WO2008039924A2

WO2008039924A2 - Touch switch with reduced susceptibility to electrical interference

Info

Publication number: WO2008039924A2
Application number: PCT/US2007/079725
Authority: WO
Inventors: David Malcolm Howie; Jeffrey Patrick Rudd
Original assignee: Illinois Tool Works Inc.
Priority date: 2006-09-27
Filing date: 2007-09-27
Publication date: 2008-04-03
Also published as: WO2008039924A3

Abstract

An electrical touch switch (10) employs an excitation signal (16) encoded with a predetermined pattern (30) to improve discrimination against electromagnetic interference (42).

Description

Touch Switch With Reduced Susceptibility To Electrical Interference

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR

DEVELOPMENT

[0001] --

[0002] CROSS REFERENCE TO RELATED APPLICATION

[0003] This application claims the benefit of U.S. provisional application 60/827,163 filed September 27, 2006 and hereby incorporated by reference.

[0004] BACKGROUND OF THE INVENTION

[0005] The present invention relates to electronic touch switches and in particular to a touch switch suitable for use in an environment with high electrical interference. [0006] Electronic touch switches provide contacts controlled not by a mechanical switch operator, but by an electrical signal coupled to a touch pad. An advantage to such touch switches is that the touch pad need not move and thus may be easily sealed from environmental contamination, for example, behind a glass or plastic surface. Because the touch pad does not move during actuation, it is also resistant to the mechanical wear of normal switch operators. [0007] A common type of touch switch detects a capacitive coupling between the touchpad and the person's finger. Capacitive coupling may be monitored by measuring a decrease in an electrical signal (detection signal) at the touchpad when that signal is coupled out of the touchpad (single antenna design). Alternatively, capacitive coupling may be monitored by measuring an increase in a detection signal coupled into the touchpad from the person, the detection signal, in this case, received by the person from a separate source or antenna (dual antenna design).

[0008] One problem with touch switches is that they are susceptible to electrical interference of sufficient strength to couple into the touchpad or its connecting circuitry without the intervention of a human operator. Such interference can trigger the touch switch or, in some cases, prevent the touch switch from triggering. Both of these situations will be termed "false touches".

[0009] The problem of false touches is particularly acute in the automotive environment where high-voltage electronic ignition signals and radio equipment including cell phones may create substantial radio-frequency interference. When the touch switch is used in this environment, for example, to control turn signals or windshield wipers, false touches may activate automotive equipment without warning, presenting a dangerous distraction to the driver.

[0010] SUMMARY OF THE INVENTION

[0011] The present invention provides a touch switch that encodes the detection signal with a predetermined pattern. The touch switch decodes the detection signal and rejects any signal that does not match the predetermined pattern. By changing the length of the coding pattern and the manner of encoding (e.g. phase, frequency, or amplitude) electrical interference may be rejected to a desired level of certainty.

[0012] Specifically then, the present invention provides a touch switch having a sending unit including an antenna providing a local radio-frequency field modulated by an encoding pattern and a touch plate accepting a touch from a human operator to receive the local radio -frequency field as coupled, at least in part, from the human operator. A decoder attached to the touch plate demodulates the local radio-frequency field to extract a received pattern. A comparator then compares the received pattern to the encoding pattern to provide a switch signal when the received pattern and the encoding pattern provide a predetermined match. A switch element having electrical terminals controls the flow of electricity between the terminals based on a state of the switch signal.

[0013] It is thus a feature of an embodiment of the invention to provide a touch switch having an arbitrarily high resistance to electrical interference. By increasing the length and complexity of the encoding, the chance of random electrical interference causing a false touch may be reduced to an acceptable level.

[0014] The encoding pattern may be expressed in the spacing of the pulses (phase), the frequency of the pulses, or the amplitude of the pulses.

[0015] Thus it is a feature of an embodiment of the invention to provide for different and possibly multiple methods of encoding the detection signal.

[0016] The sending unit may communicate with the decoding unit to allow for time sensitive decoding.

[0017] Thus it is a feature of an embodiment of the invention to provide for phase locking between the transmitter and receiver to further improve the rejection of electrical interference. [0018] The received pattern and encoding pattern represent a set of discrete data bits, wherein a comparator produces the switch signal both as a function of the bits in the encoded pattern having no corresponding bits in the received pattern and the bits in the received pattern having no corresponding bits in the encoded pattern.

[0019] Thus it is a feature of an embodiment of the invention to provide a switch system that is robust against broad-spectrum continuous electrical interference by evaluating not only missing pulses but also additional pulses that are not part of the encoded pattern.

[0020] The touch plate may be covered by an insulator and receive the local radio-frequency field through capacitive coupling to the human operator.

[0021] It is thus a feature of an embodiment of the invention to provide a switch allowing capacitive coupling such as allows a protective layer to be placed over the touch switch.

[0022] The sending unit may repeatedly send the encoding pattern and the comparator may provide the switch signal only after a predetermined number of received patterns and encoding patterns match.

[0023] It is thus another feature of an embodiment of the invention to allow the amount of rejection of electrical noise to be easily adjusted.

[0024] The encoding pattern may employ multiple signals of the same frequency and different phase.

[0025] It is thus a feature of an embodiment of the invention to provide a very simple pulse encoding system allowing narrowband filtering of the received signal for additional noise immunity.

[0026] The fundamental frequency of the radio-frequency field may be greater than 1000 Hz.

[0027] It is thus a feature of an embodiment of the invention to provide a band of operation removed from conventional power line interference frequencies.

[0028] These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention.

[0029] BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Fig. 1 is a block diagram of common touch switch topology used in the prior art and the present invention having a separate touch pad and antenna for producing the detection signal; [0031] Fig. 2 is a block diagram of the circuitry driving the antenna and touch pad showing a microcontroller receiving a detection signal from a pulse generating circuit which also communicates with a transmitter and receiving the received signal from a receiver, and operating to compare the two;

[0032] Figs. 3a and Fig. 3b are plots of the signal generated by the pulse generating circuit and a corresponding received signal having electrical interference;

[0033] Fig. 4 is a flow chart of a program executed by the microcontroller of Fig. 2 in discriminating between touches and false touches; and

[0034] Fig. 5 is a fragmentary view of an alternative embodiment of the circuitry of Fig. 2 using multiple frequencies of transmission.

[0035] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT [0036] Referring now to Fig. 1, a touch switch 10 provides a sending unit 12 having an antenna 14 for transmitting a local radio-frequency field 16 which may be received by a human operator 18 acting as an antenna.

[0037] When the operator 18 touches a touch pad 20, a portion of the received radio-frequency field 16 may be conducted via conductor 22 to a receiving unit 24. Control circuitry 26 communicating with the receiving unit 24 controls a current through contact terminals 28 as a function of any signal from the receiving unit 24.

[0038] The control circuitry 26 may implement a "normally closed" logic in which the touching of the touch pad 20 will "open" the contact terminals 28 blocking current flow there between, or a "normally open" logic in which a touching of the touch pad 20 will close the contact terminals 28 allowing current flow there between. Touch pad 20 can have an insulator over the top and receive the radio-frequency field 16 entirely by capacitive coupling.

[0039] Referring now to Fig. 2, in the present invention, the sending unit 12 includes a pulse generator 30 which produces a pulse train 32. In a first embodiment, referring also to Fig. 3a, the pulse train 32 is a set of irregularly spaced pulses 36 repeating in successive encoding cycles 34. For example, encoding cycle 34 may include a first pulse 36a and second pulse 36b having irregular spacing within the encoding cycle 34 such that space between pulse 36a succeeding pulse 36b is different from the spacing between pulse 36b and succeeding pulse 36a of the next encoding cycle 34. In this embodiment, the pulses 36a and 36b may also have different amplitudes, for example, with pulse 36a being a given percentage lower in amplitude than pulse 36b. It will be understood, in this example, that pulses 36a and 36b have the same frequency determined by the frequency of encoding cycle 34, but have different phases and amplitudes.

Thus, data is encoded in the pulses 36 in the form of phase and amplitude.

[0040] Referring again to Fig. 2, the pulse generator 30 communicates the pulse train 32 to a radio-frequency transmitter 40 which may, for example, modulate a radio-frequency signal 43 of

1 kHz to 5 kHz (in the preferred embodiment) to provide bursts of radio-frequency signals through antenna 14. Alternatively the pulses 36 may be transmitted without modulation and provide a broadband radio-frequency output.

[0041] The local radio -frequency field 16 transmitted by the antenna 14 may be received by the touch pad 20 via a human operator 18 (not shown in Fig. 2) together with electrical interference

42. The combined radio-frequency field 16 and electrical interference 42 are then received by receiving unit 24.

[0042] The receiving unit 24 may include a filter 52 to provide band limiting filtration of electrical interference outside of the frequencies of transmission of local radio-frequency field 16 and an amplifier 54 to provide a power boost to the faint received signals. The boosted and filtered signal provides received signal 44 which is received by an analog to digital converter incorporated into a microcontroller 58.

[0043] As shown in Fig. 3a the received signal 44 will generally have some pulses, for example, the pulse 46a and 46b corresponding to pulses 36a and 36b in time and relative amplitude 38.

Note the absolute amplitude of pulses 46a and 46b will normally not match absolute amplitude of pulses 36a and. 36b as a result of the unpredictable attenuation between the antenna 14 and touch pad 20; however, their relative amplitudes will largely be preserved. The received signal

44 will also contain artifact pulses 48 resulting from the electromagnetic interference 42 as well as missing pulses 50 caused by destructive interference between the interference 42 and the radio-frequency field 16.

[0044] The signal from the pulse generator 30 is also received by an analog to digital converter of the microcontroller 58. The microcontroller 58 executes a stored program 60 to compare the received signal 44 and the pulse train 32.

[0045] Referring now to Figs. 3a, 3b and 4, the program 60 of the microcontroller 58 monitors each encoding cycle 34 as triggered by the pulse generator 30 to first determine whether there is a timing match between the received signal 44 and the transmitted pulse train 32 as indicated by decision block 62. This timing match involves checking to see if there is a corresponding pulse 46a and 46b for each pulse 36a and 36b of one encoding cycle 34, and that the energy of the pulses 46a and 46b exceed by a predetermined amount the energy of artifact pulses 48 within the encoding cycle 34. If not, the program 60 loops to wait for the next encoding cycle 34. By providing the pulse train 32 from the pulse generator 30 to both the transmitter 40 and to the microcontroller 58, absolute phase locking may be had in the detection process so that phase shifted pulses 46 may be more easily rejected.

[0046] If there is a timing match at decision block 62, the program 60 proceeds to decision block 64 where the relative amplitude of the pulses 36a and 36b are compared. If the relative amplitude of pulses 46a and 46b do not match the relative amplitude of pulses 36a and 36b to within a predefined criterion, the program loops to wait for the next encoding cycle 34. [0047] If there is a match at decision block 64, an internal counter is incremented at process block 66. Then at decision block 68, it is determined whether a counter limit has been reached, for example, three, indicating that in three successive encoding cycles 34 the received signal 44 and the transmitted pulse train 32 have matched. If so the program proceeds to process block 70 and an output signal is set.

[0048] Referring to Fig. 2, this output signal may activate either by turning on or off a solid-state switch contact 72 connecting contact terminals 28.

[0049] Referring again to Fig. 4, decision blocks 62 and 64 may in an alternative embodiment be used alone, for example, with regularly spaced pulses 36 having different amplitudes used with decision block 64 or irregularly spaced pulses 36 having identical amplitudes used with decision block 62.

[0050] Referring now to Fig. 5, in an alternative embodiment each of the pulses 36 may represent a different frequency and the receiving unit 24 may provide multiple output signals 56a-56c each indicating the receipt of a different frequency. In this case the pulses 36 may be encoded with different frequencies rather than different amplitudes or different phases. [0051] It will be understood that the number of pulses 36 within an encoding cycle 34 may be increased to increase discrimination against electromagnetic interference 42. Generally, the frequency of modulation of the pulses will be such as to avoid the principal frequency of electrical interference, for example, near 60 Hz for home appliances. When multiple touch switches 10 are used in a given application, each of the different touch switches 10 may use a different encoding to provide for reduced interference among touch switches 10 that may be in proximity.

[0052] The present invention may have application in home appliances and in automotive applications, for example, in controlling windshield wipers or turn signals or the like and is particularly suited for these latter applications because of its improved resistance to electrical interference.

[0053] It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims

Claims

CLAIMS I claim:

1. A touch switch comprising: a sending unit (12) including an antenna (14) providing a local radio-frequency field (16) modulated by an encoding pattern (32); a touch plate (20) accepting a touch from a human operator (18) to receive the local radio-frequency field as coupled at least in part from the human operator; a decoder (58) attached to the touch plate to demodulate the local radio-frequency field to extract a received pattern (30); a comparator (58) comparing the received pattern to the encoding pattern to provide a switch signal when the received pattern and the encoding pattern provide a predetermined match; and a switch element (72) having electrical terminals (28) to control a flow of electricity between the electrical terminals based on a state of the switch signal.

2. The touch switch of claim 1 wherein the encoding pattern is a set of irregularly spaced pulses (36) and the encoding pattern is expressed in a spacing of the pulses.

3. The touch switch of claim 2 wherein the pulses also have different relative amplitudes (38) and the encoding is also expressed in the relative amplitudes of the pulses.

4. The touch switch of claim 1 wherein the encoding pattern is a set of pulses having different relative amplitudes (38) and the encoding is expressed in the relative amplitudes of the pulses.

5. The touch switch of claim 1 wherein the encoding pattern is a set of pulses having different frequencies (43) and wherein the encoding is expressed in the frequencies of the pulses.

6. The touch switch of claim 1 wherein the sending unit communicates with the decoder to allow for time sensitive decoding.

7. The touch switch of claim 1 wherein the received pattern and encoding pattern represent a set of discrete bits and wherein the comparator produces the switch signal both as a function of the bits in the encoding pattern having no corresponding bits in the received pattern and the bits in the received pattern having no corresponding bits in the encoded pattern.

8. The touch switch of claim 1 where in the touch plate is covered by an insulator and receives the local radio-frequency field through capacitive coupling to the human operator.

9. The touch switch of claim 1 where in the sending unit repeatedly sends the encoding pattern and the comparator provides the switch signal only after a predetermined number of received patterns and encoding patterns match.

10. The touch switch of claim 1 wherein the encoding pattern is expressed in multiple signals of the same frequency and different phases.

11. The touch switch of claim 1 wherein a fundamental frequency of the radio- frequency field is greater than 1000 Hz.