Posted by Dodd Stacy at 1/17/2001 9:35 AM EST
Engineer on team #95, Lebanon Robotics Team, from Lebanon High School and CRREL/CREARE.
In Reply to: Re: 4WD with 4 motors
Posted by Justin Stiltner on 1/17/2001 12:52 AM EST:
Josh,
Your question was a large one, and my response too short. I apologize for that. Justin provides the basic answer. The power that it takes to push the robot around over the floor is determined by the robot’s drag and speed, not by the motors themselves. The motors (however many are working on it) simply supply the power that the robot needs.
Think of riding a bicycle at a constant speed over rolling terrain. As you go from the flat to an uphill you have to push harder on the pedals (supply more torque to the wheels) at the same rpm, so you are supplying more power. Why? Because gravity - and the fact that you elect to go uphill against it - increases the load that you must satisfy. Even though you are an “engine” of, say, 1/3 HP capability, the amount of power that you actually deliver is determined by the terrain and not by your strength. If you add a lightweight 1/20 HP electric motor and battery to the bike, you can reduce your pedaling power input, but you and the motor together will provide the same total power as before - just what is demanded by the terrain.
The gnarley stuff comes in when you start looking at whether the individual motors are normally operating in a zone of torque and speed where the EFFICIENCY is high, the ratio of mechanical power output to electrical power input. At stall, with high motor torque but zero speed, the mechanical power output is zero and the motor is drawing a lot of current - high electrical power - so efficiency is zero. At free speed, mechanical power output is again zero because no torque is being delivered - the motor is running unloaded and free - so efficiency is again zero. When the motor(s) is(are) loaded by the robot somewhere between those extremes, you achieve greater than zero mechanical power output and efficiency. In this range there is a maximum of power output and of efficiency, maybe at the same loading and maybe not.
I’ll quit there before getting into too much trouble. Your question has many facets.
Dodd
: : So i know for future reference, could you explain why. I can’t seem to figure out why 4 motors would take the same amount of power that 2 motors does.
: : Josh
: : “If I were an animal, I should wish to be a platypus. I mean, how cool would that be. You walk into the bar, stroll up to the counter, lean to the girl next to you and say, ‘Hey baby, I’m a platypus.’” -Anonymous
: Well It depends on a lot of factors but basically you have 4 motors pulling 25% of the load each insted of having 2 motors pulling 50% of the load each. So depending on the motor used and its torque and other factors it could be possable to have 4 motors pulling less amperage than just 2. It would also lessen the load on the motors (as a whole) and might help to prolong the life of the motor.
: That is what I think anyway.
: Justin Stiltner
: Team #388
: Epsilon
: Grundy Va,
:
: “if it doesent fit the first time… get a bigger hammer”- just about everybody at one point or another