Position detector
First Claim
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1. A position detector comprising:
- a position indicator comprising a resonant circuit which includes at least a coil and a capacitor;
a plurality of loop coils which are parallel to position detecting directions; and
a tablet for detecting coordinate values of a position indicated by the position indicator on the basis of the levels of reception signals generated at the loop coils when electromagnetic waves at a predetermined frequency are transmitted/received relative to the position indicator;
wherein the tablet comprises;
transmission/reception control means for consecutively performing transmission/reception of electromagnetic waves relative to the position indicator at least twice while the same loop coil is being selected;
phase difference control means for controlling a phase difference of the signals in each of transmission/reception when transmission/reception of electromagnetic waves is consecutively performed at least twice;
a synchronous detector circuit for detecting synchronization by using a signal obtained by controlling the phase of the reception signal generated at the loop coil as a detection clock; and
signal level processing means for computing the average or the sum of the levels of at least two reception signals which are detected by the synchronous detection and which are based on transmission/reception of electromagnetic waves which is performed at least twice.
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Abstract
A position detector includes a position indicator. Transmission/reception of electromagnetic waves relative to the position indicator is performed at least twice while the same loop coil is being selected. With a counter circuit and a switch circuit, a phase difference less than or equal to one cycle of a resonant frequency fO is added to a signal in each transmission/reception. The phase of a reception signal generated at the loop coil is adjusted, and the phase-adjusted signal is used as a detection clock by a synchronous detector circuit to perform synchronous detection. By computing the average or the sum of the results, external noise in the reception signals is cancelled out.
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Citations
16 Claims
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1. A position detector comprising:
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a position indicator comprising a resonant circuit which includes at least a coil and a capacitor;
a plurality of loop coils which are parallel to position detecting directions; and
a tablet for detecting coordinate values of a position indicated by the position indicator on the basis of the levels of reception signals generated at the loop coils when electromagnetic waves at a predetermined frequency are transmitted/received relative to the position indicator;
wherein the tablet comprises;
transmission/reception control means for consecutively performing transmission/reception of electromagnetic waves relative to the position indicator at least twice while the same loop coil is being selected;
phase difference control means for controlling a phase difference of the signals in each of transmission/reception when transmission/reception of electromagnetic waves is consecutively performed at least twice;
a synchronous detector circuit for detecting synchronization by using a signal obtained by controlling the phase of the reception signal generated at the loop coil as a detection clock; and
signal level processing means for computing the average or the sum of the levels of at least two reception signals which are detected by the synchronous detection and which are based on transmission/reception of electromagnetic waves which is performed at least twice. - View Dependent Claims (2)
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3. A method of detecting a position of a resonant circuit with respect to a sensor comprising the steps of:
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providing a sensor having a plurality of loop coils for detecting coordinate values of a position indicated by a resonant circuit as a function of reception signals generated at the loop coils when electromagnetic waves are transmitted to and received from the resonant circuit;
sending a first signal from the sensor to the resonant circuit, said first signal having a frequency and a first phase angle;
receiving a second signal from said resonant circuit in response to said first signal;
sending a third signal from the sensor to the resonant circuit, said third signal having the frequency and a second phase angle;
receiving a fourth signal from the resonant circuit in response to said third signal; and
averaging said second signal and said fourth signal to cancel noise having frequency components close to said frequency. - View Dependent Claims (4, 5)
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6. A method of detecting a position of a resonant circuit with respect to a sensor comprising the steps of:
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a) providing a sensor having a plurality of loop coils for detecting coordinate values of a position indicated by a resonant circuit as a function of reception signals generated at the plurality of loop coils when electromagnetic waves are transmitted to and received from the resonant circuit;
b) detecting the approximate position of the resonant circuit by sending a preliminary signal to said resonant circuit from each of the plurality of loop coils and detecting the strength of the signal returned by the resonant circuit in response to each of the preliminary signals;
c) selecting a subset of the plurality of loop coils at which the signal received from the resonant circuit in response to the preliminary signal has a strength exceeding a predetermined level;
d) sending a first signal from a selected one of the subset of the plurality of loop coils to the resonant circuit, said first signal having a frequency and a first phase angle;
e) receiving a second signal from said resonant circuit in response to said first signal;
f) sending a third signal from the selected one of the subset of the plurality of loop coils to the resonant circuit, said third signal having the frequency and a second phase angle;
g) receiving a fourth signal from the resonant circuit in response to said third signal; and
h) averaging said second signal and said fourth signal to cancel noise having frequency components close to said frequency;
i) repeating steps d through h for each of the remaining loop coils in said subset of loop coils; and
j) identifying a loop coil from said subset of loop coils for which the average of said second signal and said fourth signal satisfies a given criterion.
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7. A position detector comprising:
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a position indicator including a resonant circuit;
a sensor comprising a plurality of loop coils for detecting coordinate values of a position indicated by the position indicator as a function of reception signals generated at the loop coils when electromagnetic waves at a predetermined frequency are transmitted to and received from the position indicator;
wherein the sensor comprises;
a transmitter for sending signals having the predetermined frequency and a phase angle to said resonant circuit;
a receiver for receiving signals from said resonant circuit;
a controller for controlling said phase angle; and
a processor operably associated with said receiver for computing the average of at least two out-of-phase signals transmitted by said transmitter so that noise having frequency components similar to the predetermined frequency is substantially canceled. - View Dependent Claims (8, 9, 10, 11, 12, 13, 14, 15, 16)
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Specification