Self-adjusting inductive object-presence detector

• US 4,491,841 A
• Filed: 09/14/1983
• Issued: 01/01/1985
• Est. Priority Date: 04/03/1981
• Status: Expired due to Term

• Current Assignee: PEEK Traffic Limited
• Original Assignee: Sarasota Automation Limited
• Inventors: Clark, Michael A. G.
• Primary Examiner(s): Swann, III, Glen R.
• Application Number:
  US06/532,087
• Time in Patent Office: 475 Days
• Field of Search: 340/38 L, 340/38 R, 340/551, 340/568, 340/939, 340/941, 246/249
• US Class Current: 340/939
• CPC Class Codes: G01V 3/101 by measuring the impedance of the search coil; by measuring features of a resonant circuit comprising the search coil measuring impedance or characteristics derived therefrom G01R27/00, e.g. quality factor G01R27/26
• CPC Class Codes: G08G 1/042 using inductive or magnetic detectors

Abstract

A vehicle detector has an inductive sensing loop that is a frequency determining element of an oscillator. A first counter is operated to sample the oscillator frequency or period and the count achieved is applied as a preset reference to a second down counter which is counted down in one sample period while a fresh count value is being established in the first counter. The residue in the second counter at the end of a sample period is examined by detection logic to determine the presence of a vehicle. Preferably the first counter is updated every Nth sample period whereas the second counter is activated every sample period. Upon vehicle detection the first counter is frozen at the value preceding detection, the second counter continuing operation. During vehicle detection third and fourth counters are operated in the manner of the first and second counters respectively before detection, detection logic examining the residues in the fourth, down, counter to generate control signals for incrementally adding or subtracting from the count values in the first and third counters so as to reflect ambient changes during vehicle presence. In order to detect the departure of the vehicle, the second counter is consequently preset with the predetect value of the first counter as adjusted for ambient change.

Claims

1. A method of detecting the arrival of an object in the vicinity of a sensing element sensitive to such object and subject to ambient change, comprising:

sampling the state of the element at successive occasions to obtain sample values dependent on the state of the element;

establishing at a first rate a first succession of sample values each being superseded by the next as a reference value;

establishing at a second rate greater than the first rate a second succesion of sample values and comparing each of these sample values with the existing reference value, and

indicating the presence of an object in the vicinity of the loop upon the difference between the existing reference value and a sample value of the second succession compared therewith exceeding a selected threshold.

2. A method as claimed in claim 1 in which the second rate is an integral number of times the first rate and the sampling to establish a fresh reference value is effected on the same occasion as the sampling to obtain the last sample value of the second succession that is compared with the existing reference value.

3. A method as claimed in claim 1 in which sampling of the state of the sensing element is effected by monitoring a parameter of, or associated with, the sensing element and converting the monitored parameter to a digital value with the aid of a digital counting arrangement;

and wherein the first succession of sample values establishing the reference value is realised by activating a first digital counter at said first rate, and the second succession of sample values is realised by activating a second digital counter at said second rate.

4. A method as claimed in claim 3 in which said second digital counter is a DOWN-counter and prior to each activation thereof to establish a sample value it is loaded with the existing reference value established in the first counter, whereby the residual value in the second counter after such activation is the difference between the sample value established therein and the reference value.

5. A method as claimed in claim 1 further comprising:

storing the existing reference value upon a presence indication being given;

continuing the establishing of the second succession of sample values after a presence indication is given, and comparing each of these sample values with the stored reference value; and

indicating the departure of the object upon the difference between the stored reference value and a sample value compared therewith exceeding a selected threshold.

6. A method as claimed in claim 5 further comprising establishing a succession of sample values following the indicating of the presence of an object, detecting changes in these sample values due to ambient change, and adjusting the value of the stored reference value in dependence upon the detected changes to compensate for such ambient change.

7. A method as claimed in claim 5 in which sampling of the state of the sensing element is effected by monitoring a parameter of, or associated with, the sensing element and converting the monitored parameter to a digital value with the aid of a digital counting arrangement;

and wherein the first succession of sample values establishing the reference value is realised by activating a first digital counter at said first rate, and the second succession of sample values is realised by activating a second digital counter at said second rate; and

in which said first digital counter is an UP/DOWN counter whose further activation to establish a fresh reference value is inhibited upon the presence of an object being indicated, the existing reference value being stored therein and the adjustment of the stored reference value being effected by incrementing or decrementing the UP/DOWN counter in dependence upon the sense of the detected change.

8. A method of detecting an object with the aid of a sensing element sensitive to the presence of such object in the vicinity of the element and subject to ambient change, comprising:

sampling the state of the element at successive occasions to obtain sample values dependent on the state of the element;

comparing selected sample values to detect and indicate the arrival of an object in the vicinity of the sensing element,

storing as a first reference value a sample value reflecting the state of the sensing element preceding detection upon the detection of an object being indicated;

comparing sample values obtained after detection with the first reference value to detect and indicate the departure of the object from the vicinity of the sensing element; and

compensating for ambient change during the detected presence of an object by;

storing as a second reference value a sample value reflecting the state of the sensing element following the detection of an object being indicated;

comparing sample values obtained after detection with the second reference value to detect change due to ambient change, such change being indicated by the difference between the second reference value and the sample value compared therewith having a selected magnitude, and adjusting the first reference and second reference values in accordance with said detected changes so as to maintain a difference not exceeding preselected limits between the second reference value and the sample value compared therewith.

9. A method as claimed in claim 8 further comprising:

responding to further object induced changes at the sensing element by;

monitoring the difference between the second reference value and each sample value compared therewith to detect a difference exceeding a threshold value of a magnitude not less than that of said selected magnitude and indicative of the arrival or departure of an object;

upon detection of a difference exceeding said threshold value magnitude, establishing a third reference value reflecting the existing state of the sensing element following the departure of the object from the vicinity of the sensing element;

comparing sample values obtained during the absence of an object in the vicinity of the sensing element with said third reference value to detect changes due to ambient change, such change being indicated by the difference between the third reference value and the sample value compared therewith having said selected magnitude;

adjusting the third reference value in accordance with these detected changes to maintain a difference not exceeding preselected limits between the third reference value and the sample values compared therewith;

monitoring the difference between the third reference value and each sample value compared to detect a difference exceeding said threshold value magnitude, and

upon detection of a difference exceeding said threshold value magnitude, establishing a fresh value of said second reference value.

10. A method as claimed in claim 8 in which sampling of the state of the sensing element is effected by monitoring a parameter of, or associated with, the sensing element and converting the monitored parameter to a digital value with the aid of a digital counting arrangement including first and second UP/DOWN counters for storing said first and second reference values respectively, and first and second DOWN counters for establishing a first and second succession respectively of sample values for comparisons with the stored first and second reference values respectively, each comparison being made by loading a DOWN counter with the reference value stored in its associated UP/DOWN counter, and establishing a residual value by counting down the sample value from the reference value, and a change being detected by monitoring the residual values in the DOWN counter, and an adjustment for ambient change being made by incrementing or decrementing both UP/DOWN counters in accordance with the sense of the detected change as reflected in the residual value in the second DOWN counter.

11. A method as claimed in claim 8 in which sample values for comparison with said second reference value are established at a rate less than the rate of establishment of sample values for comparison with the first reference value.

12. A method as claimed in claim 11 in which the ratio between the said rates is an integral number.

13. Presence detector apparatus for use in an installation in which the presence of an object is detected by monitoring the state of a parameter of, or associated with, a sensing element sensitive to the presence of an object in the vicinity thereof, the apparatus including a detection circuit comprising:

first means for storing a reference value representing the state of the sensing element;

second means for storing a sample value representing the subsequent state of the element;

means responsive to the difference between the sample and reference values exceeding a threshold value to provide a signal indicative of the arrival of an object in the vicinity of the sensing element; and

means for controlling the establishing of a succession of reference values in the first means and a succession of sample values in the second means such that the succession of sample values in the second means is established at a rate greater than the succession of reference values in the first means.

14. Apparatus as claimed in claim 13 in which the ratio between the two rates is an integral number, and said control means is operable to establish a fresh reference value in the first means coincidentally with a sample value in the second means for comparison with the previous reference value.

15. Apparatus as claimed in claim 13 in which said first means comprises a first digital counter in which each reference value is established as a digital value and said second means comprises a second digital DOWN counter coupled to said first counter to receive the reference value as a pre-set value therefor, said control means being operable before establishing each sample value to cause pre-setting of said second counter and then to establish the sample value as a count down from the pre-set value, and said difference responsive means being coupled to said second counter to be responsive to the residual value in said second counter following the establishment of the sample value therein.

16. Apparatus as claimed in claim 13, in which said control means is responsive to said arrival-indicative signal to inhibit the establishment of a further reference value and hold the existing reference value in said first means whereby the difference responsive means is responsive during the presence of the object to the difference between a continuing succession of sample values established in the second means and said existing reference value to cancel said arrival-indicative signal upon departure of the object from the vicinity of the sensing element.

17. Apparatus as claimed in claim 16 further comprising a second detection circuit responsive to a succession of sample values representing the state of the sensing element while said arrival-indicative signal obtains to detect changes indicative of ambient change, said second detection circuit being coupled to said first means to adjust said existing reference value stored therein to compensate for detected ambient change.

18. Apparatus as claimed in claim 17 in which said first means comprises a first digital counter in which each reference value is established as a digital value and said second means comprises a second digital DOWN counter coupled to said first counter to receive the reference value as a pre-set value therefor, said control means being operable before establishing each sample value to cause pre-setting of said second counter and then to establish the sample value as a count down from the pre-set value, and said difference responsive means being coupled to said second counter to be responsive to the residual value in said second counter following the establishment of the sample value therein;

and

in which said first counter is an UP/DOWN counter and said second detection circuit is operable to control the direction of counting of said counter to cause the existing reference value therein to be incremented or decremented to compensate for ambient change.

19. Presence detector apparatus for use in an installation in which the presence of an object is detected by monitoring the state of a parameter of, or associated with, a sensing element sensitive to the presence of an object in the vicinity thereof, the apparatus including:

a first detection circuit responsive to a succession of sample values representing the state of the sensing element to detect and signal a change therein indicative of the arrival of an object in the vicinity of the sensing element, said first detection circuit including first means for storing a first reference value representing the state of the sensing element preceding such detection, and second means responsive to the stored reference value and to sample values following detection to detect and indicate the departure of the object from the vicinity of the sensing element; and

a second detection circuit for compensating ambient change in at least one sense during the detected presence of an object comprising;

third means for storing a second reference value representing the state of the sensing element following detection;

fourth means for establishing a succession of sample values representing the state of the sensing element after detection; and

fifth means responsive to a difference between the second reference value and the sample values established by the fourth means having a selected magnitude as indicative of ambient change in these sample values, said fifth means being coupled to said first means to adjust the first reference value by a predetermined amount in dependence upon ambient change detected by said fifth means; and

said fifth means being coupled to said third means to adjust the second reference value by said predetermined amount in dependence upon ambient change detected by said fifth means so as to maintain a minimal difference between the second reference value and the sample values established by said fourth means.

20. Apparatus as claimed in claim 19 in which each of said first and third means comprises an UP/DOWN counter for establishing said first and second reference values in digital form and said fourth means comprises a DOWN counter for establishing in digital form said sample values for comparison with the second reference value, said fourth means being coupled to said third means to receive said second reference value as a pre-set value and to count down a sample value therefrom to provide a residual value, said fifth means being responsive to said residual value as the difference between the sample value and the second reference value.