2 Robot Drives

2.1.2    Motors Specifications

In our project the stepper motors provide the movement for the camera and three D.C motors for the movement of the vehicle. Stepper motors operate by producing motion in discrete steps and usually has permanent magnets on the rotor and coils on the stator. The changing magnetic field, due to the pulses supplied to different coils at different time, enables the motion of the rotor.

Motors come with extensive specifications. The meaning and purpose of some specifications obvious; below we written about the primary specifications of motors:

2.1.2.1 OPERATING VOLTAGE

All motors are rated by their operating voltage. Most motors can be operates satisfactorily at voltages lower than those specified, but it may not be as powerful as it could be, and it will run slower. Similarly, when it operates at voltage higher than those specified.

2.1.2.2 CURRENT DRAW

Current draw is more important when the specification describes motor loading. The current draw of a free running motor can be quite low .With most permanent magnet motors, current draw increases with load. If the system is designed to handle the stall current (short circuit current) it can handle anything.

2.1.2.3 SPEED

Most continuous DC motors have a normal operating speed of 4000 to 7000 rpm. However, some special purpose motors, such as those used in tape recording and computer disk drivers, operate a slow as 2000 to 3000 rpm. For just about all robotic applications, these speeds are much too high. This high speed is reduced to no more than 150 rpm by using a gear train. The speed of stepping motors is not rated in rpm but in steps (or pulses) per second. The speed of a stepper motor is a function of the number of steps that are required to make one full revolution plus the number of steps applied to the motor each second.

2.1.2.4 TORQUE

Torque is the force the motor exerts upon its load. The output torque and power from a stepper motor are functions of the motor size, motor heat sinking, working duty cycle, motor winding, and the type of driver used.

2.1.2.5 POWER

The power delivered by a motor is the product of its speed and the torque at which the speed is applied. The power of the stepper motor is a function of both the increased over-all pull-out torque and the increased stepping frequency range.

2.1.2.6    GEARS REDUCTION

The motor shaft is fitted with a gear of small radius that meshes with a gear of large radius. Depending on the mass of the robot and the performance desired, different gear ratios might be appropriate. Experimentation is probably the best way to choose the best gear train.

The DC motors that we use it in our design are already having a gear reduction box built onto the motor. When selecting gear motor, we interested in the output speed of the gearbox, not the actual running speed of the motor. Note as well that the running and stall torque of the motor will be greatly increased.