Photo-bridge
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[edit] Photo-bridges
Put simply, a Photo-bridge consists of two Light Sensors that are connected in series, and used to regulate the duty-cycle of a simple Central Pattern Generator (CPG). This CPG might be a microcore, a Monocore, or any type of bicore.
Incorporating a photo-bridge into one of these circuits will produce a robot that is able to locate, track, follow or even avoid a light source that is brighter than the ambient background light within the filed of view of the photo-bridge sensors.
There are two primary class of photo-bridge, the earliest of which to be used on a BEAM-robot was probably the Bilateral type. Later, the Voltage Divider type of photo-bridge was introduced to the BEAM community.
[edit] Bilateral Photo-bridge
A basic bilateral photo-bridge made using photo-diodes can be seen in figure 1. Note the symmetrical (cathode to cathode) configuration of the photo-diode.
As a result, a bilateral photo-bridge can be incorporated into a circuit either way around without causing a noticeable change in its effect on the current that passes through it.
The schematic in figure-2 shows how a bilateral photo-bridge is integrated into a Suspended-Bicore. By using a photo-bridge in place of, or in conjunction with the timing resistor](s) in this way, a Suspended-Bicore is transformed into a Photo-Bicore (P-Core).
[edit] How a Bilateral Photo-bridge Works?
As long as the light detected by the photo-bridge remains in balance (detected equally by both photo-diodes) the P-Core will oscillate with a 50% duty-cycle. But any difference between the amounts of light detected by the photo-diodes will result in a proportional difference in the duty-cycle of the P-Core.
It works like this, the output of inverter-A has just gone from high to low and the output of inverter-B has gone from low to high. While in this state, photo-diode PD1 is forward biased and will allow current to pass through as if it was a regular diode.
Photo-diode PD2 on the other hand is reverse biased, so it resists the flow of current through it. How much resistance PD2 will exhibit depends on just how much light it is currently detecting.
An increase in the amount of light detected by PD2, will allow more current to pass. As lower levels of light are detected, less current will be able to pass though PD2. So the light level detected by PD2 affectively determines the length of the current half of the duty-cycle.
At the end of this half cycle the output of inverter-A will go low and the output of inverter-B will go high. Now PD1 will be reverse biased and PD2 will be forward biased.
Therefore current will pass through PD2 as if it was a regular diode, and PD1 will limit the amount of current that can flow through it relative to the level of light it detects. So the light detected by PD1 will govern how long this opposite half of the duty-cycle will last.
So if the light detected by PD1 is 1½ times brighter light than that detected by PD2, the output of inverter-A will be high 40% of the time and low 60% of the time. Meanwhile the output of inverter-B will be low 40% of the time and high 60% of the time.
The photo-diodes that make up the photo-bridge seen in Figure-3 the are inverted. That is, they are configured anode to anode, rather than cathode to cathode as with the photo-bridge shown in Figure 1 As a result, the duty-cycle of a Photo-Bicore that incorporates this photo-bridge will be opposite that of the P-Core seen in Figure 2. So under the conditions described in the above paragraph, the output of inverter-A would be high 60% of the time and low 40% of the time, while inverter-B would be high 40% of the time and low 60% of the time.
Therefore, if the P-Core in Figure-2 was used to implement positivephototropic behavior (photophilia) in a robot, then using the photo-bridge in Figure-3 would result in a robot that exhibits the opposite behavior, or negative phototropism (photophobia).
There is one potential drawback to this type of photo-bridge. That is that it causes the Suspended-Bicore's frequency to vary in relationship to the absolute level of light within the robot ’s field of view. As the amount of light increases, the frequency at which the Suspended-Bicore oscillates will also increase. Lower levels of light would of course mean that the Suspended-Bicore will oscillate at a lower frequency.
This can result in a robot that may function quite well in an area of moderate light. But in an area where the light is either very bright, or very dim, the robot can (and probably will) behave erratically.
Resistors can be used to keep this deviation in frequency within a bounded range. Looking back at Figure 4, the value of resistor R1 will fix a minimum oscillation frequency. The value of resistor R2 on the other hand can be added to limit the maximum frequency of the Photo-Bicore.
This technique will improve things a bit, but care should be taken not to go overboard. Doing so will limit the sensitivity range of this simple sensory-motor circuit.
A better method of dealing with this problem is discussed in the Photo-Bicore article.
[edit] Using LDRs
Figures 5a through Figure 6b show four ways that a bilateral photo-bridge can be made using LDRs (Light Dependent Resistors, AKA Photo-Resistors, Photo-Cells or Cadmium-Sulfide Cells (CdS)).
Unlike photo-diodes, LDRs conduct equally well in either direction. As a result, diodes must be used to either by-pass (Figures 5a & 5b), or block (Figures 6a & 6b) LDR-1 during one half of the Suspended Bicore’s cycle, and LDR-2 during the other half of the cycle.
[edit] Voltage Divide Photo-bridge
Figure 7a and Figure 7b are examples of the basic [[Voltage Divider]] type photo-bridge, first introduced as part of Wilf Rigter's Power Smart Head (PSH).
Unlike a bilateral photo-bridge it does not matter whether photo-diodes or LDRs are used. No addition diodes will be required. Also, a voltage divider type photo-bridge does not introduce any sensitivity to the absolute level of the light it detects.
Instead, a voltage divider photo-bridge will produce a single output voltage that is proportional to the difference between the levels of light detected by each of its photo-sensors.
The photo-bridge seen in Figures 7a consists of two photo-diodes connected in series. The cathode of photo-diode PD1 is connected to the positive (+) side of the power supply, while the anode of photo-diode PD2 is tied to the ground or negative (-) side of the power supply.
The anode of PD1 and the cathode of PD2 are connected together. This connection is tapped as the output of the photo-bridge.
The example shown in Figure 7b is functionally the same as the one in Figure 7a except that Light Dependent Resistors are used instead of photo-diodes.
[edit] How a Voltage Divider Photobridge Works?
As long as the light detected by the photo-sensors is balanced the output voltage will be approximately equal to Vcc/2, or half the power supply voltage.
If PD1 (or LDR1) detects around 1½ times more light than does PD2 (or LDR2), then the output voltage will equal Vcc x .6, or about 60% of the power supply voltage.
On the other hand, if PD2 (or LDR2) were to detect twice as much light as PD1 (or LDR1), then the output voltage will be approximately Vcc x .3, or about 33.3% of the power supply voltage.
The output voltage from this type of photo-bridge is generally used to modulate the duty-cycle of a Monocore, also known as a High/Low Oscillator (HLO).
[edit] A Photo-bridge With Two Outputs
The photo-bridge seen in Figures 8a is similar to the photo-bridge seen Figure 7a. The only difference is that there is one or more regular diode(s) placed in between the photo-diodes.
The cathode of photo-diodes PD1 is connected to the positive (+) side of the power supply, and its anode is connected to the anode end of the diode(s) in series. The anode of photo-diode PD2 is tied to the ground or negative (-) side of the power supply, while the cathode of PD2 is connected to the cathode end of the diode(s) in series.
This is variation was also introduced by Wilf Rigter as part of an improved version of Martin Keen's Schmitt Comparator Head. Two of these are used by the PV Roller.
This type of photo-bridge functions just like the original voltage divider type, except that the junctions between the photo-diodes and the diode(s) that are in series between them produce two distinct voltage outputs or taps. The output voltages coming from these taps are approximately {N x 0.7 volts} apart.
Where N equals the number of diodes placed in series between the photo-diodes. Figure 8b shows how the same concept can be implemented usin Light Dependent Resistors instead of photo-diodes.
[edit] UCore Photo Bridge
Wilf Rigter described a voltage divider type of photo-bridge implemented as part of a microcore based walker (see figure 9 below). A description of this arrangement can be found in the BEAM Wiki article titled: "UCore Photo Bridge".
