How to choose the servo to make the robot

Submitted by Abby Brown on Wed, 06/02/2021 - 08:04

How to choose the servo to make the robot

Servo is a very common actuator in robotic systems. We can see servos in multi-legged robots, humanoid robots, camera gimbals and low-cost manipulators.

The essence of servo is a small closed-loop servo system, which receives the target signal, drives the internal DC motor, and passes through the reduction gear set to adjust the angle of the output shaft. The output angle is sampled by the potentiometer, and then the output angle is adjusted by the feedback control system to match the target value. The whole process constitutes a typical closed-loop feedback control system.

The advantages of the servo are compact structure, convenient use, and low price, but the position accuracy, load capacity and ability to maintain the position are limited, and it is suitable for occasions where the control performance is not high and the volume is small. However, it is not suitable for high-precision, high-torque, and high-speed occasions (except for a few high-end products), and can be replaced by AC or DC brush/brushless motors + rotary encoders.

Small model airplane servos are generally small in size and use plastic gear sets, such as FS90. Generally used for camera pan/tilt;

Medium-sized servos generally use metal gears, such as MG995/MG996. It can be used for the direction control of a four-wheeled vehicle; a large torque steering gear, which also uses metal gears, with a torque greater than 25kg/cm, is used for the main force-bearing joints of the mechanical arm;

High-performance servos generally use Hall-type or incremental encoders for position feedback, and use high-precision gears to improve steering stability. Such as SM40BL/SM85BL.

Regardless of analog or digital servo, it accepts PWM signals; and under the digital motor, there is another type of "serial bus servo" --- that is, on the basis of digital servo, it supports sending commands through the bus (TXD/RXD) . Bus-type servo can reduce the load of the hardware platform, and has a "maintenance" function without continuous output signals. The bus-type servo control method is more flexible, and generally can support multiple modes such as speed closed loop and position closed loop. Some digital servos also support multi-stage series connection, which can reduce the number of I/O ports occupied. The disadvantage is that the general price is very high, up to several thousand dollars. Such as Feetech Servo, just only a few hundred dollars.

Thanks for the informations.

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