
Braun Markus
Schaefer Hagen
Voigt Burkhard
Deere & Company
Oda Christine
Zaveri Subhash
LandOfFree
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