Solar Lifter

Challenge:  To design a motorized winch that can lift a small mass while powered by a solar panel.

Parameters: You will design a mechanical winch run by a small DC motor powered by a solar panel supplied by Mr. J. (output is less than 200 mW, and current draw seems to max out at 50 mA).  The lamp supplied must not be closer than 10 cm to the solar panel.  Your lifter device must be mounted to the edge of your desk, and must have the ability to lift a mass of your choosing a minimum height of 80 cm.  The team that designs the most powerful winch will win.

Scoring:  Your system will be tested by having your mass and height change independently verified by other students, and then you will get up to four timed runs.  The lamp distance must be fixed and cannot be closer than 10 cm.  You will need to optimize your system before the official trials.  The times will be measured from the moment the bottom of your mass leaves the floor to the moment the bottom of your mass passes the 80 cm mark of a meter stick.

Score = Efficiency = mg∆h / ∆t (the lifting power) / IV (the electrical power used)

Physics:  DC motors are efficient only when they are running at high speed, so it is important that your winch has a large mechanical advantage (pulleys or reducing gears).  If your thread is winding on a large diameter spool, the mass will rise quickly, but will likely have to be smaller.  The opposite is also true, but you will need to try different masses to find out the best performance of your winch.  The ideal mechanical advantage must be measured, and any tests you do with your winch should be included in your journal.  Friction is the enemy of efficiency because it wastes energy, so anything you can do to minimize friction would help.  Generally lighter winches will perform better than heavy ones.

Extensions: If your winch lifts the mass quickly, it will technically have some kinetic energy, which is not included in your score.  See if you can calculate it, and compare it to the change in gravitational potential energy.  Is it significant or not?  The electrical power consumption of your winch can be measured during the lifting process by adding an ammeter and voltmeter to your circuit.  Try taking a video of the ammeter and voltmeter during a trial, and use it to estimate the average electrical power consumption of the motor.

Help/Hints: Lego can be used to make great winches, and you can add WD-40 to reduce friction.  Thread spools, wood, elastics, and coat hangers will also work.  Feel free to “steal” ideas from Mr. J’s examples, other groups, internet, etc.

Quiz Topics: Work, energy, power, efficiency, mechanical advantage.

Online Text: chapter 7