Principle analysis and solution of common problems of digital steering gear:
1. The difference between digital steering gear and analog steering gear
Traditional analog steering gear and digital proportional steering gear (or standard steering gear) have no MCU microcontroller in the electronic circuit, and they are generally called analog steering gear. The old-fashioned analog servo is connected to a Wheatstone bridge by a power operational amplifier, etc., according to the differential voltage generated by the comparison between the received analog voltage control command and the feedback voltage of the mechanical linkage position sensor (potentiometer) to drive the brushed DC motor servo The motor rotates forward/reversely to the specified position. Digital proportional servo is the best type of analog servo. It is composed of DC servo motor, DC servo motor controller integrated circuit (IC), reduction gear set and feedback potentiometer. It is directly received by the DC servo motor control chip. Square wave, the general period is 20ms, the pulse width is 1~2 ms, the pulse width is 1 ms is the upper limit position, 1.5ms is the middle position, 2ms is the lower limit position) form of control drive signal, quickly drives the motor to perform position output until DC servo The motor control chip detects that the feedback voltage sent by the position output linkage potentiometer is equal to the average effective voltage of the PWM control drive signal, stops the motor, and completes the position output.
The MCU microcontroller in the electronic circuit of the digital steering gear is commonly called the digital steering gear. The digital steering gear has the advantages of faster response speed than the analog steering gear, small non-reaction zone, high positioning accuracy, and strong anti-interference ability. It gradually replaces the analog steering gear and is widely used in robots and aircraft models.
There are generally two digital steering gear design schemes: one is MCU + DC servo motor + DC servo motor controller integrated circuit (IC) + reduction gear set + feedback potentiometer, hereinafter referred to as scheme 1, and the other is MCU+ The scheme of DC servo motor + reduction gear set + feedback potentiometer is called scheme 2 below. Adding a digital driver board on the market to change an analog servo to a digital servo belongs to solution 1.
Second, the principle of steering gear motor speed control and how to speed up the motor speed
Common steering gear motors are generally permanent magnet DC motors, such as DC brushless coreless motors. DC motors have linear speed-torque characteristics and torque-current characteristics, good controllability, simple driving and control circuits, and driving control has two modes: current control mode and voltage control. The steering gear motor control implements a voltage control mode, that is, the speed is proportional to the applied voltage. The drive is a bipolar drive mode composed of four power switches to form an H-bridge circuit, using pulse width modulation (PWM) technology to adjust the supply to the DC motor The size and polarity of the voltage can realize the control of the speed and rotation direction (forward/reverse) of the motor. The speed of the motor depends on the average voltage applied to the motor, that is, the duty cycle of the PWM drive waveform (duty cycle is the percentage of the pulse width/period). Increase the duty cycle, accelerate the motor, and reduce the duty cycle The motor decelerates.
Therefore, speed up the motor: 1. Increase the working voltage of the motor; 2. Reduce the resistance of the main circuit of the motor and increase the current; both of them must be realized in the design of the steering gear, and both involve reselecting the rudder when the load torque is satisfied Machine motor.
3. Why is the response speed of digital servos faster than analog servos.
Many model friends mistakenly think: "The PWM driving frequency of digital servos is 300Hz 6 times higher than the 50Hz of analog servos, so the motor speed of the servos is 6 times faster. The reaction speed of the steering gear is 6 times faster than that of the analog steering gear.” Please pay attention to the concept of duty cycle. Pulse width is the effective level time per cycle, and duty cycle is the percentage of pulse width/cycle, so the size has nothing to do with frequency. The duty cycle determines the voltage applied to the motor. When the load torque is constant, it determines the motor speed, regardless of the PWM frequency.
The analog servo is a DC servo motor controller chip. Generally, it can only receive PWM external control signals with a frequency of 50Hz (period: 20ms) ~ 300Hz. If the frequency is too high, it will not work properly. If the PWM external control signal is 50Hz, the resolution time for the DC servo motor controller chip to obtain the position information is 20ms. Compare the voltage proportional to the PWM control signal with the feedback potentiometer voltage to get the difference, which is expanded by the pulse width (accounting for After the air ratio is changed, the change is proportional to the difference) and then the motor is driven. That is to say, due to the frequency limitation of the PWM external control signal, the position of the steering gear can be adjusted at the earliest 20ms.
The digital servo can receive the PWM external control signal much faster than 50Hz frequency (cycle 20ms) through MCU, it can distinguish the position information of the PWM external control signal in a shorter time, and calculate the voltage proportional to the duty ratio of the PWM signal The difference between the voltage of the feedback potentiometer and the voltage of the feedback potentiometer is used to drive the motor to make the latest adjustment of the position of the steering gear.
Conclusion: Whether it is analog or digital servo, when the load torque is constant, the motor speed depends on the duty cycle of the driving signal and has nothing to do with the frequency. Digital servos can receive higher frequency PWM external control signals, can obtain position information after a shorter cycle time, and make the latest adjustments to the position of the steering gear. Therefore, the response speed of the digital servo is faster than that of the analog servo, rather than the speed of the driving motor is faster than the analog servo.
4. Why the non-reaction zone of the digital steering gear is smaller than that of the analog steering gear.
According to the above analysis of the analog steering gear, it can be seen that the analog steering gear takes about 20ms to make a new adjustment. The digital servo drives the motor with higher frequency PWM. The acceleration of the PWM frequency makes the motor start/stop, acceleration/deceleration softer, smoother, and more effective to provide the motor with the required torque for starting. Just like the car has a smaller throttle control range, the start/stop, acceleration/deceleration performance is better. Therefore, the non-reaction zone of the digital servo is smaller than that of the analog servo.
5. The addition of a digital steering gear drive board to the analog steering gear does not increase the response speed.
According to the above analysis, it can be seen that the analog steering gear is equipped with a digital steering gear drive board to increase the response speed. PMW external control signals (such as the tail sent by the gyroscope) The frequency of the steering gear signal) must be increased. If it is still 50Hz, the reaction speed of the steering gear will certainly not increase.
6. How to choose the steering gear motor The
steering gear motor is selected according to the standard of DC servo motor, according to the requirements of motor type, load torque, speed, working voltage and so on. The steering gear generally uses a coreless motor, which is either brushed or brushless.
Coreless motors are DC permanent magnet and servo micro motors. The main difference from ordinary motors is that they use coreless rotors, also called hollow cup rotors. Has the following advantages:
- Maximum energy conversion efficiency (a measure of its energy-saving characteristics): its efficiency is generally above 70%, and some products can reach above 90% (common iron core motors are 15-50%);
- Activation and braking are fast, and the response is extremely fast: the mechanical time constant is less than 28 milliseconds, and some products can reach within 10 milliseconds. Under the high-speed operation state in the recommended operating area, the speed adjustment is sensitive;
- Reliable operation stability: strong adaptive ability, and its own speed fluctuation can be controlled within 2%;
- Less electromagnetic interference: high-quality brushes and commutator structures are used, and the reversing sparks are small, which can eliminate additional anti-interference devices;
- High energy density: Compared with the iron core motor of the same power, its weight and volume are reduced by 1/3-1/2; the corresponding parameters such as speed-voltage, speed-torque, torque-current, etc. all present a standard linear relationship.
7. How to choose the steering gear feedback potentiometer The
steering gear feedback potentiometer is selected according to the standard of type, accuracy and durability. The precision and wear resistance of the conductive plastic potentiometer are much better than other types such as wirewound potentiometers.
8. Recognition
of steering gear control dead zone, hysteresis, positioning accuracy, input signal resolution, and centering performance. For each closed-loop control system due to signal oscillation and other reasons, the input signal and feedback signal cannot be completely equal, which involves Control the dead zone and hysteresis problem, the system cannot distinguish the difference range between the input signal and the feedback signal is the control dead zone range. The automatic control system of the steering gear is always adjusted in a small range outside the control dead zone due to signal oscillation, mechanical accuracy, etc. In order to make the steering gear not adjust the oscillation in a small range, it is necessary to introduce hysteresis Worked. The hysteresis loop is larger than the control dead zone, the general control dead zone range is ±0.4%, and the hysteresis loop can be set to ±2%, the difference between the input signal and the feedback signal is within the hysteresis loop, the motor does not operate, the difference between the input signal and the feedback signal Entering the hysteresis loop, the motor starts to brake-stop. The positioning accuracy depends on the overall accuracy of the steering gear system: such as control dead zone, mechanical accuracy, feedback potentiometer accuracy, and input signal resolution. The input signal resolution refers to the minimum resolution range of the input signal of the steering gear system. The input signal resolution of the digital steering gear is much better than that of the analog steering gear. The centering performance depends on the hysteresis and positioning accuracy.