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The BEAM Circuits Collection is a BEAM Reference Library site.

The "BEAMant"
A monocore--based photopopper

The BEAMant circuit is a Mark Tilden original and dates back to 1999.

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BEAM ant circuit schematic and freeform layout diagrams courtesy of BEAM online

Here's a description of the circuit that Wilf sent to the Yahoo BEAM list back in 1999:

BEAMant - Description of circuit operation

Wilf Rigter 05/06/99 BEAMant (c) M.Tilden

Overview of circuit operation and probable behaviours
The core of the BEAMant consists of 2 coupled master Bicore oscillators. The photo Bicore will oscillate over a wide range of frequency and duty cycle and the motor Bicore has a relatively fixed frequency with a duty cycle influenced to a lesser or greater extent by the state of the photo Bicore. Under some conditions the photo Bicore will have NO influence on the motor Bicore, which is the reason for describing the BEAMant core as coupled master Bicores rather then master slave Bicores (I am not sure if the term "embedded Bicores" applies here).

The output of the photo Bicore is acting on the bias points of the motor Bicore and changes the duty cycle of the motor Bicore oscillator but not it's frequency. This coupled Bicore circuit (Unicore?) overcomes a serious limitation of the conventional photo Bicore in which both duty cycle and frequency are affected by light. There are three categories of circuit behaviour which depend on the light level, and component selection.

1. In the dark the photo Bicore oscillates many times slower than the motor Bicore.

2. In medium light the two Bicores oscillate with a small ratio between the two frequencies (ie 2:1 to 1:2)

3. In bright light the photo Bicore frequency is much higher than the motor Bicore frequency.

In the dark, with the photo Bicore at the lowest frequency, the ant will turn in clockwise and counter clockwise circles as the motor duty cycle is alternately changes. then as the light brightens it will alternately steer left and right.

As the light increases to medium some complex and even chaotic behaviour can occur as the coupled oscillators do a little dance at low harmonic ratios. This includes jittery phase locking behaviour with a chaotic quality . So in the medium light the ant may behave in complex, strange and unpredictable ways.

In brighter light the photo Bicore frequency rises and with higher frequency ratios, we see the ant moving in a straight line with moderate photo tropic behaviour. The high frequency photo Bicore output will be integrated (smoothed) by the motor Bicore inputs and the duty cycle information of photo Bicore will be extracted in the form of differential dc voltages which influence the bias points of the motor Bicore. This in turn changes the duty cycle but not the frequency of the motor Bicore.

In order to add a threshold to the point where the BEAMant changes from travelling in a straight line to turning behaviour, one shots or monostables were added to the motor Bicore outputs. This is sometimes called a "dead band" in which motor Bicore duty cycles close to 50% (ie 45%-55%) are ignored. The oneshots are really Nv stages with an "ON" time which is shorter than the shortest motor Bicore on time which you want to ignore. In that case the motor will rotate at equal speed if the motor Bicore duty cycle is near 50% This occurs because the motor bicore outputs duration's will be slightly longer than the Nv outputs and small difference will not affect the motor duty cycle. When the motor Bicore duty cycle is changed to the point where one motor Bicore output has a shorter duration than the Nv TC then this shorter pulse will be transmitted unaltered to the output of the corresponding Nv while the Bicore output with the longer duration continues to be limited by the Nv time constant. The result is that the on time of one motor output is shorter and the BEAMant will turn into that direction.

This turning is caused by several sources including low light "searching" where the slow cycle of the photocore is directly expressed as alternating left/right turns and at very low light clockwise and counter clockwise turning. At Medium light levels the turning is more photo tropic but may exhibit unusual waggles and detours ( phase slipping oscillators), In bright light the behaviour should be quite predictably photo tropic. The speed of the BEAMant should be constant except during turns. The so-called XOR Nu circuits will change this behaviour to turning or reversing or photo phobic in case of a side or head on collision and depending on the duty cycle of the photo Bicore.

Bicore details of operation
The photo Bicore (Pcore) is a dual photo to pulse width generator and the motor Bicore (Mcore) is a differential voltage controlled duty cycle generator. More precisely the Pcore outputs are complementary signals whose frequency depends on R/C and the sum of the photo currents AND whose duty cycle depends on the difference in photo current. The complementary Pcore outputs are connected to the Mcore with two equal resistors resulting in coupling currents which are dependent on the voltage difference of Pcore outputs and the Mcore bias point inputs. The Mcore outputs are complementary signals whose frequency depends on R/C and on the sum of the Pcore coupling current AND whose duty cycle depends on the difference of Pcore coupling currents.

The resulting influence of the Pcore on the Mcore output duty cycle is delightfully complex depending on the frequency ratio of the two coupled oscillators and the duty cycle of the Pcore.

For the purpose of the BEAMant, frequency ratios are dependent on light conditions as follows:

  1. In low light, the Pcore frequency is much lower than the Mcore and the ratio is high.

  2. In medium light, the frequencies are similar and the frequency ratio is low.

  3. In bright light, the Pcore frequency is much higher than the Mcore and the ratio is high.

The range of Mcore duty cycle depends on the ratio of the Pcore coupling resistors and the Mcore timing resistor.

This gives rise to 3 distinctively different modes of operation:

  1. In low light the Mcore duty cycle depends on the DC state of the Pcore outputs. ie once every few seconds the Pcore changes state and the duty cycle of the Mcore changes state correspondingly e.g. 25% to 25% to 75% etc.

  2. In medium light the Mcore duty cycle strongly depends on the phase difference of the Pcore and Mcore signals. At very low frequency ratios, the Mcore duty cycle will be modulated by the difference frequency gradually shifting between 25% and 75 % and back to 25% etc. If the 2 frequencies are nearly equal the Mcore may phase lock to the Pcore with the Mcore duty cycle dependent on the leading or lagging phase difference.

  3. In bright light, when the Pcore frequency is high, the Mcore duty cycle will depend only on the Pcore duty cycle ie the difference in photo currents.

Oneshot Nv Stage/PWM motor drivers - detail of operation
The Nv motor driver output stages are "oneshots" which is what Tilden calls them on his neural network drawing. These are not conventional oneshots however since their time constant is not independent of the input signal. For the purpose of this discussion I will call these "resettable oneshots" simply Nv. For a given rising edge on the input, the Nv output goes low for a period determined by RC but this period is shortened if a falling edge occurs on the input before the Nv times out and the output is reset on that falling edge.

More than one behaviour is possible depending on various ratios of time constants. The most obvious function of the Nv is as a duty cycle threshold detector providing a "dead band" between driving in a straight line and turning. Since the direction of a 2 motor platform is sensitive to the motor bicore (Mcore) duty cycle, tuning would be required to make it go in a straight direction under "neutral" conditions.

In practice, bicores are asymmetrical and coupled oscillators are not particularly stable. Therefore uncontrolled parameters (i.e., threshold, temperature, frequency) can shift duty cycle which would detune a "straight direction" adjustment. This can be corrected by adding a duty cycle dead band between the mcore and the motor large enough to ignore small duty cycle variations.

This is done with a "resettable oneshot" Nv on the motor Bicore outputs with Nv R/C time constant (TC (Nv)) adjusted for a "full pulse width" which is shorter than minimum Mcore output pulse width that you want to ignore. Now any pulses from the Mcore that are greater than the TC (Nv) will always be limited to the full pulse width of the Nv but pulses that are shorter than TC (Nv) will reset the output of the Nv on the falling edge and are therefore equal to the Mcore pulse width.

Other ratios between the time constants of the motor bicore and the Nv oneshot exist which give rise to a different function but I believe that this description was the reason for including a Nv in the motor Bicore outputs.

XOR Nu sensor/direction motor drivers - details of operation
The term XOR, as it is applied here, was derived from the boolean eXcluxive OR logic. In this case,it means a motor will turn only if one terminal is high and the other low. One side of each motor is driven by the one shot Nv motor drivers and the other side of each motor is connected to the output a Nu motor driver.

Together each Nv and Nu driver form a bridge with the Nv side providing fixed frequency / PWM pulses and the Nu side providing a negative or positive supply reference for motor reversing. When the motor reference supply is high it causes the motors to rotate in the forward direction and when Nu output is low, it causes the motor to rotate in the reverse direction.

Each Nu stage input is connected to a tactile sensor which detects collisions on opposite side of the motor to which it is connected as well as full frontal collisions. The function of the Nu stages is to stretch a tactile input (determined by TC (Nu)) and reverse the motor supply reference voltage for the duration of the Nu pulse width

When a collision occurs on one side the Nu, the supply reference for the motor on the other side is reversed and for the duration of the TC (Nu) that motor will now rotate in the reverse direction with each active Nv PWM pulse width determined by the normally inactive "off time" for that motor.

Assuming high light conditions, the BEAMant behaviour is reasonably predictable, and side collisions cause the BEAMant to pivot or spiral on it's axis. Unless the second tactile sensor is activated it will continue to spiral until the Nu times out. The photo effect for the time that the motor is reversed becomes photo phobic for the reversing motor and photo tropic for the forward motor.

If both tactile sensors are activated the BEAMant will exhibit it's normal waggling behaviour in reverse at a faster than normal speed and the overall photo effect will be photo phobic. Under medium and low light conditions behaviour is more complex and (for me) difficult to describe in detail (you'll just have to build a BEAMant and tell me).

Well this is my best understanding of the BEAMant circuit operation and predicted BEAMant behaviour. Has anyone seen these critters in action? I look forward to hear examples of the real BEAMant behaviour both Mark Tildens's and of course your own experiments with this design.

I think there will be immediate applications for the coupled master Bicore (Unicore?) for walkers and heads.

Since the frequency of the motor bicore is more or less fixed, it solves several design obstacles related to the conventional walker and head photo Bicores including the zero power problem for a Bicore Head.


Wilf Rigter

For more information...

BEAM Online ('though no longer actively maintained) still has a page on the BEAMant here. More-modern (and simpler) 74*240-based photopopper designs are documented here.

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Page authors: Wilf Rigter and Eric Seale
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