How do I pick out motors for a small robot?
I'm experimenting on making a small robot that will be about 5 lbs. I wanted to know what kind of motors I should be using. Should they be dc motors or servo motors? I think Legos NXT uses servo motors so I wasn't sure on what to use.
I'm currently using two 5V dc motors with 11,600 rpm no load and it's not rotating when the wheels touch the floor. Do I need a larger voltage motor? What does the 11600 rpm no load mean exactly?does it mean that 11600 rpm is the minimum rpms it needs to rotate or else it doesn't? If that's the case does the voltage even matter and I need to find one with lower rpm no load?
Essentially I just want to know what the no load rpm means and what is the significance of the voltage on dc motors as well.
Also, what is better to use, a full rotation servo motor, or a standard dc motor(if so, what are the specs on the most suitable one)?
Thanks in advance.
servo motor are motors where the angle they are rotated to is controlled. You control then by commanding them to "move to angle x". They don't have to be, but most are, limited to a certain amount of rotation. If so not good for drive motors, but can be used to position mechanisms like a steering wheel. Normal motor can be converted to a servo motor by using a quadrature encoder or similar.
no load speed means the speed the motor will reach when the motor shaft isn't power driving anything. If you did have wheels on the end, and left the motor running long enough, and there was no drag from air , the wheels would eventually spin up to 11600rpm. This is perhaps a bit faster than you need!
When an electric motor is powered, and doesn't turn, it is said to be stalled. This is because the torque you are asking the motor to provide is greater than it can output.
following rough equations might be of some use (it doesn't include efficiency factors)
Torque[N.m] *RPM/60=power[W]= Voltage[V] *Current{A]
The power of the motor is more important than the voltage.
It sound's like you are attempting to drive the wheels directly. Have you considered using some kind of gearing/pulleys to increase the mechanical advantage the motor has,as this means it will take a higher torque acting on the wheel for it to stall?
gearing will also allow the motor to run at a higher speed, which means it will be lot happier.
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You can use F=ma and drag equations to guess the force needed to move/accelerate your robot.
This force acts on circumference of the wheels and can be converted to a torque needed by each wheel by multiplying by the radius of the drive wheels, and dividing by the number of driven wheels.
gearing can for a first approximation be estimated by assuming power in=power out. Say you want your wheels to have a (max) torque of "t" and your motor has a stall torque of "m". gearing used should allow the motor to do just over "t" turns for ever"m" turns of the wheel. Wheel speed will be reduced from the motor speed. If s is the rpm of the motor, then the wheel speed will the s*m/t
If the wheel doesn't slip then the speed the robot would be doing in [m/s] is s[rpm]*2*pi()*wheel_radius[m]/60
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