Method and apparatus for detecting direction and speed using PIR sensor

A dual pyroelectric-effect sensor having the sensing elements aligned in a motion plane permits direction determinations to be made for moving IR sources. Dual sensing-element PIR sensors provide different voltage outputs depending upon a relative direction of movement of an object and the sensing elements. By alternating the effective polarizations of the sensing elements in the PIR sensor, clear direction information is available from the PIR sensor. A direction detecting circuit working in cooperation with a switch controller employing a counter and a timer, permits independent tallying of entrances and exits. Upon the counter indicating that the number of objects that exited the area equals the number of objects that entered, the lights are immediately extinguished. The timer ensures that the lights turn off should incorrect values become recorded in the counter.

BACKGROUND OF THE INVENTION 
The present invention relates generally to pyroelectric infrared (PIR) 
sensors, and more specifically to PIR sensors used to detect direction and 
speed. 
PIR sensors are well known in the art. These sensors are commonly used in 
security systems for measuring motion in a monitored area. PIR sensors use 
materials having a pyroelectric effect. One common material is 
LiTaO.sub.3, which in its crystalline form is spontaneously polarized 
(i.e. electrical dipoles in the crystal structure develop). Heating the 
LiTaO.sub.3 crystal to a temperature just below its Curie temperature in 
an electrical field causes these dipoles to line up in a direction of the 
electrical field. 
Bringing opposing electrodes into contact with the polarized crystal causes 
the surface to be electrically charged. Ions in the air neutralize this 
surface charge. Thereafter, the crystal absorbs incident infrared 
radiation (IR). The absorption of the IR causes the temperature of the 
crystal to change, altering the spontaneous polarization and thus the 
number of dipoles. The change in the number of dipoles unbalances surface 
charges on the crystal's surface. It is possible to measure this surface 
charge imbalance as a voltage change. Thus, voltage changes on the surface 
of the crystal are indications of incident IR. The voltage change and the 
subsequent detection of IR required a temperature change in the crystal. 
To include a temperature change in the crystal requires either a moving IR 
source, or a chopper disposed between the crystal and the IR source. 
FIG. 7 is a schematic presentation of the pyroelectric effect. At the top 
of FIG. 7 a source of IR is shown modulated by a chopper. The chopper 
first is closed, then opened to illuminate a pyroelectric crystal with the 
infrared radiation, and then subsequently closed. Below the status graph 
of the chopper is seen the charge distribution of the crystal surface 
corresponding to the chopper status. Position A, with the chopper closed 
prior to illumination, shows a balanced surface charge aligned with an 
electric field (not shown). Position B, opening the chopper, disturbs the 
balance of the dipoles, producing a net positive charge distribution. 
Position C occurs sometime later with the chopper open, after excess 
surface charge is neutralized and charge becomes rebalanced at the new 
dipole generation rate. Thereafter, at position D (closing the chopper), 
induces an equal but opposite charge distribution in the crystal. Ions 
become associated with the unbalanced crystal charge at position E, 
sometime later. The graph below the representations of the crystal charge 
distribution illustrates output from a sensor employing such a crystal. 
FIG. 8 is a schematic diagram of a prior art PIR detector 100 consisting of 
dual pyroelectric elements (LiTaO.sub.3) 102a,b a high-ohmic resistor 104, 
and a low-noise field effect transistor (FET) 106 built into a TO-5 
package. The TO-5 package includes a window 110 made up of a silicon 
filter which limits incident rediation to wavelengths in a prespecified 
range. 
The prior art employed dual element sensors to reduce noise signals from 
the sensor. FIG. 9 is a top view of a PIR sensor 100 having two sensing 
elements 102a and 102b. Typically, the sensing elements are one millimeter 
by two millimeters and separated by a one millimeter space. The polarities 
of the sensing elements are reversed as shown., with the sensing elements 
oriented at about a forty-five degree angle to improve the noise reduction 
feature of the sensing element. 
It is common in the prior art to employ these PIR sensors in switches to 
control room lighting, for example. In one common application, a switch 
with a PIR sensor is mounted to monitor an entrance to a room. When a 
person enters the room, the PIR sensor detects the entrance and begins 
operation of a timer. Subsequent detections of persons entering or leaving 
the room will reset the timer. When the timer expires, the switch 
automatically turns the lights off. This is a desirable energy-saving 
feature. These switches do not have an ability to detect relative motion 
of the person entering or leaving the room, so that a person leaving the 
room will reset the timer. If the switch were able to discriminate between 
a person entering or leaving, the switch could immediately turn the lights 
off rather than waiting for the timer to expire. 
SUMMARY OF THE INVENTION 
The present invention provides method and apparatus for detecting direction 
and speed of an object moving in a field of view of a PIR sensor. The 
invention permits switches incorporating the invention to determine 
whether a person enters or leaves a room, and permits immediate 
extinguishing of room lights when a person leaves. Additionally, a counter 
permits comparison of numbers of counts of objects entering and leaving. 
Upon determining that a number of exits equals a number of entrances, the 
switch extinguishes the lights. By employing a plurality of the direction 
indicating sensors, a velocity of an object, both its direction and speed, 
should be determinable. 
According to one aspect of the invention, it includes a dual element 
pyroelectric infrared sensor (PIR) with its sensing elements oriented in a 
motion plane. An electronic circuit coupled to an output of the PIR sensor 
measures for voltage levels of an output signal to determine a relative 
direction of motion for an object moving in the monitored motion plane. 
Use of multiple PIR sensors, having two or more sensing elements permits 
speed determinations of the moving object. 
The present invention permits construction of even more energy conservative 
switches than those of the prior art without additional sensing elements. 
There are many potential uses for a sensing element which is able to not 
only to detact a moving object, but also a direction or velocity of the 
moving object. 
Reference to the remaining portions of the specification and drawings may 
realize a further understanding of the nature and advantages of the 
present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS 
FIG. 1 is a perspective view of a wall-mounted switch 10 monitoring a room 
12. An object 14 in the room 12 passes within a field of view of the 
switch 10 and advances either toward or away from a door 16. Preferably, 
the switch is placed so that the field of view monitors all entrances and 
exits from the door 16. For purposes of this description, I define a plane 
of motion for the object 14 as movement in a plane extending toward and 
away from the door 16. The arrows on the object 14 roughly identify its 
plane of motion. The switch 10 includes a pyroelectric infrared (PIR) 
sensor 20 and a manual on/off control 22. 
In operation, the object 14 initially enters the room 12 through door 16. 
When the object enters the field of view of the PIR sensor 20, and if the 
on/off control 22 is on, then the switch 10 will illuminate lights 24. 
Advancing the object 14 further into the room 12 and out of the field of 
view of the PIR sensor 20 does not cause the lights 24 to extinguish. When 
the object 14 is detected moving toward the door 16, and subsequently 
leaves the field of view of the PIR sensor 20, the lights 24 are 
extinguished immediately. 
In the case where more than one object 14 enters or leaves the room 12, the 
switch 10 optionally includes a counter (not shown). The PIR sensor 
detecting an entrance into the room 12 of an object increments the 
counter. Detecting an exit of an object from the room 12 decrements the 
counter. After decrementing the counter, the switch 10 extinguishes the 
lights if the counter indicates that all the objects have exited the room 
12. 
In one preferred embodiment of the present invention, a timer is used to 
ensure that the lights 24 are extinguished properly in the event that 
switch 10 does not discriminate multiple objects entering or leaving, such 
as when two objects enter or leave together. 
In another preferred embodiment of the present invention, multiple sensors 
and switches permit resolution of multiple object entries and exits. 
Additionally, for rooms having multiple entrances and exits, positioning a 
switch, or sensor, near each such entrance or exit properly monitors these 
rooms. A master counter for the room is incremented upon any entry and 
decremented for each exit, irrespective of which entrance or exit the 
object employed. Each exit checks the counter to determine if the room is 
empty. When the last object leaves, the lights are extinguished. For 
systems employing timers, each entrance into the room of an object resets 
the timer. 
FIG. 2 is a view of the PIR sensor 20 having dual sensing elements 26a and 
26b. As shown, for the use depicted in FIG. 1, the preferred embodiment 
orients the sensing elements 26a,b in the motion plane, which is 
horizontal for the application shown in FIG. 1. 
FIG. 3 is a block diagram of the switch 10 coupled to the lights 24. In 
addition to the PIR sensor 20 and the on/off control 22, the switch 10 
includes a control circuit 30, a timer 32, a counter 34 and a direction 
detecting circuit 36. In response to movement of an object in a field of 
view of the PIR sensor 20, infrared radiation illuminates the dual sensing 
elements 26a,b. 
FIG. 4 illustrates a typical output from the PIR sensor 20 responsive to an 
IR source moving in its field of view. As an IR source enters the PIR 
sensor's field of view, the IR illuminates one particular sensing element 
before the other. In the case of the configuration generating FIG. 4, the 
IR initially illuminates a positive sensing element first. As shown, the 
sensing element produces a positive voltage output, just as illustrated in 
FIG. 7. As the IR source continues advancing, IR no longer illuminates the 
positive sensing element. Without illumination, the positive sensing 
element produces a counter negative voltage, also as illustrated in FIG. 
7. A difference between FIG. 4 and FIG. 7 is that the IR source moves on 
to illuminate the negative sensing element, whereas the FIG. 7 
illustration includes a single sensing element. The illumination of the 
negative sensing element causes an initial generation of a negative 
voltage. The negative voltage of the negative sensing element and the 
counter negative voltage of the positive sensing element produces a net 
negative voltage having a magnitude twice that of the positive voltage. 
Subsequent movement of the IR source out of the field of view of the 
negative sensing element produces a counter voltage that is positive and 
that has a magnitude equal to that of the initial positive voltage. 
Movement of the IR source in a direction opposite to that which produced 
the voltage output described above produces a different voltage output. 
Essentially, the voltage output is an inversion of the voltage output 
produced from the oppositely moving IR source. The inversion results 
because the IR source initially illuminates the negative sensing element, 
producing a negative voltage followed by its positive counter voltage as 
the IR source continues movement. The subsequent illumination of the 
positive sensing element produces a positive voltage adding to the voltage 
output, causing a positive voltage twice in magnitude to the negative 
voltage. Subsequent movement of the IR source out of the field of view of 
the positive sensing element produces the counter negative voltage. 
The direction detecting circuit 36 of FIG. 3 monitors the output of the PIR 
sensor 20 to determine which of the waveforms types illustrated in FIG. 4 
is present. The direction detecting circuit passes this information on to 
the control circuit 30. Depending upon the specific embodiment and 
configuration of the switch 10 and the sensing elements 26a,b the control 
circuit 30 increments or decrements the counter 34, depending upon whether 
the direction indicated is into the room, or out of the room. 
If into the room, the control circuit 30 initiates illumination of the 
lights 24 if they were out, resets the time 32, and increments the counter 
34. If the direction detection circuit 36 indicates that the IR source 
exited the room, the control circuit decrements the counter 34 and 
determines if a value stored in the counter 34 equals a predetermined 
value (typically 0). If this value is stored in the counter 34, the 
control circuit 30 extinguishes the lights 24. If prior to the 
decrementing of the counter 34 to the predetermined value, the timer 32 
expires, the control circuit 30 extinguishes the lights 24. 
FIG. 5 is a flow chart of operation of the switch 10 diagramed in FIG. 3. 
Initially, the method begins at Start, step 200. The control circuit 30 
initializes the counter to zero (step 202) and turns the lights 24 of FIG. 
1 off (step 204). The switch 10 waits for a pulse from the PIR sensor 20. 
Step 206 determines if the pulse is a positive pulse exceeding a 
predetermined threshold. If it not such a pulse, the system determines at 
step 208 whether the pulse is a negative pulse having a magnitude 
exceeding a predetermined threshold. If the pulse is neither a positive 
pulse or negative pulse of sufficient magnitude, the system branches back 
to step 206 to check the next pulse. If the pulse is either a positive 
pulse or a negative pulse of sufficient magnitude, the system next checks 
for a pulse of opposite polarity at step 210. This opposite polarity pulse 
must also exceed a predetermined threshold, which in the preferred 
embodiment is greater than the first pulse threshold. Failure of the 
second pulse to be of the proper polarity or insufficient magnitude 
branches the system back to step 206. If the order of the first two 
amplitude-qualified pulses is correct, the system checks, at step 212, for 
a third amplitude-sufficient pulse having a polarity like the original 
(first) pulse. If the pulse order for the three pulses is not correct, the 
system returns to step 206. Correct three pulse order advance the system 
to step 220. Step 220 checks whether the polarity of the second pulse of 
the three pulses was positive. For the signal of FIG. 4, a positive second 
pulse indicates that the IR source monitored exited the area being 
monitored. Thus, if the second pulse were not positive, the system 
advances to step 222 to increment the counter. Incrementing the counter 
indicates that a person entered the room. Upon entering the room, the 
system also resets the timer at step 224 and checks at step 226 whether 
the lights are on. If the lights are on, the system returns to step 206 to 
wait. If the lights are out, the system, at step 228, turns the lights on 
and returns to step 206. 
At step 220, if there had been a positive second pulse, the counter is 
decremented at step 230. Checking the counter at step 232 determines if 
the room is now empty. If the counter is not zero, there is at least one 
person in the room so the lights should not be turned off. If at step 232 
the counter is not equal to zero, the system branches back to step 206. 
However, if the counter value is equal to zero, the lights are turned off 
(step 234) and then the system returns to step 206. 
Expiry of the timer causes an immediate extinguishing of the lights and a 
reset of the counter. Essentially, the system returns to start. It is 
possible to implement the timer in the hardware or to include timer 
checking in the flowchart of FIG. 5. FIG. 5 does not include timer 
checking. It could be accomplished by adding a timer check process into 
the pulse checking loop of steps 206 through 212 or after the counter 
check, step 232. 
There is also an optional set of steps (not shown) after any of the steps 
206, 208, 210 have indicated presence of a pulse. This optional step would 
change the switch 10 into a `timer` mode in which the step 234 is 
deactivated. This could be desirable for situations in which the counter 
recorded an incorrect number of objects. The switch is the timer mode 
would still monitor direction and perform different actions based upon 
whether an object entered or left. Entering the room causes the timer to 
be reset and lights to be turned on. Leaving the room has no effect on the 
timer, and it continues to count down to a point where it will turn the 
lights off after a sufficient time lapse from the last detected entrance. 
FIG. 6 illustrates a detector so with multiple PIR sensor 20. The control 
circuit 30' monitors each of a plurality of direction detecting circuits 
36' corresponding to a plurality of PIR sensor 20. For multiple entrances 
into a room, the detector 50, configured so that a PIR sensor 20 monitors 
each entrance, will efficiently control the lights of the room. The 
operation of the detector 50 is similar to that of the switch 10 except 
that the control circuit 30' receives a plurality of signals indicating 
entries and exits of IR sources. For each entry, the control circuit 30' 
increments the counter 34, while it decrements the counter 34 for exits. 
The timer, reset at each entrance, will cause the control circuit 30' to 
extinguish the lights if it expires prior to the counter 34 attaining a 
predetermined value. 
The configuration of FIG. 6 is useful for more than multiple entrance 
rooms. By proper positioning of the PIR sensors 20 and their associated 
direction detection circuits 36' and including a derivative circuit 52 
within the control circuit 30', velocities of the moving IR sources is 
determinable. Velocity is a derivative of position with respect to changes 
in time. Thus, when a moving IR source passes by two PIR sensors 20, the 
control circuit 30' can determine elapsed time between detections of the 
moving IR source by the different PIR sensors 20 and determine a speed of 
the moving IR source. By understanding the physical relationship between 
the PIR sensors 20 and their relative positioning, various information 
relating to the IR source's motion are determinable. In fact, the shape of 
each of the waveforms of FIG. 4 encode speed information as well, which is 
usable by the system depending upon particular applications. 
In conclusion, the present invention offers a simple and cost effective 
mechanism to measure direction and speed of an object in addition to 
motion detection by use of PIR sensors. While the above is a complete 
description of the preferred embodiments of the invention, various 
alternatives, modifications, and equivalents are possible. For example, 
thin film sensing elements, or other materials exhibiting the pyroelectric 
effect, are substitutable for the LiTaO.sub.3 sensing elements. 
Additionally, different ways of discriminating the voltage signals to 
produce the motion information, such as determining direction by detecting 
a large voltage, plus or minus indicating direction, or using the voltage 
transitions. For example, two positive and one negative pulse identifies a 
particular direction. Additionally, checking a polarity of a `sandwiched` 
pulse will also indicate direction. Other variations include addition of 
more sensors or more sensing elements, or both. Speed and distance 
information is available from knowledge of the optics and sensor/sensor 
elements spacings. An additional sensor/sensor elements, for some 
embodiments, improves object counting, permitting confirmation of object 
counts. Addition of an audio sensor to reset the timer helps to prevent 
prematurely extinguishing the lights. In the description, the preferred 
embodiments presume a stationary sensor and moving IR sources. It is one 
variation to mount the sensors on rotating structures or to employ 
mechanical choppers. Furthermore, the described embodiment includes 
reversed polarity sensing elements. Another embodiment of the present 
invention includes two or more similarly polarized elements arranged so 
that the detection signals of each are individually detected. A barrier 
between two similarly polarized elements would enhance detection 
performance. By separately detecting each of the signals from each of at 
least two of the similarly polarized detecting elements, a direction and 
speed of an object's motion is detectable. Therefore, the above 
description does not limit the scope of the invention that is defined by 
the appended claims.