Photo-Bicore

Photo-Bicore

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Contents [hide] 1 The Phototropic Suspended Bicore 2 Uses 3 PWM controls motor bicore dutycycle 4 External References

[edit] The Phototropic Suspended Bicore

The duty cycle of a suspended bicore can easily be modulated by using a Bilateral Photo-bridge (typically a pair photo-diodes wired in series cathode to cathode) in place of, or in conjunction with the timing resistor(s). As a result, the duty-cycle of this modified suspended bicore will vary in direct relationship to the difference between the levels of light detected by each of the photo-diodes.

Figure 9

If there is half as much light falling on one of the photo-diode as compared to the other, the duty cycle of the bicore's output signals will exhibit the same ratio (e.g., 2:1 ratio). Now the motor will rotate twice as long in one direction than the other. It's a bit like taking one step back and two steps forward, on average you go forward.

The frequency at which the Photo-Bicore oscillates will also vary, but in proportion to the absolute level of the light detected by both of the photo-diodes.

As the overall level of detected light increases, the frequency at which the Photo-Bicore oscillates will also increase. Lower levels of light would of course mean that the Photo-Bicore will oscillate at a lower frequency.

[edit] Uses

At one time, the photo-bicore was commonly used as a stand alone head circuit. This was (obviously enough) called a "Bicore Head". But today, Wilf Rigter's "power smart head" has for the most part replaced it a the standard circuit for building BEAM heads.

The photo-bicore can be used as effective analog to digital converter for use with a microcontroller (uC) based robot. That is, if the particular uC is not equipped with analog inputs. One of the photo-bicore outputs could be connected to one of the microcontroller's digital inputs. The uC could then be programmed to extract the absolute level of detected light by measuring the frequency of the incoming photo-bicore waveform. It can also extract information about whether there is a brighter light source to the left or the right side of the robot, and if so determine which side is brighter by examining the duty-cycle of the waveform.

Today. a photo-bicore is most commonly used as the front end of a BEAMant, or as the Master portion of a Master/Slave Bicore based Walker. It should be noted that the previously described frequency variations could mean that although the walker might function quite well in an area of moderate light, in an area where the light is either very bright, or very dim, the robot can (and probably will) exhibit erratic behavior.

Figure 10

Resistors can be used to keep this deviation in frequency within a bounded range. Looking back at Figure 10, the value of resistor R1 will fix a minimum oscillation frequency. The Resistor R2 on the other hand can be added to limit the maximum frequency of the photo-bicore.

Utilizing these resistors will improve things a bit, but care should be taken not to go overboard. Doing so could limit the effective sensitivity of this simple sensory-motor circuit.

[edit] PWM controls motor bicore dutycycle

There is a way to deal with the above mentioned problem of frequency variation. The following is an excerpt from message #44453. It was posted to the BEAM email list by Wilf Rigter, and describes an effective way to use a separate photo-bicore as part of a walker.

Begin Excerpt

For operation over a wider range of light, try the attached variation on the unicore circuit. Unlike the unicore circuit, the operation of this circuit is designed to be more predictable.

It uses a phototropic bicore front-end which operates at a high (100Hz-100kHz) frequency. This generates complementary rectangular output waveforms with dutycycles determined by the ratio of light on the two photodiodes and frequency proportional to the absolute light level.

These two waveforms Pulse Width Modulate, the current in the two 1M timing resistors of the motor control bicore. When the light is balanced the dutycycle of these PWM signals is 50% which connects the grounded bicore timing resistors to ground 50% of the time. With 50% PWM waveforms, the effective values of the 1M resistors are therefore 2M respectively.

As the PWM duty-cycle of the two complementary signals varies, the effective values of the two 1M timing resistors goes up and down respectively, changing the dutycycle of the grounded bicore motor controller but leaving the frequency relatively unchanged.

Figure 11

The increased frequency of the phototropic PWM signals with increasing absolute light levels has no effect on the grounded bicore.

The lowest PWM frequency should be at least 100 times the grounded bicore frequency to interaction with the grounded bicore frequency.

A 1M resistor in parallel with the photodiodes sets the minimum phototropic bicore frequency to about 100Hz.

Regards

Wilf

End Excerpt

Figure 12 - Walker with ratio controlled duty-cycle

[edit] External References

PWM controls motor bicore dutycycle By: Wilf Rigter

BEAM Message #44453 Date:Aug 11, 2004 [1]