Servo constitution

Mainly by the steering rudder, a reduction gear set, the position feedback potentiometer. 5K, DC motor, a control circuit board and other components.

working principle:

The control circuit board receives a control signal from the signal line, the control motor rotation, a series of motor driven gear train, to an output of the reduction transmission rudder. An output shaft and a position servo feedback potentiometer is connected, while rotating the helm, drive position feedback potentiometer, the potentiometer output voltage signal to a control circuit board, feedback, and motor control circuit board in accordance with the location determined the velocity and direction of rotation, so as to achieve the target stop.

Servo control signal HM cycle is 20ms pulse width modulation (PWM) signal, wherein the pulse width from the position of 0.5ms-2.5ms, corresponding to 0-180 helm (steering now have more angular range using servo magnetic sensor having a controllable angle of 360 degrees), changes linearly. That is, it provides a certain pulse width, and its output shaft will remain at a corresponding angle, regardless of how the external torque change until give it an additional pulse width, it will change the output the angle corresponding to the new position. A position signal to the servo, the motor will rotate and drive the gear set, the corresponding angle to the potentiometer and the motor is stopped, when there are external forces interference contrast this angle, will change the position of the potentiometer, the steering gear internal detection the circuit will again let the motor rotates, the gear set drives the potentiometer to the position command signal, and then stop the motor, but the detection circuit detects the potential is not to have the specified location, it will directly to the motor rotation, so that noise is generated, and the greater the power of outside interference, the greater the noise, the current servo motor inside the greater the need, if the current is greater than the internal electrical steering board of the rated current, the board will burn.

Internal steering gear has a reference circuit, the generation period of 20ms, 1.5ms width of the reference signal, a comparator comparing the external signal with a reference signal, determines the direction and size, thereby generating the motor rotation signal. Thus, the position servo is a servo drive, the rotation range of not more than 180 degrees (now a variety of steering angle range, using a magnetic sensor having a servo controlled angle of 360 degrees) for those who need an angle changing and driving them can be maintained. For example joints, aircraft control surfaces such as robots.

There are three input lines of the servo, an intermediate red, power line, while black is the ground, car steering wires to provide basic energy assurance, the rotation is mainly consumed by the motor. Power in two sizes, one is 4.8V, 6.0V one, a torque corresponding to different standards, i.e., the output torque is different, 6.0V corresponding to a large number, see the specific application conditions; an additional control signal lines are line , Futaba generally white, JR is generally orange. Remember the red power supply (positive), Black for the earth (negative), it is generally not a mistake.

Servo information on specifications provided by the manufacturer, will be included Dimensions (mm), torque (kg / cm), speed (sec / 60 °), a test voltage (V) and weight (g) and other basic information. Torsion units are kg / cm, meaning that the length of the arm 1 centimeters, a few kilograms can lift heavy objects. This is the concept of the arm, so the longer arm length, the smaller the torque. Speed unit is sec / 60 °, meaning that the servo 60 ° rotation time required. Voltage directly affect the performance of the servo, e.g. Futaba S-9001 at 4.8V Torque is 3.9kg / cm, a speed of 0.22 seconds / 60 [deg.], The torque at 6.0V was 5.2kg / cm, a speed of 0.18 seconds / 60 °. Unless otherwise indicated, JR servos are test voltage to 4.8V, Futaba is based on 6.0V as a test voltage. Speed, high torque servos, in addition to expensive, will be accompanied by power-hungry features. Therefore the use of high-level steering gear, be sure to match the high-quality, high-capacity battery, can provide a stable and sufficient current is available to play its due performance steering gear.

Servo Category:

Model aircraft servos

A model aircraft servos, including Cars, ship model and other models, the robot servo housing NA

Robot Servo

Servo robot used to generate internal structure of general manufacturers and different model aircraft rudder is not only the convenience of the robot body to form a housing assembly, but in fact should not only distinction robot servo housing, in the interior of the part selection, the software the adjustment parameters have to be adapted according to the needs of the robot servo.

Analog servo

The internal member is an analog signal processor chip, PWM control signal, a low refresh rate

Digital (digital) servos

The signal processor is a digital device inside the chip, a programmable chip, PWM control signal, refresh frequency is low, but when the load is usually noisy, serious electromagnetic interference

Bus (serial) servo

The signal processing device inside the chip is programmable, the control mode TTL, RS485, etc., with a user-defined parameter adjustment, read back angle, temperature, current, voltage, etc.

FAQ principle of digital servo analysis and solution:

First, the difference between digital and analog servo servo
electronics traditional analog and digital servo steering gear ratio (referred to as standard or servo) is no microcontroller MCU, generally referred to as an analog servo. Older analog servo by a power operational amplifier connected as a Wheatstone bridge, the servo DC motor according to the received differential voltage between the analog voltage control instruction and a mechanical linkage position sensor (potentiometer) comparing the feedback voltage generated by the driving brush motor forward / reverse operation to the specified location. Digital servo ratio is the best type of analog servo, a DC servo motor, DC servo motor controller integrated circuit (IC), and the reduction gear set composed of a feedback potentiometer, which receives a PWM (pulse directly from the DC servo motor control chip square wave, typically a period of 20ms, pulse width of 1 ~ 2 ms, 1 ms pulse width upper limit position, the position of 1.5ms, 2ms drive signal to control the lower limit position) in the form of a motor driven quickly perform the position output until the DC servo motor control chip detects the position of the output sent from the interlocking potentiometer feedback voltage and the average voltage of the PWM control driving signal effectively equal, the motor is stopped, the output completion position.
Digital servo electronic circuit with a microcontroller MCU it is commonly known as a digital servo, the digital servo ratio with analog servo having a faster response, small non-reaction zone range, high positioning accuracy, has strong anti-interference advantages gradually replacing analog servo widely used in robotics, in the model aircraft.
Digital servo design generally two types: one is a DC servo motor MCU + DC servo motor controller integrated circuit (the IC) + + reduction gear set potentiometer feedback scheme, hereinafter referred to as Scheme 1, the other is the MCU + DC servo motor gear reducer groups + potentiometer feedback scheme, scheme 2 hereinafter. Digital market installation of the drive plate to change analog servo genus digital servo scheme 1.

Second, the principle of the servo motor speed motor speed and how to speed up
common servo motors are generally for the permanent magnet DC motor, such as DC brush coreless motor. DC motor with a linear speed - torque and torque - current characteristics, good controllability, and a drive control circuit is simple, the drive current control mode and voltage control of the control modes. Servo motor control voltage control mode is implemented, i.e., the rotational speed is proportional to the applied voltage, it is driven by a power switch composed of four bipolar drive mode H-bridge circuit, the use of pulse width modulation (PWM) DC motor technology regulated charge a voltage magnitude and polarity to achieve control of the motor speed and rotational direction (forward / reverse) of. Motor speed depends on the average magnitude of the voltage applied to the motor, i.e. a duty ratio depending on the PWM drive waveform (pulse width percentage duty cycle / cycle) in size, to increase the duty cycle, the motor accelerates, the duty cycle reduction motor decelerates.
Therefore, to accelerate the motor speed: 1, to increase the motor operating voltage; 2, reducing the electrical resistance of the main circuit, increased current; both in the steering gear designs to be achieved, involving both meet the load torque demand in case of reselection rudder mechanical and electrical machines.

Third, the response speed faster than the digital servo analog servo why
many friends die error that: "PWM driving frequency of the digital servo 50Hz 300Hz than analog servo 6 times higher, the servo motor speed 6 times faster, so the digital the reaction rate servos to 6 times faster "than the analog servo. Please note here the concept of duty cycle, pulse width of the active period of a week, as a percentage of the duty cycle of the pulse width / cycle, so independent of the size and frequency. Duty cycle is determined on the voltage applied to the motor, when the load torque is constant, the motor speed decided, regardless of the frequency of the PWM.
Analog servo chip is a DC servo motor controller generally only receive 50Hz frequency (period 20ms) PWM external control signal of about ~ 300Hz, the frequency is too high will not work properly. If the external PWM control signal is 50Hz, the DC servo motor controller chip resolution obtained time position information is 20ms, comparison PWM control signal proportional to the voltage of the feedback potentiometer voltage difference results, the difference pulse-width extension (% of air ratio is changed, changing the operation of the motor driving the size proportional to the difference), that is due to an external control signal PWM frequency limitation, fastest 20ms to make a new adjustment of the servo arm position.
Digital servo may be received much faster than external control signal PWM frequency of 50Hz (20ms period) by the MCU, can be resolved in less time the position information of the external PWM control signal, the duty ratio of the PWM signal is calculated proportional to the voltage and the difference between the feedback potentiometer voltage to the motor driving operation, the latest position of the rocker arm do servo adjustment.
Conclusion: whether analog or digital servo, when the load torque is constant, the duty ratio of the motor speed depends on the magnitude of the drive signal independent of frequency. Digital servo can receive a higher PWM frequency external control signal, the positional information may be obtained after a shorter period of time, to make adjustments to the latest position servo arm. So that the reaction rate is faster than the analog digital servo steering gear, instead of the drive motor speed is faster than analog servo.

Fourth, no reaction zone range than the analog digital servo why small servo
based on the analysis of the simulated analog servo servo known about 20ms to make a new adjustment. And a higher frequency at the PWM digital servo drive motor. PWM frequency to accelerate the starter motor / stop, acceleration / deceleration softer, smoother and more effective to provide the desired starting torque of the motor. Like a car to get a smaller throttle control interval, start / stop, acceleration / deceleration better performance. So digital servo-free reaction zone smaller than analog servos.

Fifth, the installation of the digital servo analog servo drive plate to enhance the reaction rate is not
in accordance with the above analysis, analog servo digital servo drive installation board, to enhance the reaction rate, the PMW external control signal (e.g., sent from the gyroscope tail servo signal) frequency must be accelerated, or if 50Hz, then of course there is no steering response speed improved.

Six servo control dead zone hysteresis appreciated, the positioning accuracy, the input signal resolution, the centering performance of
each of the closed loop control system due to the oscillation signal, the input signal and the feedback signal can not be completely equal, which involves the deadband and hysteresis problems, the system can not distinguish the difference range of the input signal and the feedback signal is to control the range of the dead zone. Automatic steering system due to signal oscillation control, mechanical control system causes accuracy in small areas outside the control range of the dead always make adjustments to the steering gear to the shock within a small range does not make adjustments, which requires the introduction of hysteresis a role. Hysteresis ratio control large dead zone, the dead zone of the control range of typically difference ± 0.4%, a hysteresis may be set to ± 2%, the difference between the input signal and the feedback signal of the motor is not operated in the hysteresis, the input signal and the feedback signal into the hysteresis, the motor starts to brake - stop. Servo positioning accuracy depends on the overall accuracy of the system: control dead zone, mechanical accuracy, the accuracy of the feedback potentiometer, the input signal resolution. Actuator input signal resolution means a minimum resolution of the input signal range, digital servo input signal resolution, much better than analog servo. Centering and hysteresis performance depends on the positioning accuracy.

Seven, servo why the old creak sound
servo positioning creak of old sent back and forth to adjust the sound, because of some regulatory function steering gear without hysteresis, deadband range set too small, as long as the input signal and the feedback signal always fluctuating, their difference exceeds the deadband, the servo signals to the drive motor. Hysteresis is not rotating step further adjustment function, if the difference between the mechanical precision steering gear set, large tooth phantom, the feedback potentiometer is driven, the step had a range exceeding the scope deadband control, it will adjust the steering stop, creak non-stop.

Eight, why some steering bombing Burnability board
power device current is large while some optional servo system design or chip comes with overcurrent protection, can detect a short circuit condition and the blocking rotated quickly stop flow motor drive signal. There may be a varistor connected in the motor circuit from transient over-voltage and capacitance of the power absorption device front-end design. Such servo stall bombing is not easy to burn the circuit board and the motor. And the steering gear teeth are metal or plastic teeth there is no absolute relationship.

IX why shaking rudder servo
control dead zone sensitive to the input signal and the feedback signal fluctuations due to various reasons, the difference exceeds the range, moving the rudder horn, rudder so shaking.

XI why shaking rudder servo
control dead zone sensitive to the input signal and the feedback signal fluctuations due to various reasons, the difference exceeds the range, moving the rudder horn, rudder so shaking.

Servo buy:

Steering gear teeth on the market, plastic, metal teeth, small size, standard size, large size, thin in addition to the standard size servos, and a low center of gravity of the model. Small steering gear generally called a micro servo, are relatively small torque, the market 2.5g, 3.7g, 4.4g, 7g, 9g refers to the weight of the other servo steering gear were in gram, the volume is gradually increased and the torsion Big. Internal micro servos mostly plastic teeth, 9g servo metal teeth models, torque than plastic teeth to be bigger. futaba S3003, Fraser MG995 servo standard, almost volume, but the former is a plastics teeth, metal teeth, the latter, both the nominal torque is also much worse. Spring sr403p, Dynamixel AX-12 + is a dedicated servo robot, except that the former is made, which is Korean, both the metal teeth nominal torque 13kg or more, but the former change to change just like the analog servo, the latter is the RS485 serial communication, position feedback, velocity feedback but also has a digital temperature servo feedback, vary greatly both in performance and price.

In addition to selecting different size, shape and torque, steering speed of the reaction and the phantom should also consider, the reaction speed of the nominal general common servos 0.22 seconds /60°,0.18 sec / 60 °, better servo 0.12 seconds / 60 °, etc., the reaction soon small value.

Dummy bit generator and servo steering torque and related manufacturing processes, a large load range corresponding torque is large, the dummy bit accordingly small, ordinary steering when new dummy bit generally small, i.e. half a tooth angle, this issue should be a precision machining, good steering is relatively small. But use for some time in the future, especially large torque servos, the phantom is getting bigger, this should not be a cause of gear wear, dissected several steering gear, steering gear are found insufficient cover of plastic material hard, shaft gears are mounted directly on the plastic cover, over time, these holes are expanded into the oval. A pull rocker arm shaft of the gear will be side to side, the dummy bit is produced.

Now cohabitation servo market, in general, not as good as genuine imitation goods, as good as expensive cheap, plastic teeth, metal teeth, not as good, not as good as new and old, domestic as good as imported and so on, we need not be too pursuit of perfection, according to their own select the purchasing power enough on the line.