View Single Post
  #26   Spotlight this post!  
Unread 11-12-2012, 11:25
apalrd's Avatar
apalrd apalrd is offline
More Torque!
AKA: Andrew Palardy (Most people call me Palardy)
VRC #3333
Team Role: College Student
 
Join Date: Mar 2009
Rookie Year: 2009
Location: Auburn Hills, MI
Posts: 1,347
apalrd has a reputation beyond reputeapalrd has a reputation beyond reputeapalrd has a reputation beyond reputeapalrd has a reputation beyond reputeapalrd has a reputation beyond reputeapalrd has a reputation beyond reputeapalrd has a reputation beyond reputeapalrd has a reputation beyond reputeapalrd has a reputation beyond reputeapalrd has a reputation beyond reputeapalrd has a reputation beyond repute
Re: Why not to shift?

Quote:
Originally Posted by DampRobot View Post
I hope everyone posting on this thread understands the diferamce between power, torque, and pushing force. When a robot is in low gear, it generates moretorque at a given power, which resuts in more robot pushing force.
To add to that:

Power = torque * speed
Power is work / time
Work is force * distance
So, torque * rotations / seconds = power

Acceleration requires torque. To accelerate faster, you need more torque (unless you're traction limited, then you need more controlled torque output or less torque).

For DC motors, since the power curves are linear (or extremely close), peak power usually occurs at 1/2 speed (which is also 1/2 torque), so peak power is indicated as (1/4)(torque)(speed) if you convert units correctly. You have 0 power at stall torque or free speed. This point splits the 'good side' and the 'bad side' of the power curve. Motors are very very unhappy with the bad side of the curve (lower speed, less torque). Don't design to be on the bad side for too long.

For DC motors, speed is inversely proportional to torque and proportional to voltage. That means, at free speed, you are producing no torque. Since you always require torque due to friction losses, you will never be at free speed.

Torque is also proportional to current, and motor heat output.

Voltage = current * resistance. The entire electrical system can be represented as a resistor, so you can estimate voltage drop based on motor current.

A gear reduction adjusts speed and torque while maintaining power. If you cut speed by 1/3, you get 3x torque and power remains the same (neglecting the slight efficiency loss)

If you gear for too high of a speed, you will see several issues:
-Increased torque at the motor during acceleration increases current draw, draining the battery faster, possibly tripping breakers, and increasing voltage drop in the electrical system.
-Running too long on the 'bad side' of the power curve could thermally damage the motor. Some motors are more notorious for letting out the smoke than others.
-You will take longer to accelerate if you gear higher than a certain point, as you will be torque limited by the motor at a lower speed. Essentially, you need torque to accelerate, if you don't have enough, it will take longer to accelerate to the same speed (even if your top speed is higher)

In a car, shifting is done for several reasons:
-Engine power curves are very nonlinear, they usually want to stay in the efficient operating region and sometimes in the peak power or peak torque region
-Acceleration is faster if you're shifting at the optimal points. Imagine graphing the wheel torque vs speed in all gears on the same graph. There are intersections where it becomes more efficient to shift as torque drops off. If you shift at the right place, you will stay in the most efficient place. A CVT would always stay at the optimal place, transmissions with many gears also shift often, but it's a tradeoff between staying in the optimal operating region and shifting a lot.

In a FRC robot, where shifting is generally manual, shifting is done for slightly different reasons:
-To optimize pushing power, to become traction limited at a point where you can't trip the breakers in match play
-To optimize speed so you can still drive fast during the rest of the game

You could alternatively optimize two gears to accelerate faster, if you shift at the correct points.

With so much power in the motors now, it's getting more and more reasonable to design around a moderately high single speed (10fps-11fps) rather than use a 2-speed of ~5fps and ~13fps and shifting to accelerate. There's always something to be said for throwing more power at the problem.
__________________
Kettering University - Computer Engineering
Kettering Motorsports
Williams International - Commercial Engines - Controls and Accessories
FRC 33 - The Killer Bees - 2009-2012 Student, 2013-2014 Advisor
VEX IQ 3333 - The Bumble Bees - 2014+ Mentor

"Sometimes, the elegant implementation is a function. Not a method. Not a class. Not a framework. Just a function." ~ John Carmack