ABSTRACT
We have seen the various types of actuator motors and drive systems to power the robot motion. The most prominent form of drive used in robotics is electrical motors, which also come in various types such as DC motors (with commutator), brushless DC motors, DC servomotors, stepper motors, and so on. During the design phase, suitable motors have to be decided for the robot. This selection is usually done based on the experience and the specific needs of the robot. For example, if the desired robot motion is continuous, a DC motor can be selected and if there are motions in steps, then stepper motors can be chosen. Once the motor type is chosen, the next task is to decide power and torque requirements. In general, robots driven by such motors may need a high torque up to several newton-meters, even though they need to move relatively slow. We have seen the torque equation of motors in Chapter 5 on drives. The power developed by a motor is the product of the angular speed and the torque developed. If the motor develops a certain torque of τ Nm and runs at a speed of n revolution/s, then the equivalent power, in watts, is given as 2πnτ. To keep the robot at a reasonable weight, the motors should be light and small. Such small motors inherently develop low torque measured in milli-newton-meters. The motor needs to rotate faster, up to a few tens of thousands of revolution per minute (rpm), to achieve the high power required. This presents a conflicting situation where we have to use low-torque high-speed motors to power robot loads that move relatively slow, but require high torque. Therefore, the primary reason for using gears in any system is load matching since the high-speed low-torque motors have to drive low-speed highinertia/friction loads requiring heavy torque. For example, it is not a good idea to drive a car up a slope in fourth gear. Car drive systems provide many selectable gear ratios, so that the driver can choose a ratio according to the circumstance. In robotic systems, it is quite difficult to have a gear-changing mechanism since it will make mechanical design cumbersome and complicated. In addition, limitations on robot size will preclude this approach. In robotics, only one gear ratio is used as shown in Figure 6.1, and a proper gear ratio is often decided by a trial-and-error method. This chapter aims to discuss the methods that may be useful in choosing the appropriate gear ratio (Kanniah, Ercan et al., 2004).
