Hey, we are trying to build a robot that runs on two DC motors, and weighs between 8-10lbs. We were wondering what the specs of the motors we should use would be, because the larger the voltage requirement, the heavier the power source will be. Currently we have two DC geared motor with these specs:
Product Name : DC Geared Motor;
Rated Voltage : DC 6V;
Rated Current : 0.45A ( no load)
Speed : 300 RPM;
We deemed that these motors run at a good speed, but have far too little torque to support our robot. In addition, we have no idea what the gear ratio is, so we aren't sure how to backtrace these values to find an appropriate substitute.
Please help, because we are noobs who have no experience with motors!!
-How does gearing affect rpm? In fact, how does free RPM relate to the gearing and the actuall speed? For example, given a free RPM of 300 and a gearing of 300:1.
-For a robot of approximately 8-10 lbs, what sort fo torque should we look for in a motor? Speed isn't important but we don't want it moving at a snail's pace either.
You need to figure out what is the steepest incline the robot must negotiate. How quickly you intend for it to accelerate to its top speed. The rolling resistance of the wheels, and the additional rolling resistance of the worst surface it must negotiate.
Look up and purchase the book "Build Robot Drive Trains" - it will have everything you need to understand what you are trying to do. Well worth it, and should be on every robot hobbyist's bookshelf.
Hello there.
Generally to move larger objects you need larger motors, or motors with a high gear ratio.
A gear ratio is basically how much the motor has been reduced and how much more torque it has achieved.
e.g a motor with 5000rpm ungeared speed and lets say a rated torque of 100g/cm was coupled to a gearbox with a 5:1 ratio.
torque is measured in kg/cm or g/cm. You would want a motor that produced more torque than the robot weighs.
end result, your speed has been decreased by 5 times and torque has increased by 5 times. for a 8-10 pound robot you need something that has about 5kg/cm of torque per motor to move your robot.
The higher the ratio, the more torque you have.
here is a good example of a decent speed and torque:
WK95:
-For a robot of approximately 8-10 lbs, what sort fo torque should we look for in a motor? Speed isn't important but we don't want it moving at a snail's pace either.
Attach a weigh scale to the front of the robot and pull it around -- up inclines, across grass/carpet (or wherever you plan on traveling), and from a complete stop (to test acceleration). The amount of force that you are using to pull the robot is the tangential force that MarkT described below.
tawandapro:
torque is measured in kg/cm or g/cm. You would want a motor that produced more torque than the robot weighs.
Torque is a product (not a ratio) of force and distance. Even the advertising from large companies seems to get this wrong sometimes, but torque is measured in units like kgcm or gcm (not kg/cm or g/cm). This is important since you can basically figure out what torque means from the units. A given torque on a 1 cm wheel will produce a force at the rim which is twice as large as the same torque on a 2 cm wheel.
Given that, it's meaningless to say that you want more torque than the robot's weight. They're different physical quantities. Weight is a force. Torque is force*distance.
If the torque you're interested in is the torque at a wheel, the relevant distance is the radius of that wheel. It is reasonable to demand that the available force exerted by the tires be comparable to the robot's weight. So the torque required would be (weight*wheel radius). This is actually a good bit more than you'll need for most applications, but it's a reasonable starting point.
A gearbox can be viewed as having an input shaft and an output shaft. The gear ratio describes how fast one shaft rotates with respect to the other. It also describes the torque on one shaft with respect to the other. The faster you want things to turn, the less torque will be available. The product torque*rotational speed is pretty much constant when using different gearboxes. That product is referred to as the power.
Torque is a product (not a ratio) of force and distance. Even the advertising from large companies seems to get this wrong sometimes, but torque is measured in units like kgcm or gcm (not kg/cm or g/cm). This is important since you can basically figure out what torque means from the units. A given torque on a 1 cm wheel will produce a force at the rim which is twice as large as the same torque on a 2 cm wheel.
kg and g aren't units of force, they are units of mass.
kgf and gf (kilogram-force, gram-force) are the units you actually mean.
I strongly recommend this approach: convert all forces to newtons and
calculate with torque in N-m (newton-metres - despite the '-' sign this means
newtons x metres).
Once you are using N-m then the equations are trivial to remember.
power = torque x angular velocity.
(watts, newton-metres, rad/s)
You can also (possibly more logically) measure torque in J/rad (joules per radian).
Compare to force - force (newtons) is the same as J/m (joules per metre). Torque
is like force but with angles rather than linear distances. All intuitive and no
conversion factors!