In this chapter we will discuss the optical sensing techniques that will be integrated into the robot itself, to enable it to locate and successfully dock with the charger beacon. There are two steps involved with this process. First, the robot must become aware that it is within range of the charger beacon with a non directional Omni type sensor that will detect the beacon no matter which direction it is relative to the robot. This is called "Proximity Detection". Once the robot has determined it is near the charger, a less sensitive narrow angle sensor allows it to home in on the beacon and direct it to the charging base. This is called the " Directional Sensor". Here we will cover both topics, and give you a sense of the approaches that are commonly used to get our bot to its base. In these diagrams, sensors are usually color coded red, as is the source IR beams.

Proximity Detection

 A
            very simplistic approach to proximity detection is to enclose
            a sensitive IR detector inside a translucent dome. When the surface
            of the dome is struck with infrared modulated light, the sensor
            will see the domes underside light up and a crude detection is
            accomplished. While fairly low in sensitivity, if one uses a
            Panasonic PN4602 integrated sensor, the range can be six feet
            or more in a normal room brightness.

 An
            overhead view of how this works is seen here. The beam will sweep
            over the robots central detector as the robot moves forward only
            when it is within the narrow confines of the beam. The robot
            would then immediately stop, and start its routine for determining
            the beams point of origin.


 With
            a centrally located multi-beam array on the beacon, the robots
            chances for detecting its proximity to the charger are greatly
            increased. Such an extreme approach is suitable for office or
            factory environments, however the single beam is more suitable
            for a home environment.

Directional Sensing

 This
            is how the basic analog and digital vane sensors work. When the
            vane points to the left of the beam, the right side is in the
            light and the left side is in the shadow of the vane and sees
            no light. By rotating the vane back and forth by rocking the
            body of the robot or moving the sensor on a servo motor, we can
            determine when the vane is directly aiming at the source when
            both sensors just see light at the same time. In the case of
            an analog system, cadmium sulfide photo cells can give a linear
            voltage reading which indicates the source direction by comparing
            the two output voltages. For a digital sensor such as the PN4602,
            when the vane is exactly pointed at the source, both will have
            an output. Proper filtering of the digital outputs is ESSENTIAL
            for giving the microcontroller a clean signal. More on this later.

 Another
            type of directional sensor used is the tube type. Here, a single
            photocell is mounted at the bottom of a black tube. Only when
            the tube is aimed directly at the source will there be an output.
            The robot then rotates 360 degrees slowly and when the beam is
            detected stops rotation. It then drives forward until it looses
            the beam, then does its search all over again. This simplistic
            approach can work fairly well, but the robot uses up a lot of
            energy spinning every few feet to get to the charger!


 A
            rotating cylinder with a single photocell in the center or back
            of the tube opposite of the slit can also be used to locate the
            beacon. The idea is either the entire robot or just the cylinder
            rotate, and when the slit in the side of the cylinder is aimed
            exactly at the source, the photocell will become active. I rather
            like this approach because the cylinder can be easily turned
            by a servo, and the slit can be so narrow as to get directional
            accuracy needed for a precise navigation to the charger.

 An
            example of a four way vane sensor in action. This was one of
            our experiments with the PicBot5 and provided first, an crude
            direction for the robot to turn, and second the vanes can be
            used with two of the sensors to home right in on the robot. Later
            we will have lots of images and some movies of this technique
            in action.



 With
            an analog approach, and two IR phototransistors, we can simply
            measure or compare the voltages from two sensors mounted about
            where the eyes are in the graphic on the right. You can either
            use a compartor to do the comparison and put out a digital signal
            to the processor or send both analog voltages directly to the
            analog in ports on the micro.


 A
            very cool approach to beacon location is to rotate the IR beam
            on the charger, and encode its rotation angle in the data stream
            on the IR beam. The robot then knows its polar angle from the
            charger and perhaps using sonar can get the radius to the charger
            to get its exact position in the room.

 The
            "Roomba" docking technique is to encode in the beam
            what the robot should do if it encounters the beam. So the robot
            will drive along, into one of the side beams and start turning
            toward the charger as it is instructed to do so. Such a system
            must be perhaps in a hall way to limit the robot so it does not
            cross the center beam first.


 Another
            thought at encoding the beams with data, if we simply number
            the beacon beams which are active all of the time, we can get
            directional information to the charger. This allows us to COMBINE
            a charger beacon with a complete navigation system in a small
            room or office area in one unit.