Working principle of servo in robot application

The steering gear, also called servo, was first used on ships to achieve steering functions. Since its turning angle can be continuously controlled by a program, it is widely used in various joint movements and in smart cars to achieve steering, as shown in Figures 1 and 2.

Figure 1 Servo used in robots

Figure 2: Servo used in smart car

The servo is the control mechanism for the car's steering. It has the characteristics of small size, large torque, simple external mechanical design, and high stability. Whether in hardware design or software design, the servo design is an important part of the car's control part. Figure 3 is the appearance of the servo.

Figure 3: Steering gear appearance

Composition of the servo

Generally speaking, the servo is mainly composed of the following parts: steering wheel, reduction gear set, position feedback potentiometer, DC motor, control circuit, etc., as shown in Figure 4 and Figure 5.

Figure 4 Schematic diagram of the composition of the servo

Figure 5: Servo components

There are three input lines for the servo, as shown in Figure 6. The red one in the middle is the power line, and the black one on the side is the ground line. This line provides the most basic energy guarantee for the servo, mainly for the rotation consumption of the motor. There are two specifications of power supply, one is 4.8V and the other is 6.0V, which correspond to different torque standards, that is, the output torque is different, and the 6.0V one corresponds to a larger one, depending on the application conditions; the other line is the control signal line, Futaba's is generally white, and JR's is generally orange. In addition, it should be noted that the power line of some models of SANWA servos is on the side instead of in the middle, and needs to be identified. But remember that red is the power line and black is the ground line, and you will not make a mistake.

Figure 6 Output line of the servo

Working principle of servo

The control circuit board receives the control signal from the signal line, controls the motor to rotate, and the motor drives a series of gear sets, which are decelerated and transmitted to the output steering wheel. The output shaft of the servo is connected to the position feedback potentiometer. When the steering wheel rotates, it drives the position feedback potentiometer. The potentiometer will output a voltage signal to the control circuit board for feedback. Then the control circuit board determines the direction and speed of the motor rotation according to the position, so as to achieve the target stop. Its working process is: control signal → control circuit board → motor rotation → gear set deceleration → steering wheel rotation → position feedback potentiometer → control circuit board feedback.

The control signal of the servo is a pulse width modulation (PWM) signal with a period of 20MS, in which the pulse width varies from 0.5-2.5MS, and the corresponding steering wheel position is 0-180 degrees, which changes linearly. In other words, if a certain pulse width is provided to it, its output shaft will maintain a certain corresponding angle, no matter how the external torque changes, until it is provided with a pulse signal of another width, it will change the output angle to the new corresponding position as shown in Figure 7. There is a reference circuit inside the servo, which generates a reference signal with a period of 20MS and a width of 1.5MS. There is a comparator that compares the external signal with the reference signal to determine the direction and size, thereby producing the rotation signal of the motor. It can be seen that the servo is a position servo drive, and the rotation range cannot exceed 180 degrees. It is suitable for drives that need to be constantly changed and can be maintained, such as robot joints, aircraft control surfaces, etc.

Figure 7 Relationship between servo output angle and input pulse

Steering gear selection

The servos on the market have plastic teeth, metal teeth, small size, standard size, large size, and there are also thin standard size servos and low center of gravity models. Small servos are generally called micro servos, and the torque is relatively small. The 2., 3.7g, 4.4g, 7g, 9g servos on the market refer to the weight of the servos in grams, and the volume and torque are gradually increasing. Most of the micro servos have plastic teeth inside. The 9g servos have metal teeth, and the torque is also greater than that of plastic teeth. Futaba S3003 and Huisheng MG995 are standard servos, with similar volumes, but the former has plastic teeth and the latter has metal teeth, and the nominal torque of the two is also very different. Spring SR403P and Dynamixel AX-12+ are robot-specific servos. The difference is that the former is made in China, while the latter is made in South Korea. Both have metal teeth with a nominal torque of more than 13kg, but the former is just an analog servo with a modified appearance, while the latter is a digital servo with 485 serial communication, position feedback, speed feedback, and temperature feedback functions. The two are very different in performance and price.

In addition to the different choices of size, shape and torque, the reaction speed and dead space of the servo must also be considered. The nominal reaction speed of a general servo is commonly 0.22 seconds/60°, 0.18 seconds/60°, and some better servos have 0.12 seconds/60°, etc. The smaller the value, the faster the reaction.

The specifications of the servos provided by the manufacturer will include the overall dimensions (mm), torque (kg/cm), speed (seconds/60°), test voltage (V) and weight (g). The unit of torque is kg/cm, which means that at a swing arm length of 1 cm, an object weighing several kilograms can be lifted. This is the concept of the lever arm, so the longer the swing arm length, the smaller the torque. The unit of speed is sec/60°, which means the time required for the servo to rotate 60°. The voltage will directly affect the performance of the servo. For example, the torque of Futaba S-9001 is 3.9kg/cm and the speed is 0.22 seconds/60° at 4.8V, and the torque is 5.2kg/cm and the speed is 0.18 seconds/60° at 6.0V. If not otherwise specified, JR's servos are all tested at 4.8V, while Futaba's are tested at 6.0V. Fast-speed, high-torque servos are not only expensive, but also consume a lot of power. Therefore, when using advanced servos, be sure to use high-quality, high-capacity batteries that can provide stable and sufficient power.

There are a lot of mixed goods on the market now. Generally speaking, imitations are not as good as the originals, cheap ones are not as good as expensive ones, plastic teeth are not as good as metal teeth, old ones are not as good as new ones, domestic ones are not as good as foreign ones, etc. You don't have to pursue perfection too much, just choose what is sufficient according to your own purchasing power.

Matters needing attention when using the servo

1) The rated working voltage of commonly used servos is 6V. Chips such as LM1117 can be used to provide 6V voltage. If 5V power supply is used directly to simplify the hardware design, the impact is not great, but it is best to separate the power supply from the microcontroller, otherwise the microcontroller will not work properly.

2) Generally speaking, the signal line can be connected to any pin of the microcontroller. For the 51 microcontroller, the PWM output of the module is required for control. However, if you connect a chip like Freescale, since Freescale has a PWM module inside, it can directly output the PWM signal. In this case, the signal should be connected to the dedicated PWM output pin.

Editor: Huang Fei