Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
2000-11-28
2003-02-25
Strecker, Gerard R. (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S207160, C331S1170FE, C331S167000, C455S041300
Reexamination Certificate
active
06525530
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to electronic circuits for driving the transmitter winding of an inductive position transducer.
2. Description of Related Art
Inductive position transducers are widely used to measure relative displacements between one or more receiver windings and one or more windings or disrupting elements that modulate the inductive coupling between the receiver windings and a transmitter winding. In various conventional inductive position transducers, such as those disclosed in U.S. Pat. No. 6,005,387 to Andermo et al. and 6,011,389 to Masreliez et al., each incorporated by reference herein in its entirety, a lower power, intermittent drive circuit is used to supply a time-varying drive signal to the transmitter windings. In the 389 and 387 patents, the intermittent drive circuit discharges a capacitor through the inductor formed by the transmitter winding. This causes the transmitter winding to “ring”. That is, the current released by connecting the charged capacitor to ground through the inductor formed by the transmitter winding and a serially-connected resistor oscillates and exponentially decays.
This circuit provides a clean sinusoidal signal having a single fundamental frequency that is directly dependent on the inductance of the transmitter winding. However, to use this decaying ringing signal, the peak amplitude of the largest peak in the signal must be carefully sampled to be able to accurately determine the relative position between the receiver windings and the disrupting elements and/or coupling loops. Moreover, because the ringing circuit quickly decays, only a single sample can be taken of this signal each time the capacitor is charged and then subsequently discharged through the inductor formed by the transmitter winding.
In contrast, in various other conventional systems, the transmitter winding is continuously driven. U.S. Pat. No. 4,737,698 to McMullin et al. discloses a system that uses a continuously driven inductive transducer. For example, the 698 patent discloses a power oscillator that runs at a frequency of 10 kHz to 1 MHz. This low frequency range indicates that the load inductance on the power oscillator is large. As is well-known in the art, large load inductances, and therefore large load impedances, are easier to drive than inductive transducers having small inductances, and therefore small impedances.
As disclosed in the 698 patent, a single capacitor can be connected in parallel with the transmitter winding to form a resonant tank circuit that increases the impedance. This is shown, for example, in FIG.
9
. However, the 698 patent indicates this is optional, suggesting that for the transmitter windings disclosed in the 698 patent, the impedance need not be specifically tuned to resonate at the oscillation frequency, and/or that inductance of the transmitter winding need not participate in determining the oscillation frequency. The 698 patent also discloses that the parallel capacitor is located at the transmitter winding.
However, the 698 patent does not provide any suggestion of the location of the power oscillator, implying that the location of the power oscillator is not critical. Since a power oscillator located remotely from the transmitter winding must drive relatively unpredictable wiring impedances in addition to the circuit elements at the transmitter winding, this again suggests that for the transmitter windings disclosed in the 698 patent, the impedance need not be specifically tuned to resonate at the oscillation frequency and/or that inductance of the transmitter winding need not participate in determining the oscillation frequency.
In yet other various conventional systems, the inductive position transducer is incorporated into a readhead, such as those used in hand-held calipers, linear scales and other position transducing systems that measure distances to relatively high accuracy and resolution.
FIG. 7
shows a block diagram of the transducer, signal processing circuit and transmitter driver of one such conventional position transducer
600
. As shown in
FIG. 7
, a program microcontroller
610
, which includes program memory and RAM, a calibration memory
670
and a gate array
680
are connected to a data bus
695
. The gate array
680
is connected to and controllably drives a transmitter driver
685
. The transmitter driver
685
is connected to a dual-scale transducer
620
over a pair of drive signal lines
686
and
687
.
The dual-scale transducer
620
includes a first scale having a first transmitter winding and a first set of receiver windings and a second scale having a second transmitter winding and a second set of receiver windings. The first set of receiver windings are connected over the signal lines
622
to an input multiplexer
630
, while the second set of receiver windings are connected over the signal line
624
to the input multiplexer
630
. The input multiplexer
630
selectively connects the first or second receiver windings to a synchronous demodulator
640
over a pair of signal lines
632
and
634
. The synchronous demodulator
640
synchronously demodulates the induced signal in the first or second set of receiver windings generated by continuously driving the first or second transmitter winding. The synchronous demodulator
640
outputs the synchronously demodulated received signal over a signal line
642
to an amplifier and integrator
650
.
The amplifier and integrator
650
amplifies the synchronously demodulated received signal and integrates it to generate a position signal corresponding to the relative position between the set of receiver windings used to generate the synchronously demodulated receiver signal and either or both of a set of disruptive elements or a set of coupling windings. The amplifier and integrator
650
outputs an amplified and integrated position signal over a signal line
652
to an analog-to-digital converter
660
that converts the analog signal to a digital signal. The digital signal is then output over the databus
650
to the microcontroller
610
. The microcontroller
610
analyzes the digital signal to determine a relative position for the inductive position transducer
620
.
This relative position is then output over the databus
695
to the gate array
680
. The gate array
680
then outputs the position signal, either in quadrature form or as a numeric value, to the input/output interface
690
. The input/output interface
690
then outputs the signals to a signal line
699
, which can be connected to a display device for displaying the numeric value of the position signal or to a control system, such as a numerically-controlled machine tool, that uses the quadrature signals as control signals.
FIG. 8
shows one exemplary embodiment of a digital drive circuit
700
that imposes a square wave on an impedance-adjusted serially-connected inductive-capacitive circuit
720
. In this case, the inductor of the serially-connected inductive-capacitive circuit
720
is formed by the transmitter winding
122
of the transducer
620
. This is shown in
FIG. 8
for a digital drive circuit that is used to drive the transmitter winding
122
of the transducer
620
, using an oscillating power source
710
that is connected between ground
702
and the impedance-adjusted serially-connected inductive-capacitive circuit
720
. In particular, the impedance-adjusted serially-connected inductive-capacitive circuit
720
comprises a capacitor
750
connected in series with the first transmitter winding
122
between the output of the oscillating power source
710
and ground
702
. The digital drive circuit
700
shown in
FIG. 8
relies on frequency discrimination provided by this impedance-adjusted serially-connected inductive-capacitive circuit
720
to convert the square wave imposed on the impedance-adjusted serially-connected inductive-capacitive circuit
720
into an approximate sine wave.
FIG. 9
shows a second exemplary embodiment of a digital drive circuit
700
that imposes a square wave on an impedance-adj
Jansson Bjorn
Nahum Michael M
Mitutoyo Corporation
Strecker Gerard R.
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