Solarbotics motor question
My robot is going to compete in a competition that will require it to compete in different competitions. For one competition, it has to be very efficient, and basically keep following a black line longer than everyone else. Another competition that it has to compete in is a tug of war. The rule is, it cannot change its hardware in between competitions. Naturally, for the endurance competition, you want to hook up a smaller voltage to the motor. But I was thinking of using the following circuit in order to give the motor more voltage for the tug of war competition:
It is actually a simplified circuit. There really is a microcontroller and h-bridge in there, but this schematic should give everyone an idea of what I'm trying to do. In endurance mode, the robot will run off of the 3.3V regulator. But for the tug of war, the relay will kick in and give the motor 9V. The diode will keep the 9V battery from shorting directly to the 3.3V regulator. We plan on using the 5V solarbotics motors.
And now, we finally get to my question. My question is...Has anyone ever ran those solarbotics motors at a voltage higher than 5V? And if so, how long did they last? I've done a similar scheme with huge motors for my solar boat. They were 24V motors, and they were able to withstand 36V across them. But I am unsure about the quality of the solarbotics motors. Thanks in advance for any info anyone can give me.
@ Thu, 2008-11-20 17:59
If these are those standard
If these are those standard little motors found everywhere, they won't last too long with 9 volts. I did a lot of testing a while ago on different voltages with these motors, at 9v they tend to run so fast they scream. They also get extremely hot, and will probably melt their windings pretty soon.
Also, you may want to look for voltage regulators with an enable pin, that way you can run two in parallel and enable them with the microcontroller.
@ Thu, 2008-11-20 18:14
Wow, I didn't know there
Wow, I didn't know there were regulators with enable pins. Thanks for the tip!
Also...I figured they wouldn't last very long. I'm hoping they'll last long enough for the competition though. Compared to our robot, all the other robots look prett big, so we need a way to be competitive in the tug of war. I think we have the endurance in the bag though..
Gabe
@ Thu, 2008-11-20 14:56
Efficiency
Using a regulator like that is very unefficient, it will waste twice as much energy as heat as what the motor uses. Simply use PWM to reduce the dutycycle / power usage. If you put a bipolar electrolytic capacitor (100uF or more) across the motor it will smooth out the voltage and may improve efficency. A recent experiment I did with small motors running at less than 10% dutycycle showed that the capacitor improved the ability for the motor to start.
This method also allows you to run for example a 6V motor off of 12V by running at 50% duty cycle. In situations such as competition, this also allows you to temporarily exceed the normal power rating of the motor for brief periods of excessive strength. Twice the voltage means twice the current equals four times the power.
@ Thu, 2008-11-20 17:38
Yes, we thought of the
Yes, we thought of the efficienty of the regulator. We are actually using a switching regulator which can get efficiencies of up to 80%. In addition, running a higher voltage batter means we draw less current from the batteries for the same amount of power we draw from the regulator. And in terms of battery life, we are more concerned about the amount of current we draw from the battery. For example, if we are drawing lets say...1 amp from the regulator, that would mean that we are trying to get:
P = IV = 1A*3.3V = 3.3W
To get the same amount of power from the batteries, and taking into account the inefficiency of the regulator, we get:
P = 3.3W = Pbattery*Regulator Efficiency = Pbattery * 0.8
=> Pbattery = 3.3W / 0.8 = 4.125W
So we are in fact taking 4.125W from the battery in order to make the regulator give us 3.3V. Since we are using a 9V battery, the current draw from the battery is:
I = P/V = 4.125W / 9V = 0.45A
So in essence, the regulator gives the motor 1A, but we are only drawing less than half an amp from the batteries. So yes, you are right we are probably inefficient in terms of the efficiency between the power going to the motor, and power coming from the batteries. But...for the endurance event, I believe current draw is more important.
I hope that makes sense. My group and I spent a bit of time analyzing this design.
Gabe
@ Fri, 2008-11-21 00:56
Fair enough
@ Thu, 2008-11-20 14:15
Maybe
The short answer is maybe...
The motors are so cheap that I recommend you buy one and try. Add some load to the motor, so it doesnt spin free, and hook up a 9volt battery and let it run until one of them dies. either the motor or the battery.
I dont know which of the solarbotic motors you plan to use, but several of them use motors with a standarized shape and size. So it should be possible to find different replacement motors with other voltage and load capasities...
/ vzz-clck-"Maneuver"
@ Thu, 2008-11-20 17:35
Yeah, that's probably what
Yeah, that's probably what I'll do. Thanks for the idea.
Gabe