One of the basic concepts of robotics is fitting high power components in the smallest space possible, or power density. This makes the machines both small and capable. Small, high torque motors are usually used in robots because of their high-power density. When choosing a motor for your application, a few options are available:
High Torque DC Motors
The most affordable high density motor is the High Torque Direct Current (DC) motor. These motors come in both brush and brushless variations. The high torque in their name refers to high start-up torque, although the overall torque is relatively low compared to other types of motors. This problem can be solved using gear systems with high reduction ratios. Gearing keeps the advantages of high-speed motors while mechanically increasing the torque. These motors can also be used for linear motion using a system of spindles and cable, belts, or chain drive systems. Brushless DC motors are also used in servomotors like the Rozum R-Drive series. These motors use a DC motor with an AC core, with an encoder, a controller, a harmonic gear, and the AC/DC hybrid motor. These hybrid AC/DC motors allow the systems to have the high durability and life expectancy of AC motors with the higher torque output of DC motors.
An improvement on standard DC motors is the introduction of multiple poles. Additional poles are created by adding iron slots to brushless high torque DC motors. Adding these extra poles causes the motor to produce higher torque at lower speeds than conventional motors. Multipole motors also require higher switching frequencies than conventional DC motors and could lead to higher heat production and energy usage.
EC Flat Motors
EC flat motors are multipole motors with an external rotor. The external rotor allows the motor to create relatively high power at the edge of the rotor at medium speeds. Because of the external rotor design, there is a significant amount of inertia which decreases dynamic. Dynamics are needed to allow the robot to make quick movements in different directions. Because of this, these motors are more suited to robots designed to move in single directions like the underwater cleaning robot Hullberg. In cases like this, the high inertia helps the robot keep moving in the original direction using less energy. The motors also use simple construction with a single wound iron core and a permanent isotropic magnetic ring. This design eliminates complex parts and processes and makes these motors very affordable.
A major improvement in motor technology is the introduction of torque motors. These motors can start, stop, and switch directions almost immediately. Instead of a solid shaft, they are built with a hollow shaft that allows them to fit almost anywhere without needing any gears or belts to connect it to the machine. Torque motors have been designed with a huge number of poles to create extremely high torque compared to the other motor types. These motors usually run with high torque and low speeds. The hollow shaft design eliminates any inertia in the system and allows very high dynamic rates because there is very little mass in the shaft itself.
A major disadvantage of these motors is the cost. Because the motors are designed for each specific application, there is no price reduction due to mass production. They also require high switching frequencies to power all the poles and this causes a large amount of heat to be generated. High Torque Motor systems need to be liquid-cooled to keep them from overheating. The advanced cooling requirement adds even more complexity to the design of both the machine and the motor when compared to other motor options. The design complexity adds more cost to the system. High torque motors are often overlooked for linear drive applications due to the high cost and design complexity.