Servomotoren | Prüfungen Informatik

Servo motors

A servo motor is a -mechanical device that rotates with high efficiency and precision, through an output

shaft that can be moved to a particular angle, position and velocity and uses position feedback to control its

motion and final position. The motor is controlled with an electric signal, which represents the final

command position for the shaft. Types of servo motors are classified based on current type: AC and DC

servo motors.

- For DC motor, speed is directly proportional to supply voltage. DC servo motors give precise speed control

(nature of digital signal).

- For AC motor, speed is determined by frequency of the applied voltage. Therefore, AC motors have

smooth control in the entire speed zone, almost no oscillation, and high repetitions and high precision.

Servos come in many sizes and three basic types: positional rotation, continuous rotation, and linear servo.

Positional rotation: most common type, output shaft rotates 180 degrees. Physical stops prevent turning

beyond limits to protect sensor.

Continuous rotation: can turn in either direction indefinitely, rotate clockwise or counterclockwise as

desired.

Linear servo: like the positional rotation servo motor but with additional gears to change the output from

circular to back-and-forth.

Size of the servo motor is another factor for choosing a specific motor. Larger motors have larger power

requirements but can obviously shift larger weights.

The advantages of servo motors are precise control and accuracy, stable operation, fast response and

lightweight. Disadvantages: due to the complex circuit, reliability is less; when stopped, motor’s rotor

continues to move back and forth one pulse: not suitable if you need to prevent vibration

→ Open and Closed-loop systems

A major difference between servo motors and other motors is that the servomotor is a closed-loop system.

Most other motors like DC motors and stepper motors are open-loop. Main difference between open-loop

and a closed-loop is that closed-loop system has the ability to self-correct.

Closed-loop systems are often called feedback control systems. In an open-loop control system, the control

is only an input, for example: I keep turning the handle of a dynamo. Closed-loop system uses knowledge of

the output to decide the control action, for example: I turn the dynamo, I see I have turned it 45 degrees

and I turn it back. To have a closed-loop system, we need some sensor to get the information of 'where the

system is'. Information on state in the moment is provided by the sensor and 'fed back' to determine what

to do next. Control is determined by the feedback received.

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Servo motors A servo motor is a -mechanical device that rotates with high efficiency and precision, through an output shaft that can be moved to a particular angle, position and velocity and uses position feedback to control its motion and final position. The motor is controlled with an electric signal, which represents the final command position for the shaft. Types of servo motors are classified based on current type: AC and DC servo motors.

For DC motor, speed is directly proportional to supply voltage. DC servo motors give precise speed control (nature of digital signal).
For AC motor, speed is determined by frequency of the applied voltage. Therefore, AC motors have smooth control in the entire speed zone, almost no oscillation, and high repetitions and high precision. Servos come in many sizes and three basic types: positional rotation, continuous rotation, and linear servo. Positional rotation : most common type, output shaft rotates 180 degrees. Physical stops prevent turning beyond limits to protect sensor. Continuous rotation : can turn in either direction indefinitely, rotate clockwise or counterclockwise as desired. Linear servo : like the positional rotation servo motor but with additional gears to change the output from circular to back-and-forth. Size of the servo motor is another factor for choosing a specific motor. Larger motors have larger power requirements but can obviously shift larger weights. The advantages of servo motors are precise control and accuracy, stable operation, fast response and lightweight. Disadvantages : due to the complex circuit, reliability is less; when stopped, motor’s rotor continues to move back and forth one pulse: not suitable if you need to prevent vibration → Open and Closed-loop systems A major difference between servo motors and other motors is that the servomotor is a closed-loop system. Most other motors like DC motors and stepper motors are open-loop. Main difference between open-loop and a closed-loop is that closed-loop system has the ability to self-correct. Closed-loop systems are often called feedback control systems. In an open-loop control system, the control is only an input, for example: I keep turning the handle of a dynamo. Closed-loop system uses knowledge of the output to decide the control action, for example: I turn the dynamo, I see I have turned it 45 degrees and I turn it back. To have a closed-loop system, we need some sensor to get the information of 'where the system is'. Information on state in the moment is provided by the sensor and 'fed back' to determine what to do next. Control is determined by the feedback received.

Both closed and open loop systems contain an actuator. The actuator: changes electrical energy to physical movement. In a closed loop system, the sensor output drives the level of the actuator input. For any system where the system can vary from a desired outcome, a closed-loop system is required. Each control type has its advantages and is best suited to types of control operations. The advantages of closed-loop control: process can be kept on set point within a given accuracy, corrections to process disturbances are automated and unstable processes can be stabilized. When the system's cost is a priority, or the process is erratic (has a sticking valve or erratic sensor) it is better to run something open loop. The servo motor's sensor is the potentiometer reference signal. Servo motor position is controlled by electrical signal. The width of the control signal pulse is used to vary shaft position of servo motor. A comparator compares the control signal and potentiometer reference signal to generate error signal. The DC motor connects with a gear mechanism which provides feedback to a position sensor. The It is connected to the central shaft and informs the angle in which the motor’s shaft is available. From the gear box, the output of the motor delivers via servo spline to the servo arm. The standard servo motor uses the plastic gear whereas the high-power servo motor uses the metal gear. A control circuit allows for control over the motor’s motion by sending electric pulses. Potentiometer is attached to the rotating shaft. As the motor rotates, the potentiometer’s resistance changes. Change in resistance gives a precise measure of current shaft position. Incoming pulse applies current to the motor to turn the shaft until the potentiometer indicates that the position corresponds to the incoming pulse width. Motor receives the desired position from the pulse width. To control speed of the servo we need a variable voltage Ac. power source. If power switched on and off quickly, achieve some speed between zero and full speed, the PWM (Pulse Width Modulation) controller sends a certain pulse rate to motor rate that corresponds to a certain angle. Minimum pulse, a maximum pulse, and a repetition rate. Length of the pulse will determine how far the motor turns, for example, a 1500 ms pulse will make the motor turn to the 90° position. The power applied to the motor can be controlled by varying the width of these applied pulses. By changing or modulating the timing of these pulses the speed of the motor can be controlled. The longer the pulse is “ON”, the faster the motor will rotate and likewise, the shorter the pulse is “ON” the slower the motor will rotate. Wider pulse width applies more average voltage to motor terminals. The stronger the magnetic flux inside the armature windings the faster the motor rotates.

Choice of software systems and frameworks, depends on robot complexity. The Middleware provides services to software applications: "software glue". In our case is the system, i.e., Arduino microcontroller + IDE (Integrated Development Environment). The middleware basically allows us to abstract hardware from the software. Work in terms of software for all robot hardware. Implies you can create programs for robots without having to deal with hardware. Creates software version of whole robot including all the interconnections between parts. The 'screws' and 'wires' attaching an external device to the main system in hardware, are device drivers in the software. A device driver is a program that operates a device and integrates it with the operating system. Sensors and actuators both need drivers. A driver provides a software interface to devices, enabling operating systems and other computer programs to hardware functions. Device drivers simplify programming by acting as translators between a hardware device and the operating system. A driver communicates with the device through the microcontroller communications subsystem. When a calling program calls a routine in the driver, the driver issues commands to the device. Once the device sends data back to the driver, the driver may invoke routines in the original calling program. Drivers are hardware dependent and operating-system-specific. Drivers can help with the following tasks: Software can also require drivers, for example: need to write a tool that accesses core operating system data, which can be accessed only by code running in kernel mode. You can do that by splitting the tool into two components: first component runs in user mode and presents the user interface; second component runs in kernel mode and has access to the core operating system data. Component that runs in user mode is an application, and component that runs in kernel mode is a software driver. Device drivers can be generic, or equipment-related. There are device drivers for almost every device associated with a computer and can be classified into two categories: Kernel Device Drivers and User Device Drivers. A major source of Arduino device driver libraries is the 'Contributed by the community' libraries. Over time the Arduino has outgrown its role as a teaching product and now it’s used to program thousands of different types of devices. Arduino shields are another form of obtaining device drivers. These are add-on boards than can be plugged on top of an Arduino board. They provide additional capabilities and functionalities to an Arduino board. A shield is similar to an expansion board/breakout board. A shield may have several chips engineered and built into a complete plug-and-go sub-system. The breakout board is to connect to a chip or other device without having to solder up your own board. A breakout board usually has the sole purpose of making a single chip available by "breaking out" its signal lines on to dedicated pins. Shield is a larger board built for a dedicated more complex function, like a complete GPS system or cellular communication system. -Interfacing directly -Writing to or reading from a device control register -Using another lower -level device driver (file system drivers using disk drivers) -simulating work with hardware -allows the operating system direct access to hardware resources

Servomotoren, Prüfungen von Informatik

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