Method and apparatus for eliminating an offset signal in an electrostatic digitizing tablet

• US 5,902,967 A
• Filed: 02/09/1996
• Issued: 05/11/1999
• Est. Priority Date: 02/09/1996
• Status: Expired due to Term

• Current Assignee: Avago Technologies General IP PTE Limited (Broadcom, Inc.)
• Original Assignee: LSI Logic Corporation (Broadcom, Inc.)
• Inventors: Teterwak, Jerzy A.
• Primary Examiner(s): Shankar, Vijay
• Application Number:
  US08/599,609
• Time in Patent Office: 1,187 Days
• Field of Search: 345/173, 345/174, 345/175, 345/179, 345/182, 382/315, 178/18.01, 178/18.03, 178/18.04, 178/19.02, 178/19.04, 178/20.02, 178/20.03, 178/20.04, 178/18.05, 178/18.06
• US Class Current: 178/18.01
• CPC Class Codes: G06F 3/044 by capacitive means
• CPC Class Codes: G06F 3/0443 using a single layer of sensing electrodes

Abstract

A method and apparatus for detecting when a second object touches a digitizing panel while a first object is touching the digitizing panel is disclosed. The method includes the steps of generating a first composite signal when the first object touches the digitizing panel, the composite signal including a first touch-down signal component and an offset signal component, converting the first composite signal to a first digital value indicative of an amplitude of the first composite signal, filtering the first digital value to filter out the offset signal component and to output the first touch-down signal component, generating a second composite signal when the second object touches the digitizing panel, the second composite signal including a second touch-down signal component and the first composite signal, converting the second composite signal to a second digital value indicative of an amplitude of the second composite signal, and filtering the second digital value to filter out the first composite signal and to output the second touch-down signal component.

Claims

1. A method for eliminating an offset signal in a digitizing panel, comprising the steps of:

contacting the digitizing panel with an object so as to generate a composite signal having an offset signal component and a touch-down signal component indicative of position of the object relative to the digitizing panel;

converting the composite signal to a direct current (DC) signal indicative of the composite signal; and

filtering the DC signal to filter out the offset signal component and to output the touch-down signal component to a signal line.

2. The method of claim 1, wherein the filtering step includes the step of:

passing the DC signal through a band-pass filter to filter out the offset signal component and to output the touch-down signal component.

3. The method of claim 2, wherein the band-pass filter is implemented as a second order band-pass filter.

4. The method of claim 1, wherein:

the converting step includes the step of generating a plurality of digital values to represent the DC signal, and

the filtering step includes the step of passing the plurality of digital values through a second order digital band-pass filter.

5. The method of claim 2, wherein the band-pass filter has an upper cut-off frequency and a lower cut-off frequency set to frequencies less than approximately 20 Hz.

6. The method of claim 2, wherein the band-pass filter has a transfer function of approximately:

##EQU2##

7. The method of claim 1, further including the step of amplifying the composite signal with a current-to-voltage amplifier, wherein:

the converting step includes the step of rectifying the amplified composite signal in order to obtain the DC signal.

8. The method of claim 1, wherein the contacting step includes the step of:

generating the touch-down signal component when a finger contacts the digitizing panel.

9. A digitizing panel comprising:

a resistive layer having an offset signal component flowing therethrough;

means for generating a composite signal when an object capacitively couples to said resistive layer, said composite signal including the offset signal component and a touch-down signal component indicative of position of said object relative to said resistive layer;

means for converting said composite signal into a direct current (DC) signal indicative of said composite signal; and

means for filtering said DC signal to filter out said offset signal component and to output said touch-down signal component on a signal line.

10. The digitizing panel of claim 9, wherein said means for filtering includes a band-pass filter for filtering out said offset signal component and outputting said touch-down signal component on said signal line.

11. The digitizing panel of claim 10, wherein the band-pass filter is a second order band-pass filter.

12. The digitizing panel of claim 10, wherein the band-pass filter is a second order digital band-pass filter.

13. The digitizing panel of claim 10, wherein said band-pass filter has an upper cut-off frequency and a lower cut-off frequency set to frequencies less than approximately 20 Hz.

14. The digitizing panel of claim 10, wherein said band-pass filter has a transfer function of approximately:

##EQU3##

15. The digitizing panel of claim 9, further including:

a signal rectifier coupled to said signal converting means; and

a current-to-voltage amplifier coupled to said signal rectifier.

16. The digitizing panel of claim 9, wherein the object is a finger.

17. A method for determining position of a second object relative to a digitizing panel while a first object is touching the digitizing panel, comprising the steps of:

generating a first composite signal when the first object touches the digitizing panel, the composite signal including an offset signal component and a first touch down signal component indicative of position of the first object relative to the digitizing panel;

converting the first composite signal to a first direct current (DC) signal indicative of the first composite signal;

filtering the first DC signal to filter out the offset signal component and to output the first touch-down signal component;

generating a second composite signal when the second object touches the digitizing panel while the first object continues to touch the digitizing panel, the second composite signal including the first composite signal and a second touch-down signal component indicative of position of the second object relative to the digitizing panel;

converting the second composite signal to a second DC signal indicative of the second composite signal; and

filtering the second DC signal to filter out the first composite signal and to output the second touch-down signal component.

18. The method of claim 17,

wherein the first DC signal filtering step includes the step of filtering the first DC signal with a band-pass filter;

and

wherein the second DC signal filtering step includes the step of filtering the second DC signal with the band-pass filter.

19. The method of claim 18, wherein the band-pass filter is a second order band-pass filter.

20. The method of claim 19, wherein the band-pass filter is a second order digital band-pass filter.

21. The method of claim 18, wherein the band-pass filter has an upper cut-off frequency and a lower cut-off frequency set to frequencies less than approximately 20 Hz.

22. The method of claim 18, wherein the band-pass filter has a transfer function of approximately:

##EQU4##

23. The method of claim 17, further including the steps of (1) amplifying the first composite signal with a current-to-voltage amplifier, and (2) amplifying the second composite signal with the current-to-voltage amplifier, wherein:

the first composite signal converting step includes the step of rectifying the amplified first composite signal, and

the second composite signal converting step includes the step of rectifying the amplified second composite signal.

24. The method of claim 17, wherein the first object is a first finger and the second object is a second finger.