What is the principle of smart car steering gear?

In the early morning, the first ray of sunlight shone obliquely into the laboratory. There is an unfinished smart car parked on the laboratory table. Its front wheels are slightly deflected, forming an angle. This angle is as subtle as the dancer's state of thinking, as if waiting for the signal to take off.

Have you ever thought about where this precise deflection comes from?

The answer hidden in the black box called the "servo" is not a simple motor, but a precision angle control system.kpowerServo symbolizes the art of converting commands into precise displacements. In essence, the core of the steering gear mechanism is a closed-loop servo system with position feedback. The principle can be broken down as follows.

The brain-like control chip sends out a pulse width modulation signal, that is, a PWM signal, and the duty cycle of the signal is directly mapped into the target corner.

Signal comparison: The circuit board inside the servo receives the signal and compares it with the current angle fed back by the potentiometer (that is, the position sensor).

Due to error driving, an error voltage is generated. If there is no deviation, the motor is at rest, but if there is a deviation, it will start.

The DC motor starts to rotate, and the torque is amplified with the help of a reduction gear set, and then drives the output shaft, which is the power output.

The potentiometer slides driven by the output shaft until the feedback value reaches the same value as the command value. The error becomes zero and the motor stops. This is feedback correction.

In common teaching-level smart car cases, developers are often confused. Why can't stable tracking be achieved by using an ordinary DC motor and a linkage mechanism? The reason is that there is a huge difference between "open-loop control" and "closed-loop control". The steering gear relies on its internal "potentiometer-comparator" closed loop to achieve angle memory and correction.

Imagine that you are walking with your eyes closed (open loop). This is different from walking with your eyes open and always looking at the target (closed loop). The steering gear is the precise traveler that always keeps its eyes open.

How to achieve the magic from "swing" to "turn"?

When this closed-loop system is placed at the front end of the car, its meaning changes. The output arm of the steering gear (generally in the shape of a cross or a disc) is directly connected to the steering wheel through a tie rod or connecting rod. The wheel's rotation angle does not rely on complex differential calculations, but is simplified into a precise linear relationship: PWM signal value → steering gear angle → wheel deflection angle.

The situation is like connecting a ballet dancer's nerves directly to his toes. Every command is a precise punctuation mark of muscle memory.

A collection of frequently asked “frequently asked questions” (Q/A):

Q: Why does the servo make a "sizzling" noise?

There is a closed-loop system here, which performs high-frequency fine-tuning near the target position. This is a normal sound. If there is continuous and severe shaking, then the load may be too large.

Q: What is the essential difference between digital servos and analog servos?

A: The digital servo is equipped with a higher refresh frequency, has a faster response, has a greater holding torque state, and is more suitable for high-speed smart cars.

Q: How to determine the length of the steering tie rod?

When the steering gear is in the neutral position, it is necessary to ensure that the wheels are in an upright state, and there is no mechanical jamming when the steering gear reaches full rudder. This is the basis of Ackermann's steering geometry.。

Q: Why does the standard servo only turn 0-180 degrees?

Limit the physical stroke of the potentiometer and continuously rotate the servo with the limit removed, and apply it to a special chassis.

Q: How to prevent the servo from being damaged?

It can be rewritten as follows: A. If you need to install a soft limiter or change the control program, if you directly use a physical hard limiter, it will easily cause the motor to be burned or the teeth to sweep.

If you look away from a single component and focus on the "nervous system" of the entire smart car, you will find that the steering gear and steering mechanism are the joints in its skeleton.It receives the "will" from the controller, that is, the PWM signal, and then converts it into an "action" that drives changes in the chassis, that is, deflecting the tires.. This process is not only a manifestation of strict physical laws, but also a demonstration of the philosophy of elegant automation.

The real challenge has never been the principle itself, but how to incorporate millisecond-level signal delays into your short control code, how to incorporate inertial drift caused by load changes into your concise control code, and how to incorporate nonlinear changes caused by ground friction into your concise line of control code.

The core point is repeated: the key to the steering of the smart car is not the rotation of a single motor, but a closed cycle of "setting-measurement-comparison-correction" that is constantly being performed. Its essence is to transform human vague instructions into a precise angular language that is universal for mechanical energy.

If you want the car in your hand to change from a "running" state to a "running" state, to achieve this goal, you need to take the following recommended actions:

1. For the calibration zero operation before physical installation, first send a high-level pulse with a duration of 1.5 milliseconds (this is the neutral signal), and then confirm the vertical installation of the servo arm.

2. The hardware and software are combined, and the steering angle set in the program will change gradually, that is, the S-curve change, rather than a sudden change. This can protect the gears and make the movement smoother.

3. In a balanced state of force and position: Check whether the ball head of the tie rod is smooth. If the virtual position is too large, it will cause "understeer". If it is too tight, the steering gear will be burned.

4. Practice has proved that on a car chassis weighing 2kg, when using a steering gear with a torque of 12kg·cm (likekpowerLike the Servo standard type), it can achieve 90% of the sharp cornering response.

Return to the beam of sunshine at the beginning of the article, and when you truly understand the inside of that small square box, an endless war around "deviation and return", you will master the spell that gives the machine a "sense of direction". Every exquisite turn of the smart car is not a rigid execution of code, but also a sonnet about control written by electronics and steel.

At this moment, is your car ready to dance?