[Dprglist] Light Sensors, Moth/Anti-Moth Behaviours, and PID steering

Thu Feb 11 19:28:57 PST 2021

Murray,
      I used a moth robot to occupy my grandkids time (and my own) watching
it move room to room to find the brightest light and then circle it just
like a moth. Really fun stuff. It is amazing to see the behavior based on
such a simple algorithm. It is best to start the robot in a dark room
that has a door to a lighted room. The longer you can make the light
gradient (several rooms if possible), the more fun it is to watch. It is an
excellent project.

Regards,
Doug P.

On Thu, Feb 11, 2021 at 9:17 PM Doug Paradis <paradug at gmail.com> wrote:

> Murray,
>      Moth / Anti-Moth (i.e., Cockroach) behavior robots are really fun for
> children and are very simple to make. All you need for sensors are 2 CdS
> light sensors and 2 A/D pins. The Build More Robot tutorial series had a
> session devoted to them. They were also demonstrated using the Tiny Wander
> DPRG club robot (see Make Magazine vol 29) which used a Attiny13 8 pin
> processor.
> Regards,
> Doug P.
>
> On Thu, Feb 11, 2021 at 5:50 PM Murray Altheim via DPRGlist <
> dprglist at lists.dprg.org> wrote:
>
>> Hi,
>>
>> Among my other side projects is a moth/anti-moth behaviour. Rather than a
>> passive sensor I thought to use one that provides a better very-low to
>> bright ambient light range. From Pimoroni I ordered a pair of Adafruit
>> GA1A12S202 Log-scale Analog Light Sensors, which are cheap and tiny 3-pin
>> boards that produce an analog output. They arrived yesterday and I built
>> a small holder for them on the front of the robot, wired them up to a pair
>> of analog inputs, then began writing a Python support class.
>>
>> But when I thought to look into more details about the sensor I found out
>> that Adafruit have discontinued these sensors because they don't actually
>> work so well as described. They intended to replace them with a Vishay
>> VEML7700 lux sensor wired up as an I2C device, but sadly the designer
>> never
>> thought to provide a way to alter the fixed I2C address, so one can only
>> mount a single sensor on a robot without resorting to multiplexing, which
>> I'd rather avoid. A real oversight. Nice little board otherwise.
>>
>> Investigating the VEML7700 it turns out that its support library has some
>> serious bugs at higher light levels, such as what one would encounter
>> outdoors (!), and that part of the problem may be the sensor itself.
>>
>> Vishay make an improved sensor called the VEML6070, and Adafruit even make
>> a breakout board for it (since the sensor is SMT), but, but, but, they
>> again didn't provide for altering the I2C address.
>>
>> So for now I'll be sticking with the GA1A12S202 since I have two of them
>> installed and my robot (for now) is indoors. But since the board has been
>> discontinued and apparently isn't good for outdoor use I'm looking for
>> alternatives.
>>
>> Adafruit list 26 different sensor boards (!), but it's unclear which have
>> good performance and suitable output across a wide light range, and of the
>> 26 many have been discontinued. I'm not sure if a UV sensor would work as
>> well for a moth/anti-moth sensor; I'm guessing not so well in a dark room.
>> There's a TSL2561 that permits altering the I2C address but has also been
>> discontinued. From what I can see, almost all of the I2C light sensor
>> boards (e.g., TSL2561) that haven't been discontinued have fixed
>> addresses.
>>
>> The one exception to this is the BH1750,
>>
>>    https://www.adafruit.com/product/4681
>>
>> which has a solder bridge to change the address. I may order a pair of
>> these,
>> as their lux range looks suitable. It's a bit larger than I'd like (at
>> 25x18mm
>> it's about 3x bigger than the pair of sensors I'm using now) but seems to
>> be
>> the only one Adafruit sell that fits the bill. Is it any better than the
>> little GA1A12S202 boards I have?
>>
>> Anyone have any experience with this or could recommend a good light
>> sensor?
>> And preferably small and relatively cheap? Most of these boards are well
>> under US$10, e.g., $2.50, $4.50.
>>
>> ----
>>
>> This is all in mind of developing that moth/anti-moth behaviour, where the
>> sensor is supported by a Python class that has various possible outputs:
>> an
>> Orientation enum indicating where the brightest light is coming from (an
>> output of PORT, STBD, or NONE, with a hysteresis setting to permit a
>> center
>> point); a bias value from -MAX_V to +MAX_V; a pair of raw floating point
>> values from 0.0 to 3.3 (volts); and a pair of ints from 0 to MAX (which is
>> calculated as round(v) * 100, in this case 330).
>>
>> I've got this up and running, supported off two analog input pins on a
>> Pimoroni IO Expander board (the little Nuvoton MS51 on the front of my
>> robot), so far so good:
>>
>>     https://github.com/ifurusato/ros/blob/master/moth_test.py
>>     https://github.com/ifurusato/ros/blob/master/lib/moth.py
>>
>> ----
>>
>> With our ongoing discussion of PID loops and the use of a PID controller
>> for steering a robot, using velocity+rotation rather than left and right
>> velocity as the steering model. The idea I had was that if one were using
>> a simple proportional PID for steering, to implement a moth or anti-moth
>> behaviour it'd be a matter of adding the output from get_bias() to the
>> rotation value, multiplied by some kind of fixed constant to control how
>> forcefully the robot reacts to changes in light level. I thought about
>> having its reaction slow as it got closer and closer to bright light,
>> until at a certain threshold it would stop (in moth mode). A bit like my
>> cat. This could be used to charge the batteries from a solar panel.
>>
>> Is this something that anyone in the group has done before? If so I'd be
>> interested in hearing of your experience and any advice.
>>
>> Cheers,
>>
>> Murray
>>
>>
>> ...........................................................................
>> Murray Altheim <murray18 at altheim dot com>                       = =
>> ===
>> http://www.altheim.com/murray/                                     ===
>> ===
>>                                                                     = =
>> ===
>>      In the evening
>>      The rice leaves in the garden
>>      Rustle in the autumn wind
>>      That blows through my reed hut.
>>             -- Minamoto no Tsunenobu
>>
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