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Unread 05-12-2003, 02:37
Matt Adams's Avatar
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Post Re: Shifting Gears

Quote:
Originally Posted by MBosompra
Please offer any and all words of advice it's greatly appreciated. We are rookies at building gear boxes. We're not sure which equations to use or even were to start. Please help.
Alrighty, it appears that you're in sort of a bind (bad pun intended) with where to start, so I'll sort of give you some (or a lot) of background on transmissions. Please don’t take my words as absolute truths, but instead use them balanced with some other veterans to come to your own conclussions. Some people more talented than myself may disagree with some of what I have to say.

Thoughts on Transmissions in FIRST.

When choosing a transmission design, you have to weigh a few different aspects before making your choice: The most important aspect is your justified purpose of having the transmission in the first place. Robots can maneuver reasonably well without the potential headache, cost, and design time needed to create a transmission.

The foremost purpose of any transmission is to effectively provide an adequate balance of speed and torque for a given situation. Typically in FIRST, a transmission implies more than one torque and speed setting which is adjustable during a round.

In FIRST robotics, your drive train must be able to do two things effectively:

1. Move from one point to another as quickly as possible, typically in a specific orientation.
2. Maneuver in "high torque situations" against field playing objects and opposing robots.

Without going into the details of the analyses that I’ve performed, the amount of torque required in "high torque situations" is approximately 2.5 to 3 times as much torque as required for standard and high speed maneuvering situations. Essentially, pushing a robot requires significantly more torque than is required for simple turning, which requires more torque than just moving forward and back.


Some quick facts on transmissions are written below. When I spit out numbers, these are calculated. Please ask questions about any and all of it if you’d like more information.

1. The proper gear ratio for using the drill motor in low speed, with 6 inch wheels, a 130 lbs robot with a coefficient of friction of 1.2, while not pulling more that 37 Amps is about 2.5 to 1. Using any less than this ratio with those assumptions will lead your robot to trip the 40 Amp circuit breaker while in pushing matches, no questions asked.

2. The limiting factor in almost all drive trains using a single pair of motors is tripping the circuit breakers of the individual motors. The circuit breakers for the chips and drill motors are 40 amps, while the stall current, (the amount of current that a motor is using while it is exerting is maximum (stall) torque), is over twice that of the breaker. Thus, even if the wheels are spinning due to slip, you can still be pulling more current than is permitted by the breakers.

3. If you choose to couple a drill motor and an Atwood motor in single side of a drive train design, (using 4 motors total) your danger lies in pulling more than 120 Amps which would trip the master circuit breaker.

4. Transmissions are (relatively) engineering and manufacturing intensive, as gears have much tighter tolerances than most other features of a robot, such as some sort of macro level manipulator. They are also much more detrimental when the fail, since when your drive train is not working, neither is your robot. Hence, special care and adequate resources must be applied such that a transmission performs effectively. Failure of your drive train and transmission can completely ruin an entire competition or potentially an entire season.

5. Transmissions, obviously, have a weight associated with them. Typical transmissions have weights between 3 and 8 lbs. per side. Often, this is primarily determined by the quantity of steel required in the gearing and meshing components. Using more than a single pair of motors can further increase weight by 8 to 10 lbs., depending on mounting and gearing required.

6. Transmissions have relatively high financial costs. Since each gear average ~$25, and you’ll be requiring between 6 and 10 gears per side, the total cost adds up quickly. Often times, for fiscially conservative teams, this means that having replacement gears may not be an option.

With those facts put out there in the open, I’ll talk about my personal thoughts on transmissions. Again, please comment as you agree or disagree.


It should be noted that there is a finite (limited) amount of force that a robot can push on the field. In a four-wheel drive system, this is equal to the weight of the robot times the coefficient of friction (stickiness of the wheels to the ground). Even if you have a max output torque by your motors equal to that of a V8 automotive engine, you won’t be able to push more than the weight of the robot times that coefficient of friction. (which is typically in ideal situations, about 1.1 to 1.5)

With all of this being said, I have calculated that you can move at a relatively high speed, ~10-12 feet per second, as well as push other robots with the use of only a single pair of motors, and a pair of properly chosen gear ratios.


To further extrapolate on this, I believe that it is (though admittedly very debatable) wasteful to use a second pair of motors. Below is my justification.

As I stated, a drive train with a single motor pair using proper gear ratios can reach the maximum amount of torque required, which means that a second pair of motors will only provide a different (higher) maximum speed. With that said, I would like to state the primary disadvantages of using a second motor pair.

1. The additional actual weight of a motor pair, the gearing needed to mesh with the first set of motors, as well as the required mounting of these motors could add between 8 and 15 lbs to your robot.

2. You run the risk of tripping your main (120 A) circuit breaker, which means that the entire robot will shut down for the rest of the round. Two motors could pull a maximum current no higher than 80 Amps, while 4 motors could pull 160 amps, dangerously above what will trip the main breaker.

3. At higher speeds, it is more difficult for the driver to accurately control the robot.

4. Twice the motor count means at least twice the complexity. Calculation requirements double, nearly twice the machining for motor mounts double, and at least twice as many pieces will require the high tolerances needed for an efficient drive train.

Hence, it is in my opinion that with the current rules that are in place with FIRST robotics, a second motor pair is a waste of resources, since the primary purposes of a transmission can be achieved with only one pair.

However, YOU need to decide: is it worthwhile to spend twice (or more) the machining, twice (or more) the cost, twice (or more) the design time, and twice (or more) the weight simply so that your robot can move FASTER. You need to decide what is fast enough.


Currently, Team 461 is working on a design that could potentially completely level the playing field for many financially strapped and machining limited teams by designing a transmission that:

1. Requires no machining beyond a drill press and bandsaw.
2. Has a total cost for a PAIR of transmissions less than $250
3. Has weight total less than 4.5 lbs per side.

More details are coming soon regarding that design.

Feel free to ask questions, make comments, or corrections.

Matt
 


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