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

Thu Feb 11 19:37:34 PST 2021

Murray,
     Here is a moth robot run from the Build More Robots tutorial series.
It doesn't circle a light because there is no local maximum in the
different room lightings, but it found the "outdoors".(window with
sunlight). Note that it starts in a darkened room.
https://www.youtube.com/watch?v=G08ctK1rGfs

Regards,
Doug P

On Thu, Feb 11, 2021 at 9:28 PM Doug Paradis <paradug at gmail.com> wrote:

> 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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>>
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