Motor Characteristics Graph + controlling motor

[lil'gal]

Hi, can i know where can i get websites for the speed-torque graph of a DC motor? The motor is just a simple one, those that can be bought cheaply off the shelves..

Also, suppose i use a DC motor for my robot, how can i change the direction of the motor's turning? I am not supposed to control the robot manually once it runs.

Lastly, does the gear ratio come into the picture in calculating power needed in cells? Suppose i already have my output power calculated, and gears would be used in transmitting the power to the motor, do i have to use the gear ratio when calculating power supplied by the batteries?
a) if gear ratio is needed, how do i use it?
b) if gear ratio not needed, is it used in any part of calculations? or rather, when do i need to use gear ratio in the calculations?

Eerr... in formal calculations for robots, after calculating the output power needed by the mechanism, what are other aspects of calculations do i need??

Appreciate any replies posted

little gal

 

[bmcculla]

Most DC motors will only give you the stall torque (the shaft isn't turning at all) which is the maximum torque the motor can produce with a given voltage. The torque should drop linearly until the motor reaches its maximum speed. if you have the stall torque and the maximum speed you should be able to just draw a line between the two and you'll have a fair aproximation of the speed-torque graph. DC motors are current devices the torque produced is proportional to the input current. As the motor spins faster it works like a generator creating a voltage that opposes the input voltage reducing the curent through the motor. That opposing voltage is linearly proportional to velocity.

To change motor direction an H bridge is usualy used. An H bridge is a set of 4 transistors that switch so you can put current through the motor in both directions.

Gear motors reduce the output speed but increace the output torque. So for the same output torque you have to put less current into the motor. But you will have a much reduced motor speed. Gear boxes are given in ratios like 50:1 so 50 turns on the input gives you one turn on the output. You'll also have about 50 times more torque.

For a basic robot you are probably better off just measuring most things and only using calculations to give you a ballpark estimate. Try just hooking up your motor to the supply and just feel how much torque you get. You can also measure how much current it takes. Watch the current move around as you hold the shaft vs. letting it spin free.

Brent

 

[subhasis]

well, u have to use a computer or microcontroller to change the direction, in between the motion and program the desired motion. computer is pretty easy.... i can provide u simple ckt diagrams to control stepper motors in case u want to work with computers.
i suggest u to go for stepper motors, they give more control over the system..... the torque is almost constant.
for the power needed in the cells, u don t have to calculate that....first u have to decide, the motor. that will depend upon the maximum load it has to carry. u can calculate the maximum torque of a motor by a simple test. make a small setup.
attach a string to the shaft of the motor.
then attach a small pan at the other end....
hold the motor fixed.
add weights to the pan..... put as much weight that u thing is beyond the load of te motor.....
then start the motor and release the weights one by one....
till, the motor starts rotating......
this simple experiment can give u the max torque. the principle used in brake dynamometer.
chose the motor, well beyond the required torque.... the motor sud not be stressed to the limit all the times....
then for the cell power..... see the rating of ur chosen motor....
the voltage need is to be satisfied first.... then see the required current for running the motor. u can check the current passing through the motor, by using a multimeter.
ur cells sud at least provide that much amount of current. they sud have that much ampere rating, i think, it is clear...
u an use a power supply (AC to DC converter.... they r easily found in labs)
then there are various calculations that u need to do in robotics.... like how many links, how many degrees of freedom etc. that all depends on ur design of robot. i suggest keep it simple, and keep the number of linkages small.....
Subhasis



ps: well, tell me which standard u r in or r u in university??