Servo
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In electrical engineering, a servo ( Latin servus , "servant, slave") denotes a combination of control and drive unit. This can be, for example, an electric motor and its control electronics. In common parlance, servos are often equated with servomotors .
Model building servos
In the model specific model servos are used. Their compact and lightweight design is particularly advantageous for aircraft models.
Assembly and connections
For assembly, classic model making servos have fastening devices on two sides with which they can be screwed onto a base. Model building servos are connected directly to the receiver or controller. The cable consists of three lines: the supply (V CC ), ground (GND) and a signal line (PWM). The receiver supplies the servo with supply voltage and a control signal. Connector shapes, pin assignments and wire colors are not the same among the various manufacturers, which can lead to compatibility problems.
Control
Model building servos are controlled via pulse width modulation (PWM). The angle to which the servo arm is to be set is controlled via the width of the pulses. A 50 Hz signal (20 ms period length) is common, which is between 500 microseconds (left stop, 0 degrees) and 2500 microseconds (right stop, 180 degrees) at high level and the rest of the period length at low level. However, many servos have not used their full freedom of movement in this value range and / or can move between other angles. Example: 0 degrees at 1 millisecond and 180 degrees at 2 milliseconds. The values at which the servo is fully left or right can also be below 1 millisecond or above 2 milliseconds. The period of 20 ms is not a critical value and does not have to be adhered to during activation.
Position control
With model making servos, the angle of the output shaft is regulated. To determine the angle there is a potentiometer inside the servo , which is connected to the output shaft. The servo electronics determine the actual angle of the output shaft via this potentiometer. This is compared with the target angle, which is determined from the PWM signal. If there is a discrepancy between the actual and target angle, the electronics adjust the angle of the output shaft via the motor and the gearbox. A disadvantage of model building servos is that it cannot be queried whether the servo is overloaded. This means that it cannot be determined whether the servo is even able to move to the desired position.
Analog and digital servos
The electronics of model building servos can be digital or analog. Accordingly, servos are classified as digital or analog servos. Digital servos have a higher angular resolution and the servo position is controlled faster and more precisely. Digital servos can often also be reprogrammed, which means that (depending on the servo model) the speed, overload protection and the like can be adapted to the application. However, the need for auxiliary power and the price of digital servos are higher than that of analog servos.
Servo modification
A common modification to model making servos is conversion into a geared motor without position control and an endless angle of rotation. To do this, a mechanical stop on the final axis gear must be removed in the servo housing and the potentiometer must be replaced by two fixed resistors. In hobby electronics, chopped servos are often used as wheel drives for mobile robots. Advantages compared to commercially available gear motors are: lower purchase price, easy installation on the chassis, easy installation of the wheels on the servo axis, the motor driver is integrated in the servo. Disadvantages are the weak plastic gears in cheap servos and the often not quite even rotational movement that can result from improper modification of the servomechanics.
Automobile manufacturing
In automotive technology, more and more application variants of servos are being developed. The retractable radio antenna, the window lifters , seat adjustments etc. are operated with servos. To control and monitor these servos, they are usually connected to control devices . For simple cases such as window regulators, no potentiometer is used for position feedback, but only end position contacts.
Hard drives
In the English-speaking world, the control circuit consisting of a mechanical drive and magnetic position control for positioning the read / write heads of hard drives is also referred to as a servo . First of all, stepper motors with a threaded spindle were used, nowadays practically exclusively moving coil systems ("Voicecoil") with position control.
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Web links
- Description of the servo control signal and the position control
- Tutorial servo hacking