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How to choose a robot motor?

Date:2023-11-25   Author:XINDA MOTOR

In robots , actuators are generally divided into two types: hydraulic drive and motor drive, of which motor drive is the most common. This article mainly discusses the selection of motors in general robot drive mechanisms.

Motors commonly used in robots are divided into brushed DC motors , brushless DC motors, permanent magnet synchronous motors, stepper motors , etc. Among small and medium-sized robots (size 15cm-30cm), DC motors and brushless DC motors are the most common due to their low price and high positioning accuracy.

Introduction to DC motor:

In the market, there are many kinds of DC motors. According to incomplete estimates, there are thousands of motor manufacturers in China alone. Not to mention Japan, Germany and other established motor production powers. Even for a small DC motor, it has a complex internal structure and a large amount of product information . Here, we only consider its requirements for motor performance from the perspective of the robot.

The choice of motor is often a compromise between our requirements for motor performance and the price we can accept. Therefore, in the motor selection process, we need to consider the various parameters of the motor as a whole, select the parameters that are important to us, and select the appropriate motor based on our needs.

For a motor, it often has multiple operating characteristic curves. Here we will list some of the most important characteristics and then give some explanations of these characteristics. For a motor, from the perspective of robot driving, our main concerns are:

a) Working voltage - For a motor, there may be multiple voltage parameters; the most commonly used one is the rated voltage under continuous operating conditions; some motors can exceed the rated speed and torque above the rated voltage. Operation, but local overheating problems may occur after running for a period of time; that is to say, partial overvoltage can only operate for a short period of time, but not for a long time;

b) Rotation speed - motor rotation speed, generally expressed in revolutions per minute, sometimes expressed in radians per second or angles per second;

c) Torque - the ability of the motor to change the rotation speed; for example, when a wrench is used to turn a screw, the torque of the wrench causes the screw to rotate; in the field of robotics, torque is generally used to make the robot move or to make the mechanical arm complete various actions . ;Torque is equal to the result of force and moment arm, its unit is Nm;

d) Current - For a motor, there may be multiple current parameters, such as no-load current, rated current, locked rotor current, etc.;

e) Physical parameters - such as motor size, motor shaft size, cross-sectional size and location of fixing holes, etc.;

f) Other parameters - some motors also provide other components, such as encoders , brakes, gearboxes, bases, etc.;

DC reduction motor:

Obviously, the power supply current of a DC motor is DC, so it can be powered by a battery; this is also one of the reasons why DC motors are widely used in robots; small DC motors may be different in size, but their basic parameters are generally the same. ;The direction of rotation of the DC motor can be changed by changing the sign of the supply voltage;

Small DC motors generally run in the high-speed and low-torque operating range, which is contradictory to the motor drive requirements in robots; the robotics field usually requires the motor to run in the low-speed and large-torque range; therefore, in order to reduce the motor speed while increasing the motor speed, Generally, a shaft system, that is, a reducer, is added between the motor and the output shaft. By combining different reducers, the motor can obtain different rated speeds and rated torques. Many DC motors currently purchased on the market already have reducers when they leave the factory. , so it often becomes a DC reduction motor;

The advantages of geared motors are: simple use, high output torque, low speed, and a wide range of options.

The main disadvantage is that the accuracy is low. Even if the reduction motors of the same batch produced by the same manufacturer are applied with the same voltage or current, the output of the reduction motor may be different; therefore, in robot applications, when controlling the reduction motor, it is generally It is necessary to introduce speed closed-loop control instead of open-loop control;

Here are some common factors to consider when selecting a geared motor:

a) Versatility - Generally speaking, the more versatile the motor, the cheaper the price;

b) Working voltage - Generally, the working range of small reduction motors is 6-24V;

c) Output torque - Generally, the output torque of small reduction motors is between 0.1-0.5Nm; it can generally be used to drive robots weighing 20-30kg;

d) Rotation speed - the motor speed will determine the maximum operating speed of the robot together with the tire size; in general wheeled robots, the tire size is between 5-20cm, with 6-10cm being the most common;

Although most geared motors can rotate forward and reverse, some geared motors can only rotate in one direction. In addition, although reduction motors generally have many parameters, from a robot perspective, many parameters such as the moment of inertia of the motor shaft are generally not of concern to us. The most important parameters for reduction motors in robot applications are motor speed, torque and rated voltage.