Displacement sensing system

• US 6,933,846 B2
• Filed: 04/18/2001
• Issued: 08/23/2005
• Est. Priority Date: 04/18/2000
• Status: Expired due to Term

• Current Assignee: Tyco Fire & Security GmbH (Johnson Controls International plc), Rubicon Holding B.V.
• Original Assignee: Visonic Incorporated (Johnson Controls International plc)
• Inventors: Zhevelev, Boris, Moldavsky, Mark
• Primary Examiner(s): Lee, Benjamin C.
• Application Number:
  US10/257,501
• Publication Number:
  US 20040124981A1
• Time in Patent Office: 1,588 Days
• Field of Search: 340/568.1, 340/547, 340/551, 340/686.2, 340/686.6, 32420711-20726
• US Class Current: 340/568.1
• CPC Class Codes: G01D 5/2033 controlling the saturation of a magnetic circuit by means of a movable element, e.g. a magnet
• CPC Class Codes: G01D 5/243 influencing the phase or frequency of ac
• CPC Class Codes: G01V 3/08 operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices with electromagnetic waves G01V3/12
• CPC Class Codes: G08C 19/16 in which transmission is by pulses

Abstract

A displacement sensing system including an oscillator circuit having a characteristic frequency which varies as a function of a magnetic field in the vicinity thereof, an element whose position relative to the oscillator circuit is varied by a displacement, the element being operative to vary the magnetic field in the vicinity of the oscillator circuit as a function of the displacement and circuitry for sensing a change of frequency of the oscillator circuit over time (delta f/delta t) and for providing an output indication when the change in frequency of the oscillator circuit over time exceeds a threshold.

Claims

1. A protected object comprising:

an object having associated therewith a displacement sensing system including;

an oscillator circuit having a characteristic frequency which varies as a function of a magnetic field in the vicinity thereof;

an element whose position relative to said oscillator circuit is varied by a displacement to be sensed, said element being operative to vary the magnetic field in the vicinity of said oscillator circuit as a function of said displacement; and

circuitry for sensing a change of frequency of said oscillator circuit over time (Δ

f/Δ

t) and for providing an output indication when the change in frequency of said oscillator circuit over time exceeds a threshold; and

a locator operative for determining the location of at least one of said oscillator circuit, said element and said circuitry for sensing, wherein

one but not both of said oscillator circuit and said element is fixedly mounted onto said object.

2. A protected object according to claim 1 and wherein said oscillator circuit is fixedly mounted onto said object.

3. A protected object according to claim 2 and wherein said threshold varies as a function of a currently perceived threat.

4. A protected object according to claim 1 and wherein said element is fixedly mounted onto said object.

5. A protected object according to claim 4 and wherein said currently perceived threat is determined by a pattern of past outputs of said circuitry for sensing a change of frequency.

6. A protected object according to claim 1 and wherein said threshold is variable.

7. A protected object according to claim 1 and wherein said oscillator circuit and said circuitry for sensing a change of frequency are operated in accordance with a duty cycle.

8. A protected object according to claim 7 and wherein said duty cycle is variable.

9. A protected object according to claim 8 and wherein said duty cycle is adaptive.

10. A protected object according to claim 8 and wherein said duty cycle varies as a function of a currently perceived threat.

11. A protected object according to claim 10 and wherein said currently perceived threat is determined by a pattern of past outputs of said circuitry for sensing a change of frequency.

12. A protected object according to claim 1 and wherein said threshold varies in accordance with long time scale changes in said characteristic frequency.

13. A protected object according to claim 1 and wherein said threshold comprises first and second frequency thresholds, respectively above and below said characteristic frequency and which are normally generally equally separated therefrom in the frequency domain.

14. A protected object according to claim 1 and wherein said circuitry for sensing a change of frequency over time provides at least first and second different output indications when said change in frequency of said oscillator circuit over time exceeds respective at least first and second different thresholds.

15. A protected object according to claim 1 and also comprising wireless communication circuitry operative to transmit said output indication to a remote receiving location.

16. A protected object according to claim 15 and wherein said wireless communication circuitry is operative to transmit said output indication for at least a predetermined time following sensing of said change in frequency.

17. A protected object according to claim 1 and wherein said element is a magnet.

18. A protected object according to claim 17 and wherein said element is an electromagnet.

19. A protected object according to claim 17 and wherein said element is a permanent magnet.

20. A protected object according to claim 1 and wherein said circuitry for sensing a change of frequency comprises a microprocessor which receives an output of said oscillator circuit as a clock input.

21. A protected object according to claim 1 and wherein said circuitry for sensing a change of frequency is operative to count oscillator pulses over variable time periods which represent variable sensitivity thresholds.

22. A protected object according to claim 1 and wherein said circuitry for sensing a change of frequency comprises a microprocessor which provides said output indication.

23. A protected object according to claim 1 and wherein said protected object is an art object.

24. A protected object according to claim 23 and wherein said art object is a painting and wherein one but not both of said oscillator circuit and said element is fixedly mounted onto a frame of said painting.

25. A protected object according to claim 23 and wherein said art object is a painting and wherein one but not both of said oscillator circuit and said element is fixedly mounted onto a canvas of said painting.

26. A protected object according to claim 1 and also comprising wireless communication circuitry operative to transmit said output indication to a remote receiving location.

27. A protected object according to claim 26 and wherein said wireless communication circuitry is operative to transmit said output indication for at least a predetermined time following sensing of said change in frequency.

28. Apparatus according to claim 1 and wherein said displacement is a vibrational displacement.

29. Apparatus according to claim 1 and wherein said oscillator circuit and said element are physically attached to each other.

30. Apparatus according to claim 1 and wherein said oscillator circuit and said element are not physically attached to each other.

31. A displacement sensing system comprising:

an oscillator circuit having a characteristic frequency which varies as a function of a magnetic field in the vicinity thereof;

an element whose position relative to said oscillator circuit is varied by a displacement to be sensed, said element being operative to vary the magnetic field in the vicinity of said oscillator circuit as a function of said displacement; and

circuitry for sensing a change of frequency of said oscillator circuit over time (Δ

f/Δ

t) and for providing an output indication when the change in frequency of said oscillator circuit over time exceeds a threshold; and

a locator operative for determining the location of at least one of said oscillator circuit, said element and said circuitry for sensing.

32. A displacement sensing system according to claim 31 and wherein said circuitry for sensing a change of frequency over time provides at least first and second different output indications when said change in frequency of said oscillator circuit over time exceeds respective at least first and second different thresholds.

33. A displacement sensing system according to claim 32 and wherein said threshold is variable.

34. A displacement sensing system according to claim 31 and wherein said threshold is variable.

35. A displacement sensing system according to claim 31 and wherein said threshold varies as a function of a currently perceived threat.

36. A displacement sensing system according to claim 35 and wherein said currently perceived threat is determined by a pattern of past outputs of said circuitry for sensing a change of frequency.

37. A displacement sensing system according to claim 31 and wherein said oscillator circuit and said circuitry for sensing a change of frequency are operated in accordance with a duty cycle.

38. A displacement sensing system according to claim 37 and wherein said duty cycle is variable.

39. A displacement sensing system according to claim 38 and wherein said duty cycle is adaptive.

40. A displacement sensing system according to claim 38 and wherein said duty cycle varies as a function of a currently perceived threat.

41. A displacement sensing system according to claim 40 and wherein said currently perceived threat is determined by a pattern of past outputs of said circuitry for sensing a change of frequency.

42. A displacement sensing system according to claim 31 and wherein said threshold varies in accordance with long time scale changes in said characteristic frequency.

43. A displacement sensing system according to claim 31 and wherein said threshold comprises first and second frequency thresholds, respectively above and below said characteristic frequency and which are normally generally equally separated therefrom in the frequency domain.

44. A displacement sensing system according to claim 31 and also comprising wireless communication circuitry operative to transmit said output indication to a remote receiving location.

45. A displacement sensing system according to claim 44 and wherein said wireless communication circuitry is operative to transmit said output indication for at least a predetermined time following sensing of said change in frequency.

46. A displacement sensing system according to claim 31 and wherein said element is a magnet.

47. A displacement sensing system according to claim 46 and wherein said element is an electromagnet.

48. A displacement sensing system according to claim 46 and wherein said element is a permanent magnet.

49. A displacement sensing system according to claim 31 and wherein said circuitry for sensing a change of frequency comprises a microprocessor which receives an output of said oscillator circuit as a clock input.

50. A displacement sensing system according to claim 31 and wherein said circuitry for sensing a change of frequency is operative to count oscillator pulses over variable time periods which represent variable sensitivity thresholds.

51. Apparatus according to claim 31 and wherein said displacement is a vibrational displacement.

52. Apparatus according to claim 31 and wherein said oscillator circuit and said element are physically attached to each other.

53. Apparatus according to claim 31 and wherein said oscillator circuit and said element are not physically attached to each other.

54. A displacement sensing system comprising:

an oscillator circuit having a characteristic frequency which varies as a function of a magnetic field in the vicinity thereof;

an element whose position relative to said oscillator circuit is varied by a displacement to be sensed, said element being operative to vary the magnetic field in the vicinity of said oscillator circuit as a function of said displacement; and

circuitry for sensing a time duration over which a change of frequency of at least a given amount takes place and for providing an output indication when said time duration exceeds a threshold; and

a locator operative for determining the location of at least one of said oscillator circuit, said element and said circuitry for sensing.

55. A displacement sensing system according to claim 54 and wherein said circuitry for sensing a time duration provides at least first and second different output indications when said time duration exceeds respective at least first and second different thresholds.

56. A displacement sensing system according to claim 55 and wherein said threshold is variable.

57. A displacement sensing system according to claim 54 and wherein said threshold is variable.

58. A displacement sensing system according to claim 54 and wherein said threshold varies as a function of a currently perceived threat.

59. A displacement sensing system according to claim 58 and wherein said currently perceived threat is determined by a pattern of past outputs of said circuitry for sensing a time duration.

60. A displacement sensing system according to claim 54 and wherein said oscillator circuit and said circuitry for sensing a time duration are operated in accordance with a duty cycle.

61. A displacement sensing system according to claim 60 and wherein said duty cycle is variable.

62. A displacement sensing system according to claim 61 and wherein said duty cycle is adaptive.

63. A displacement sensing system according to claim 61 and wherein said duty cycle varies as a function of a currently perceived threat.

64. A displacement sensing system according to claim 63 and wherein said currently perceived threat is determined by a pattern of past outputs of said circuitry for sensing a time duration.

65. A displacement sensing system according to claim 54 and wherein said given amount remains generally constant notwithstanding long time scale changes in said characteristic frequency.

66. A displacement sensing system according to claim 54 and also comprising wireless communication circuitry operative to transmit said output indication to a remote receiving location.

67. A displacement sensing system according to claim 66 and wherein said wireless communication circuitry is operative to transmit said output indication for at least a predetermined time following sensing of said time duration.

68. A displacement sensing system according to claim 54 and wherein said element is a magnet.

69. A displacement sensing system according to claim 68 and wherein said element is an electromagnet.

70. A displacement sensing system according to claim 68 and wherein said element is a permanent magnet.

71. A displacement sensing system according to claim 54 and wherein said circuitry for sensing a time duration comprises a microprocessor which receives an output of said oscillator circuit as a clock input.

72. A displacement sensing system according to claim 54 and wherein said circuitry for sensing a time duration is operative to count oscillator pulses over variable time periods which represent variable sensitivity thresholds.

73. A displacement sensing system according to claim 54 and wherein said circuitry for sensing a time duration comprises a microprocessor which provides said output indication.

74. Apparatus according to claim 54 and wherein said displacement is a vibrational displacement.

75. Apparatus according to claim 54 and wherein said oscillator circuit and said element are physically attached to each other.

76. Apparatus according to claim 54 and wherein said oscillator circuit and said element are not physically attached to each other.

77. A displacement sensing system comprising:

an oscillator circuit having a characteristic frequency which varies as a function of a magnetic field in the vicinity thereof;

an element whose position relative to said oscillator circuit is varied by a displacement to be sensed, said element being operative to vary the magnetic field in the vicinity of said oscillator circuit as a function of said displacement;

circuitry for sensing a change of frequency of said oscillator circuit over time (Δ

f/Δ

t) and for providing an output indication when the change in frequency of said oscillator circuit over time exceeds a frequency threshold; and

circuitry for sensing a time duration over which a change of frequency of at least a given amount takes place and for providing an output indication when said time duration exceeds a time threshold; and

a locator operative for determining the location of at least one of said oscillator circuit, said element and said circuitry for sensing.

78. A displacement sensing system according to claim 77 and wherein said circuitry for sensing a change of frequency over time provides at least first and second different output indications when said change in frequency of said oscillator circuit over time exceeds respective at least first and second different thresholds.

79. A displacement sensing system according to claim 78 and wherein said frequency threshold is variable.

80. A displacement sensing system according to claim 77 and wherein said frequency threshold is variable.

81. A displacement sensing system according to claim 80 and wherein said frequency threshold varies as a function of a currently perceived threat.

82. A displacement sensing system according to claim 81 and wherein said currently perceived threat is determined by a pattern of past outputs of said circuitry for sensing a change of frequency.

83. A displacement sensing system according to claim 77 and wherein said frequency threshold varies in accordance with long time scale changes in said characteristic frequency.

84. A displacement sensing system according to claim 77 and wherein said frequency threshold comprises first and second frequency thresholds, respectively above and below said characteristic frequency and which are normally generally equally separated therefrom in the frequency domain.

85. A displacement sensing system according to claim 77 and wherein said time threshold is variable.

86. A displacement sensing system according to claim 85 and wherein said time threshold varies as a function of a currently perceived threat.

87. A displacement sensing system according to claim 86 and wherein said currently perceived threat is determined by a pattern of past outputs of said circuitry for sensing a change of frequency.

88. A displacement sensing system according to claim 77 and wherein said oscillator circuit and said circuitry for sensing a time duration are operated in accordance with a duty cycle.

89. A displacement sensing system according to claim 88 and wherein said duty cycle is variable.

90. A displacement sensing system according to claim 89 and wherein said duty cycle varies as a function of a currently perceived threat.

91. A displacement sensing system according to claim 90 and wherein said currently perceived threat is determined by a pattern of past outputs of said circuitry for sensing a change of frequency.

92. A displacement sensing system according to claim 88 and wherein said duty cycle is adaptive.

93. A displacement sensing system according to claim 77 and wherein said given amount remains generally constant notwithstanding long time scale changes in said characteristic frequency.

94. A displacement sensing system according to claim 77 and wherein said circuitry for sensing a time duration provides at least first and second different output indications when said time duration exceeds respective at least first and second different thresholds.

95. A displacement sensing system according to claim 77 and also comprising wireless communication circuitry operative to transmit said output indication to a remote receiving location.

96. A displacement sensing system according to claim 95 and wherein said wireless communication circuitry is operative to transmit said output indication for at least a predetermined time following sensing of said change in frequency.

97. A displacement sensing system according to claim 77 and wherein said element is a magnet.

98. A displacement sensing system according to claim 97 and wherein said element is an electromagnet.

99. A displacement sensing system according to claim 97 and wherein said element is a permanent magnet.

100. A displacement sensing system according to claim 77 and wherein said circuitry for sensing a time duration and said circuitry for sensing a change of frequency comprises a microprocessor which receives an output of said oscillator circuit as a clock input.

101. A displacement sensing system according to claim 77 and wherein said circuitry for sensing a time duration is operative to count oscillator pulses over variable time periods which represent variable sensitivity thresholds.

102. Apparatus according to claim 77 and wherein said displacement is a vibrational displacement.

103. Apparatus according to claim 77 and wherein said oscillator circuit and said element are physically attached to each other.

104. Apparatus according to claim 77 and wherein said oscillator circuit and said element are not physically attached to each other.

105. A protected object comprising:

an object having associated therewith a displacement sensing system including;

an oscillator circuit having a characteristic frequency which varies as a function of a magnetic field in the vicinity thereof;

an element whose position relative to said oscillator circuit is varied by a displacement to be sensed, said element being operative to vary the magnetic field in the vicinity of said oscillator circuit as a function of said displacement; and

circuitry for sensing a time duration over which a change of frequency of at least a given amount takes place and for providing an output indication when said time duration exceeds a threshold; and

a locator operative for determining the location of at least one of said oscillator circuit, said element and said circuitry for sensing, wherein

one but not both of said oscillator circuit and said element is fixedly mounted onto said object.

106. A protected object according to claim 105 and wherein said oscillator circuit is fixedly mounted onto said object.

107. A protected object according to claim 105 and wherein said element is fixedly mounted onto said object.

108. A protected object according to claim 105 and wherein said threshold is variable.

109. A protected object according to claim 108 and wherein said threshold varies as a function of a currently perceived threat.

110. A protected object according to claim 109 and wherein said currently perceived threat is determined by a pattern of past outputs of said circuitry for sensing a time duration.

111. A protected object according to claim 105 and wherein said oscillator circuit and said circuitry for sensing a time duration are operated in accordance with a duty cycle.

112. A protected object according to claim 111 and wherein said duty cycle is variable.

113. A protected object according to claim 112 and wherein said duty cycle varies as a function of a currently perceived threat.

114. A protected object according to claim 113 and wherein said currently perceived threat is determined by a pattern of past outputs of said circuitry for sensing a time duration.

115. A protected object according to claim 111 and wherein said duty cycle is adaptive.

116. A protected object according to claim 105 and wherein said given amount remains generally constant notwithstanding long time scale changes in said characteristic frequency.

117. A protected object according to claim 105 and wherein said circuitry for sensing a time duration provides at least first and second different output indications when said time duration exceeds respective at least first and second different thresholds.

118. A protected object according to claim 105 and also comprising wireless communication circuitry operative to transmit said output indication to a remote receiving location.

119. A protected object according to claim 105 and wherein said wireless communication circuitry is operative to transmit said output indication for at least a predetermined time following sensing of said time duration.

120. A protected object according to claim 105 and wherein said element is a magnet.

121. A protected object according to claim 120 and wherein said element is an electromagnet.

122. A protected object according to claim 120 and wherein said element is a permanent magnet.

123. A protected object according to claim 105 and wherein said circuitry for sensing a time duration comprises a microprocessor which receives an output of said oscillator circuit as a clock input.

124. A protected object according to claim 105 and wherein said circuitry for sensing a time duration is operative to count oscillator pulses over variable time periods which represent variable sensitivity thresholds.

125. A protected object according to claim 105 and wherein said protected object is an art object.

126. A protected object according to claim 125 and wherein said art object is a painting and wherein one but not both of said oscillator circuit and said element is fixedly mounted onto a frame of said painting.

127. A protected object according to claim 125 and wherein said art object is a painting and wherein one but not both of said oscillator circuit and said element is fixedly mounted onto a canvas of said painting.

128. A protected object according to claim 105 and also comprising wireless communication circuitry operative to transmit said output indication to a remote receiving location.

129. A protected object according to claim 128 and wherein said wireless communication circuitry is operative to transmit said output indication for at least a predetermined time following sensing of said time duration.

130. Apparatus according to claim 105 and wherein said displacement is a vibrational displacement.

131. Apparatus according to claim 105 and wherein said oscillator circuit and said element are physically attached to each other.

132. Apparatus according to claim 105 and wherein said oscillator circuit and said element are not physically attached to each other.

133. A protected object comprising:

an object having associated therewith a displacement sensing system including;

an oscillator circuit having a characteristic frequency which varies as a function of a magnetic field in the vicinity thereof;

an element whose position relative to said oscillator circuit is varied by a displacement to be sensed, said element being operative to vary the magnetic field in the vicinity of said oscillator circuit as a function of said displacement;

circuitry for sensing a change of frequency of said oscillator circuit over time (Δ

f/Δ

t) and for providing an output indication when the change in frequency of said oscillator circuit over time exceeds a frequency threshold;

circuitry for sensing a time duration over which a change of frequency of at least a given amount takes place and for providing an output indication when said time duration exceeds a time threshold; and

a locator operative for determining the location of at least one of said oscillator circuit, said element and said circuitry for sensing, wherein

one but not both of said oscillator circuit and said element is fixedly mounted onto said object.

134. A protected object according to claim 133 and wherein said oscillator circuit is fixedly mounted onto said object.

135. A protected object according to claim 133 and wherein said element is fixedly mounted onto said object.

136. A protected object according to claim 133 and wherein said frequency threshold is variable.

137. A protected object according to claim 136 and wherein said frequency threshold varies as a function of a currently perceived threat.

138. A protected object according to claim 137 and wherein said currently perceived threat is determined by a pattern of past outputs of said circuitry for sensing a change of frequency.

139. A protected object according to claim 133 and wherein said oscillator circuit and said circuitry for sensing a change of frequency are operated in accordance with a duty cycle.

140. A protected object according to claim 139 and wherein said duty cycle is variable.

141. A protected object according to claim 140 and wherein said duty cycle varies as a function of a currently perceived threat.

142. A protected object according to claim 141 and wherein said currently perceived threat is determined by a pattern of past outputs of said circuitry for sensing a change of frequency.

143. A protected object according to claim 139 and wherein said duty cycle is adaptive.

144. A protected object according to claim 133 and wherein said frequency threshold varies in accordance with long time scale changes in said characteristic frequency.

145. A protected object according to claim 133 and wherein said frequency threshold comprises first and second frequency thresholds, respectively above and below said characteristic frequency and which are normally generally equally separated therefrom in the frequency domain.

146. A protected object according to claim 133 and wherein said circuitry for sensing a change of frequency over time provides at least first and second different output indications when said change in frequency of said oscillator circuit over time exceeds respective at least first and second different thresholds.

147. A protected object according to claim 133 and wherein said time threshold is variable.

148. A protected object according to claim 147 and wherein said time threshold varies as a function of a currently perceived threat.

149. A protected object according to claim 148 and wherein said currently perceived threat is determined by a pattern of past outputs of said circuitry for sensing a change of frequency.

150. A protected object according to claim 133 and wherein said oscillator circuit and said circuitry for sensing a time duration are operated in accordance with a duty cycle.

151. A protected object according to claim 150 and wherein said duty cycle is variable.

152. A protected object according to claim 151 and wherein said duty cycle varies as a function of a currently perceived threat.

153. A protected object according to claim 152 and wherein said currently perceived threat is determined by a pattern of past outputs of said circuitry for sensing a change of frequency.

154. A protected object according to claim 150 and wherein said duty cycle is adaptive.

155. A protected object according to claim 133 and wherein said given amount remains generally constant notwithstanding long time scale changes in said characteristic frequency.

156. A protected object according to claim 133 and wherein said circuitry for sensing a time duration provides at least first and second different output indications when said time duration exceeds respective at least first and second different thresholds.

157. A protected object according to claim 133 and wherein said element is a magnet.

158. A protected object according to claim 157 and wherein said element is an electromagnet.

159. A protected object according to claim 157 and wherein said element is a permanent magnet.

160. A protected object according to claim 133 and wherein said circuitry for sensing a change of frequency is a microprocessor which receives an output of said oscillator circuit as a clock input.

161. A protected object according to claim 133 and wherein said circuitry for sensing a change of frequency is operative to count oscillator pulses over variable time periods which represent variable sensitivity thresholds.

162. A protected object according to claim 133 and wherein said circuitry for sensing a change of frequency comprises a microprocessor which provides said output indication.

163. A protected object according to claim 133 and wherein said protected object is an art object.

164. A protected object according to claim 163 and wherein said art object is a painting and wherein one but not both of said oscillator circuit and said element is fixedly mounted onto a frame of said painting.

165. A protected object according to claim 163 and wherein said art object is a painting and wherein one but not both of said oscillator circuit and said element is fixedly mounted onto a canvas of said painting.

166. A protected object according to claim 133 and also comprising wireless communication circuitry operative to transmit said output indication to a remote receiving location.

167. A protected object according to claim 166 and wherein said wireless communication circuitry is operative to transmit said output indication for at least a predetermined time following sensing of said change in frequency.

168. Apparatus according to claim 133 and wherein said displacement is a vibrational displacement.

169. Apparatus according to claim 133 and wherein said oscillator circuit and said element are physically attached to each other.

170. Apparatus according to claim 133 and wherein said oscillator circuit and said element are not physically attached to each other.

171. A displacement sensing system comprising:

an oscillator circuit having a characteristic frequency which varies as a function of a magnetic field in the vicinity thereof;

an element whose position relative to said oscillator circuit is varied by a displacement to be sensed, said element being operative to vary the magnetic field in the vicinity of said oscillator circuit as a function of said displacement; and

circuitry for sensing a change of frequency of said oscillator circuit over time (Δ

f/Δ

t) and for providing an output indication when the change in frequency of said oscillator circuit over time exceeds a threshold, wherein

said threshold comprises first and second frequency thresholds, respectively above and below said characteristic frequency and which are normally generally equally separated therefrom in the frequency domain.

172. A displacement sensing system comprising:

an oscillator circuit having a characteristic frequency which varies as a function of a magnetic field in the vicinity thereof;

an element whose position relative to said oscillator circuit is varied by a displacement to be sensed, said element being operative to vary the magnetic field in the vicinity of said oscillator circuit as a function of said displacement; and

circuitry for sensing a time duration over which a change of frequency of at least a given frequency threshold takes place and for providing an output indication when said time duration exceeds a time threshold, wherein

said frequency threshold comprises first and second frequency thresholds, respectively above and below said characteristic frequency and which are normally generally equally separated therefrom in the frequency domain.

173. A displacement sensing system comprising:

an oscillator circuit having a characteristic frequency which varies as a function of a magnetic field in the vicinity thereof;

an element whose position relative to said oscillator circuit is varied by a displacement to be sensed, said element being operative to vary the magnetic field in the vicinity of said oscillator circuit as a function of said displacement;

circuitry for sensing a change of frequency of said oscillator circuit over time (Δ

f/Δ

t) and for providing an output indication when the change in frequency of said oscillator circuit over time exceeds a frequency threshold; and

circuitry for sensing a time duration over which a change of frequency of at least a given amount takes place and for providing an output indication when said time duration exceeds a time threshold, wherein

said frequency threshold comprises first and second frequency thresholds, respectively above and below said characteristic frequency and which are normally generally equally separated therefrom in the frequency domain.