[0001] This invention relates to improvements in inductive loop vehicle detectors.
[0002] It is frequently the case in inductive loop vehicle detection installations, for
example for operating traffic signals, that a plurality of detector loops are arranged
to operate in association with one another or are coupled to a common detection signal
processing network on a time-share basis.
[0003] Fixed period scanning detectors or scanning detectors with a similar scanning period
can produce poor cross-talk performance when more than one detector is used in loop
installations with a large amount of mutual inductance between the loops.
[0004] If a multi-channel scanning detector is employed that only energises one loop at
a time, and only one such detector is used in a loop installation with inductive coupling,
the cross-talk performance is good, since little energy can be transferred between
the loops.
[0005] However, in a multi-detector loop installation the scanning periods from detector
to detector are asynchronous. When the scanning periods are similar, channels with
mutual inductive coupling can be coincident or near coincident for a long time, and
this can result in a spurious signal that will nevertheless be seen by a detector
as a detect signal.
[0006] It is an object of this invention to overcome that problem.
[0007] Most inductive loop vehicle detectors have an environmental tracking system whereby
apparent small changes in loop inductance can be tracked and discriminated against,
so that they are not seen as detect signals nor can low rate environmental changes
build to a point at which the accumulated change appears as a detect signal. Nevertheless,
if cross-talk coupling produces apparent changes in inductance, as seen by the scanning
detector, which exceed the environmental tracking rate the detector will give spurious
detect outputs.
[0008] A facility which our prior inductive loop vehicle detectors have is a detection counter.
Every time the detector sees a prime detect signal it will increment this counter.
If a prime detect signal is then not seen in a following period the count is decremented.
When the count in the counter accumulates to a fixed preset value due to persistence
of the detect signal, the detector will give a detect output. The function of this
counter is to prevent spurious noise interference causing random detections.
[0009] According to the present invention, the detector scanning periods in a multi-detector
inductive loop installation are adjusted so that the possibility of two scanning periods
of different detectors being coincident to produce an interference signal long enough
to exceed the environmental tracking rate is remote. This can be achieved by means
of an adjustable interscanning period generator to modify the scanning period of each
detector, as appropriate, from a nominal fixed scanning period.
[0010] In this way, the cross-talk performance of the scanning detectors can be considerably
improved. Also if scanning interference nevertheless causes a prime detect signal,
appearing randomly, the noise-suppression detection counter referred to above will
inhibit a detect output.
[0011] An arrangement according to the invention will now be described by way of example
with reference to the accompanying drawing.
[0012] The basic prior art detection technique is described in U.S. Patent No. 4,668,951,
especially Figure 5 thereof, which is incorporated herein by reference. By modifying
the software contained in the micro-computer of the system we obtain a flow chart
as shown in the drawing hereof. A comparison between the Figure 5 flow chart of the
U.S. patent and the present flow chart shows that the modifications to the program
are principally at the "blocks" marked (1), (2), (3) and (4).
[0013] At (1), after each channel has been run for a fixed time and the number of loop cycles
stored as in the prior system, the interscanning period timer now provided in the
present system is initialised, the timer period N being calculated to exceed the normal
oscillator running period as provided at (4). At (2), after transfer of the detected
states to the outputs, as in the prior system, a wait loop is provided simply to wait
for the interscanning period N to expire before the system steps to the next channel.
At (3), the required operation period is calculated as in the prior system and also
the interscanning period N is calculated to exceed that operation period. At (4),
the oscillator runs for a period just to exceed the required operation period as in
the prior system and then it is disabled before the expiry of the interscanning period
N. This greatly decreases the possibility of two interfering oscillators being time
coincident thereby leading to crosstalk.
[0014] The period N to reduce crosstalk can be derived in a number of ways.
(a) From the store loop cycle number "LC".
(b) From the detection counter state of the system previously described.
(c) From on board hardware switches.
(d) From data received from serial communications to the detectors
[0015] We have constructed detectors using each of (a), (b) and (c) of these possibilities
and all have good crosstalk suppression performance. The only embodiment needing further
hardware beyond that of U.S. Patent No. 4,668,951 is (c), the switches required being
additional.
1. A method of reducing cross-talk between the plurality of detector loops of a multi-channel
inductive loop vehicle detector system having environmental tracking, comprising adjusting
the respective scanning periods of the different detectors, from a nominal fixed scanning
period, so that the possibility of two such scanning periods being coincident to produce
an interference signal long enough to exceed the environmental tracking rate is substantially
avoided.
2. A method according to Claim 1, wherein after each channel has been run for a fixed
time to store a number of loop cycles, an interscanning period timer is initialised
having a timer period N calculated to exceed the normal loop oscillator running period,
and after transfer of the detect state of the channel to the output a wait occurs
while the interscanning timer period N expires before the operation steps to the next
channel.
3. A method according to Claim 2, wherein a required operation period is calculated,
the interscanning period N is calculated to exceed that operation period, and the
loop oscillator is run for a number of cycles just to exceed the calculated operation
period but is disabled before the expiry of the interscanning period N.
4. A method according to Claim 3, wherein the interscanning period N is calculated
from the stored loop cycle number.
