Robotic Plant 5 – Really Final Report

TOC

Introduction

This is the second, updated version of my final report, which I was allowed to submit again, since the deadline was extended.  I was successful in making the mechanism work, using a different MOSFET and some additional batteries to power the motor.

Description

This project is a solar-engine-based “roboplant”, that remains dormant with its blossom retracted until its solar panels have absorbed enough light for the plant to grow and bloom, at which point the stem is extended and the blossom opens.  It is based around a Miller Solar Engine circuit, in which the solar panels charge a bank of supercapacitors monitored by a voltage detector.  When the voltage detector determines the voltage across the capacitors has reached the desired level, it turns on a MOSFET that is hooked up to power a load of some kind.  In this case, the load is a motor designed to extend the plant’s stem, and it is driven by a pack of 4 AA batteries.  When the motor drives the stem to its full extension, a jumper is pulled off of the line between the batteries, removing power from the motor.

The blossom is a big mess of paper clips, paper, brads, and double-stick tape, attached to a drinking straw for the stem.  It is pulled open by tension from thread attached to the base near the solar cells.

URL and Photos

The introduction to the series is at Robotic Plant 0 – Introduction, and the photo gallery is at Robotic Plant 2 – Gallery.  This is the final schematic:

The motor is represented by R2, the load.  The battery pack is G1, and the solar panel comprises cells G2-G5.  See Solar Engine Design for more details.

Note:  I have taken some videos of this plant in operation, I’m waiting for my cousins to extract them from the recorder, so I’ll post them as soon as they’re available.

Bill of Materials

Designator: Description: Quantity: From:
D1-D7 1N5817 Schottky diode 6 Digikey
D8 1N4004 flyback diode 1 Digikey
C1 10uF 16V capacitor 1 Digikey
C2-C3 10F 2.5V capacitor 2 SparkFun
R1-R2 100K ohm resistor 2 Digikey
IC1 Microchip TC74VC3002EZB 3.0V voltage detector (TO-92) 1 Digikey
Q1 FQPF13N06L Enhancement Mode N-channel MOSFET 1 Mouser
G1 AA battery 4 Sparkfun
G2-G6 SolarBotics solar panel SCC2433B 5 SolarBotics
JP1 2-pin jumper 1 This Sparkfun one will do, but one with a handle makes it easier to attach the string.
- 4xAA battery holder 4 Sparkfun
- misc. perfboard 3 ebay
- hookup wire several feet ebay
- terminal blocks a bunch ebay
- flower pot 1 Ikea
- 8″ diameter brown fabric 1 my wife’s scrap box
- 8″ diameter cardboard 1 an old box
- sturdy string for stem extender 1 ft. my junk drawer
- sturdy thread for blossom 2 ft. my wife’s scrap box
- Meccano erector set with motor and gears 1 my childhood
- white drinking straw, with red stripe 1 a movie theatre
- colored paper, for the blossom 2 sheets my paper drawer

Challenges

With no microcontroller involved, I couldn’t have any software challenges, but as it turns out, electrical and mechanical challenges more than made up for it.  I spent a lot of time early on trying to get my articulated furling and unfurling stem (depicted in the initial design concept).  However, none of my piddly toy motors running at 3V (or even 4.5V, or 6V) could muster enough oomph to fully extend it.  Part of the problem was the way the articulation and tension on the string was structured, since the first part to pull tight and be extended was the very end, by the time it had unfurled it had to haul the entire length of the arm up.  So, I redesigned it with a vertically rising stem, hidden when retracted in the base of the flower pot.

The next trick was controlling the extension of the stem, since I don’t have a servo, and was using a dumb toy motor.  I had tried using a set of batteries and having the stem pull a switch to turn itself off once it reached its fully-extended height, but I couldn’t find any switches that the motor was strong enough to pull.  After that, I tried putting a jumper in line with the power to the motor, which would be pulled away by the extending stem.  This, finally worked, when everything was placed just right.  But I spent a lot of time trying different things, without success.

Finally, selecting the MOSFET was challenging, since I haven’t made my own design around one before, I didn’t know what kind of current and voltage capability I’d need.  I spent a lot of time trying to use a 2N7000 (see the first entry), and ended up using the FQPF13N06L (the only other PTH MOSFET I had available).  I did not take any time to do experimentation to see if the 2N7000 would work for me under other conditions, I just applied the shotgun debugging approach, since I had so little time to spare.

Improvements

Since there is no longer a significant load on the capacitors (because the motor is powered by batteries), the voltage detector will remain “on” until the self-discharge drains the capacitors beneath the lower threshold (about 2.94V).  It would be nice to have a 555-driven blinking LED built-in to the blossom, or some other visual element powered from the capacitors, that would run until they were drained and then turn on again periodically when they were recharged (since the bloom would remain open until manually reset).

As I mentioned in the design discussion, it might be good to have a full H-bridge, and a few buttons to allow me to momentarily reverse the motor, to reset the stem and retract the blossom without pulling everything apart and doing it by hand.

Finally, I would have liked some more analysis of the performance of the supercapacitors, to see what kind of self-discharge and charge-up times could be expected, so I could tell the recipient of the plant when it might be good to pay extra attention to it.  Some kind of buzzer or other audible warning that a bloom is about to occur would be nice, too, especially if it were a more spectacular bloom than the one I have planned.

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2 Responses to Robotic Plant 5 – Really Final Report

  1. Pingback: Robotic Plant 4 – Followup (Gallery) | blogsville

  2. Pingback: Robotic Plant 2 – Gallery | blogsville

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