Many people ask the question: servo - what is it? The classic servo design includes a motor, a position sensor and a three-loop control system (position, speed and current regulation).

The word "servo" is of Latin origin "servus", literally translated as "slave", "assistant", "servant".

In the engineering industry, devices acted as auxiliary components (feed drive in a machine tool, robot, etc.). However, today the situation has changed, and the main purpose of the servo lies in the implementation in the field of servo mechanisms.

The installation of a servo is justified in the case when conventional ones do not sufficiently regulate the accuracy of work.

The use of high quality instruments is necessary in equipment with a high level of performance.

This article will talk about the servo, what it is and how it functions.

Areas of use of the device

AT modern world When automation took a strong position in all areas of mechanical engineering, the design of all mechanisms was noticeably unified. In this case, modern individual drives are used.

In order to understand what a servo is, you need to know the scope of the device.

The devices contain precision structures for maintaining speed in and machine tools with high accuracy. They are mounted on drilling equipment, in various systems transport and auxiliary mechanisms.

The devices are most widely used in the following areas:

• production of paper and packaging;
• production of metal sheets;
• material handling;
• production of transport equipment;
• production of building materials.

Servo for car trunk

There are many models of car trunk servos from different manufacturers. Consider the functionality of such a device as a trunk servo from domestic manufacturer"Autozebra". The device is designed for Russian cars, but not only. For example, it can be used in a Renault Logan car.

According to user reviews, this design is convenient. It allows you to open and close the trunk without leaving the car.

The device is controlled by means of a button mounted in the passenger compartment or in

The reason for the widespread use of the device

The reason for the frequent use of servo drives was:

• the possibility of obtaining control, characterized by high accuracy and stable operation;
• wide range of speed control;
• high level resistance to interference;
• small size and weight of the device.

The principle of operation of the servo

How does the device work? A servo, based on feedback from one or more system signals, regulates an object. The output indicator of the device enters the input, where it is compared with the setting action.

Movement features

The servo drive device has two main features:

• the ability to increase power;
• providing feedback information.

Amplification is required for the purpose that the energy required at the output is very high (comes from an external source), and at the input its indicator is insignificant.

Feedback is nothing more than a closed circuit in which the signals are not matched at the input and output. This process is used for management.

The conclusion follows from this: the circuit in the forward direction serves as an energy transmitter, and in the reverse direction it serves as a transmitter of information that is needed for control accuracy.

Power supply and pinout of device connectors

A servo that is applicable to RC configurations typically has three wires:

1. Signaling. A control pulse is transmitted through it. As a rule, the wire is colored white, yellow or red.
2. Feeding. Its power indicator is from 4.8 to 6 V. Often, this is a red wire.
3. Grounding. The wire is black or brown.

Actuator dimensions

Aggregates are divided into three categories:

• microdrives;
• standard modifications;
• large devices.

There are servos with other dimensions, but the above types make up 95% of all devices.

Main characteristics of the product

The operation of the servo is characterized by two main indicators: the speed of rotation and the force on the shaft. The first value serves as an indicator of time, which is measured in seconds. The force is measured in kg / cm, that is, what level of force the mechanism develops from the center of rotation.

In general, this parameter depends on the main purpose of the device, and only then on the number of gearbox gears and the nodes used in the device.

As already mentioned, mechanisms are now being produced that operate at a supply voltage of 4.8 to 6 V. More often this figure is 6 V. However, not all models are designed for a wide voltage range. Sometimes the servo motor only runs at 4.8V or only at 6V (the latter configurations are extremely rare).

Analog and digital modifications

A few years ago, all servo circuits were analog. Now there are also digital designs. What is the difference between their work? Let's turn to official information.

According to the Futaba report, over the past decade, servo drives have become more technically superior than before, as well as small size, high rotational speed and torsion elements.

The latest round of development is the emergence of a digital device. These units have significant advantages even over collector-type motors. Although there are some downsides.

Externally analog and digital devices indistinguishable. Differences are fixed only on device boards. Instead of a microcircuit on a digital unit, you can see a microprocessor that analyzes the receiver signal. He controls the engine.

It is completely wrong to say that the analog and digital modifications are fundamentally different in operation. They may have the same motors, mechanisms and potentiometers.

The main difference is the method of processing the incoming signal of the receiver and engine control. Both servos receive the same power signal from the radio receiver.

Thus, it becomes clear, servo, what is it?

The principle of operation of the analog modification

In the analog modification, the received signal is compared with the current position of the servomotor, and then the amplifier signal is sent to the motor, causing the motor to move to given position The rate of the process is 50 times per second. This is the minimum response time. If you reject the handle on the transmitter, then short pulses will begin to flow to the servo, the interval between which will be equal to 20 m / s. Between pulses, nothing enters the motor, and external influences can change the functioning of the device in any direction. This time interval is called the "dead zone".

How the digital design works

Used by digital devices special processor operating at high frequencies. It processes the receiver signal and sends control pulses to the motor at a rate of 300 times per second. Since the frequency indicator is much higher, the reaction is noticeably faster and holds the position better. This causes optimum centering and a high level of torsion. But this method requires a lot of energy, so the battery used in the analog movement will be discharged much faster in this design.

However, all users who have ever encountered a digital model at least once say that its difference from the analog design is so significant that they would never use the latter again.

Conclusion

Digital analogs will be your choice if you need:

• high level ;
• minimum number of "dead zones";
• precise positioning level;
• quick response to the command;
• constant force on the shaft when turning;
• high power level.

Now you know what a servo is and how to use it.

Servo(lat. servus - servant, assistant; servo drive)— drive with control through negative feedback , which allows precise control of motion parameters.

A servo is most commonly found in robotics. It is impossible to do without it, especially when it comes to solving the problem of accurately moving goods or objects. This task occurs when performing any mechanical work(painting, welding, grinding, moving products on a conveyor, etc.). Manipulators that look like mechanical hands perform such work. In fact, the famous industrial robotics, which is used to automate production around the world, is represented primarily by manipulators. And not one such manipulator can do without servos that drive its links. Why?

It's all about the properties of the servo. A servo drive is a drive that uses negative feedback, which allows you to accurately control the motion parameters of the actuator (output) link of the drive (most often this is the output shaft). To create such feedback, a servo output link position sensor is usually used, but speed, force, etc. sensors can also be used. It turns out that a servo is a drive that receives a signal indicating to advance or turn into certain position. It is installed in this position and “waits” until a command is received to change the position. For example, a signal is given to set the shaft to an angular position of 90 degrees. The shaft turns to this position and holds it until the new position is signaled. Such control possibilities seriously distinguish a servo drive from a conventional geared motor, which can only rotate continuously as long as voltage is applied to it. As a result, if a robot is equipped with such drives, then it can move like a human hand and do all the work that we can do.

There are many varieties of servo drives in the industry. In this article, we will consider rotary electric servo drives. Simply put, for such servo drives, the output actuator is a rotating shaft. For simplicity, we will consider the SG-90 hobby servo device (Fig. 1), which is actively used to create educational models of robots and other floating, flying or walking mechanisms. A hobby servo drive, unlike an industrial one, is much smaller in size, develops less force, is controlled differently, but general principle action is absolutely identical to the industrial counterpart.

Picture 1

The hobby servo device is shown in Figure 2. It consists of an electric motor, a gearbox with a set of gears, a potentiometer (acts as a position sensor for feedback), an electronic motor control board and a case that contains all the contents. The same figure shows the wire through which the servo is powered and controlled. It consists of 3 cores: power "plus", power "minus" and a wire to which a control signal is applied. On the different models Hobby servo wires may have different colors. But almost always the “plus” power wire is colored red, and the “minus” power wire is black. With regard to the signal wire (for transmitting a control signal), there are no clear color standards. Depending on the servo manufacturer, the signal wire may be white, orange, or yellow.

Figure 2

To control such motors, a control signal standard has been adopted. It represents constantly repeating impulses or, as we say, a series of impulses (Fig. 3). The frequency of these pulses remains constant all the time and is 50 Hz. It turns out that the time periodpulses (the time between the leading edges of adjacent pulses) is 1s / 50 = 0.02 seconds, i.e. 20 milliseconds.

Figure 3

Interestingly, the angular position of the output shaft of the servo is set by the duration of the applied pulse. For clarification, Figure 4 shows the approximate ratio of the pulse width in time coordinates and the angle of rotation of the servo shaft. The rotation of the servo shaft is controlled by pulses with a duration of 1 to 2 ms (milliseconds).

Figure 4

As you can see from the graph, nothing more than a pulse-width modulated signal - PWM is used to control the servo. What is PWM can be found in the corresponding article on our website.

And how does the pulse width turn into shaft angle at the output?

As shown in Figure 2, there is also an electronic motor control module in the servo housing. The signal applied to the servo goes to this board. But what happens next with this signal is shown in the block diagram of Figure 5, which we will analyze in stages. Each stage is depicted by a rectangle or a circle and numbered. Inside these rectangles are the devices on which the signal is converted or processed.

Figure 5

So, the input control signal Supr with PWM modulation comes to a special chip with logical elements, with the help of which it is converted into voltage Ucontrol (stage No. 1). After that, the signal Ucontrol (control voltage) is fed to the voltage comparison element. This element is called an adder, but in fact it subtracts the voltage Uobr (feedback voltage) from the input signal Ucontrol, which comes through feedback from variable resistor(stage number 2).

The resulting difference Ucorr (corrective voltage) is amplified by the built-in amplifier (stage No. 3) and fed to the electric motor. The motor rotates (step #4) and drives the output shaft of the servo, and with it the feedback sensor in the form of a potentiometer. When the potentiometer knob is rotated, the voltage changes and it turns out that the rotation of the shaft is converted into voltage Uobr (stage No. 5). This voltage Uobr is compared (again stage No. 2) with the voltage Ucontrol, and the difference in the form of Ucorr again goes to the amplifier (stage No. 3) and so on. The signal "walks" along the chain with feedback until the ratio Ucontrol = Uobr is fulfilled. Then Ucorr will become equal to 0, and the engine will stop. This will happen when the servo drive shaft takes a position corresponding to the input control signal Supr.

Let's summarize everything that has been said. The servo shaft is mechanically connected to the potentiometer knob. Because of this, along with the rotation of the servo shaft, the potentiometer rotates, as a result of which its resistance changes and output voltage Uarr. Accordingly, the output voltage from the potentiometer Uobr directly depends on the angle of rotation of the servo. At the same time, the signal Scontrol input to the servo drive with a pulse duration from 0.001 to 0.002 seconds sets the voltage level Ucontrol, which determines the angle by which the servo shaft must turn. Stopping the motor at the moment when the servo shaft is exactly in the desired position is achieved by subtracting the feedback signal Uobr from the signal Ucontrol. And the amplifier of stage No. 3 is necessary so that amplified voltage is applied to the electric motor and the engine moves the servo drive shaft to a predetermined position as quickly as possible.

Servo motor control examples

As mentioned above, PWM with certain parameters is used to control the servomotor. You can generate such a PWM different ways. Let's show some of them.

1. Servo motor control by 555 timer . The 555 timer chip can operate as a pulse generator (for more information about this chip, read the corresponding article). Therefore, it is possible to select such parameters for the operation of this microcircuit, so that it would give out the pulses we need. By changing the duty cycle of these pulses, that is, changing the duration of the pulses from 0.001 to 0.002 seconds, we will set the angle of rotation of the servo shaft.

In order to implement a PWM signal, it is necessary to use a circuit with adjustable pulse duty cycle at a constant frequency of 50 Hz. The parameters of the components in the diagram (Fig. 6) are selected in such a way as to ensure these conditions. But in order for the control signal to satisfy all conditions, it must be inverted. The transistor in the circuit is necessary for this. To control the duty cycle within the given limits, a potentiometer with a maximum resistance of 20 kΩ would be required. We will use two 10 kΩ potentiometers (since these are the potentiometers used in the Evolvector Level 1 Basic Set, where this circuit is described in detail. The working stroke of the servo motor is 180 degrees. In this case, when turning the knob of one potentiometer, the servo will turn 90 degrees, and with additional rotation of the other - by the second 90 degrees.

Figure 6

You can study this scheme in more detail, as well as assemble it, by purchasing the Basic set of the 1st level Evolvector.

2. Servo motor control by controller. FROM you can also generate the desired PWM signal using the controller. For example, you can use a programmable controller on the Arduino platform. To simplify the programming of the servomotor control algorithm (PWM generation) as much as possible, pre-written programs called libraries are used. Their complex programming code hidden from the user, it is only offered to call the functions we need using short commands when connecting the library to our main program. All this makes the control of such devices as a servo motor, complex from an algorithmic point of view, extremely simple and convenient.

Wiring diagram andThe sketch (program) for controlling the servo motor with the Arduino controller is shown in Figure 7.

Figure 7

ATTENTION: Connecting the servomotor power to the board directly, as in our example (Figure 7), is undesirable. In the figure, we have one servo motor from the “mini” category connected, which consumes very small currents, which is why it works quite normally, powered directly from the board. A standard size servo requires more power, which can cause overheating and damage to the controller. Motors should only be connected to a separate power supply, especially if several servo drives are to be controlled simultaneously.

#include<Servo .h>- this command means connecting the library to control the servo. This library is included on the Evolvector CD that comes with our Level 2 kits. You can also find it on the Internet and put it in the "libraries" folder of your Arduino IDE.
The library we connected has a large number of commands, we will consider only those that are used in the program.

Servo drive; is a declaration of a variable of a special type. move- this is a variable (we choose the name arbitrarily). Servo is the type of the variable (a special type that is defined in the linked library). You can set up to 12 variables of this type, i.e. to control 12 servos. In other words, with this command we told the board that we have a servo, which we named move.
move.attach(9);- this command means that the servo ( move) is connected to pin 9 (output).
move.write(90); - this command causes the servo ( move) turn to the middle position (90 degrees).
move.write(0); - turns the servo to the 0 degree position.
dvig.write(180); - rotates the servo to a position of 180 degrees.

You can find what the rest of the lines in the program mean on the pages of our website or learn from the tutorials that are included

Servo Drive - A servo motor is an electric motor that performs work based on the principle of feedback. From the motor rotor, rotation is transmitted through the gearbox to the control mechanism, feedback is provided by the control unit, which is connected to a sensor that controls the angle of rotation.
Servo motors are used in automobiles to provide linear and angular movement of elements, the exact position of which is highly demanded. The principle of operation of the servo is based on adjusting the operation of the electric motor in order to execute the control signal.

Servo drive - composition and purpose

If the control signal specifies the angle at which the output shaft of the motor is rotated, it is converted into an applied voltage. For feedback, a sensor is used that measures one of the output characteristics of the motor. The readings collected by the sensor are processed by the control unit, then the operation of the servomotor is corrected.

The design of the servo drive consists of an electromechanical unit, the elements of which are located inside one housing. The servo drive includes a gearbox, an electric motor, a control unit and a sensor.

The main characteristics of the servo drive are the operating voltage, torque, rotational speed, materials and construction used in a particular model.

Servo drive - design and operating features

Modern servo drives use two types of electric motors with a hollow rotor and a core. Core motors have a wound rotor and magnets direct current placed around. The peculiarity of these electric motors is the occurrence of vibrations during the rotation of the pendulum, which leads to a decrease in the accuracy of angular movements.

Hollow rotor motors do not have this disadvantage, but are more expensive due to the complex manufacturing technology.

Servo gearboxes are needed to reduce the speed and increase the torque of the output shaft. Many servo gearboxes include spur gears, gears made of polymer materials and metal. Metal gearboxes are characterized by high cost, but at the same time they are strong and durable.

Depending on the accuracy required, servos can use plastic bushings or ball bearings to align the output shaft with respect to the housing.

The servo drive also differs in the type of control unit used, which are analog and digital. Numeric blocks provide more accurate positioning of the main element of the servo and a high response speed.

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Servo motors (servomotors) are specialized electric motors equipped with the so-called negative feedback, with the help of which precise control of all motion parameters is carried out. Its essence lies in the fact that in the process of operation of these devices there is a constant comparison of the output parameters of the functioning with the initially set input parameters. This happens on the basis of control signals generated in real time by servo controllers that have encoders in their design, that is, feedback sensors.

Thus, the design of all modern servomotors includes the actual electric motor and the control unit. Collectively, they are servo drives, with the help of which designers technical devices manages to solve whole line important tasks. Most often, servomotors (servo drives) are used in cases where it is required in automatic mode carry out precise positioning of some working structural elements of a variety of equipment (for example, machine tools with numerical program management, press and stamping equipment, robotic assembly lines, etc.) relative to others.

All manufactured by the world's leading manufacturers servomotors can be divided into two large groups: with brushes and without brushes. Servo drives can use both synchronous and asynchronous electric motors, as well as synchronous linear motors. In addition, both cased and frameless electric motors can be used in servo drives, and in the second version, the package of stator plates plays the role of the case, which allows the most efficient use of their entire profile, while significantly reducing the size and weight of the devices as a whole.

Most modern feedback servomotors are controlled by signals generated by the encoder from several system signals. One of the main features of servo systems is that they are able to amplify output signals that typically have much less power than the input ones (this is necessary so that they can be compared). Thus, during the operation of servo systems, their circuits transmit energy in the forward direction, and in the reverse direction - the information required for precise control.

Main technical specifications servomotors are their dynamics, uniformity of movement and energy efficiency. AT last years synchronous servomotors are increasingly being used, which favorably differ from asynchronous ones in higher dynamics, the possibility of long-term operation on low speeds without forced cooling and higher overload resistance. At the same time, asynchronous motors used in servo drives have the advantage over synchronous motors that complete absence pulsation during rotation.

The third component of the control equipment is the servo. In this article, we will try to explain to you what this component is, what its purpose is, the device and the principle of operation of the servo.

Servo Definition

Steering servo - a device with an electric motor that allows you to achieve precise control of the movement format radio controlled model through negative feedback. Any servo drive in its device has a sensor and a control unit that maintains certain values ​​on the sensor in accordance with an external parameter.

Let's describe in simpler terms how a servo works:

• The servo receives a pulse signal - a control value that determines the angle of rotation of the servo arm,
• The control unit starts comparing the incoming parameter with the value on its sensor,
• Depending on the result of the comparison, the VU returns a signal that predetermines what action must be performed: turn, accelerate or slow down so that the compared indicators become the same.

Servo device

Most modern steering machines are built on the same principle and consist of such constituent parts: output shaft, reducer gears, DC motor, potentiometer, printed circuit board and control electronics.

The gearbox together with the motor form a drive. To transform the incoming voltage into a mechanical rotation, an electric motor is needed. The gearbox, on the other hand, is a gear structure that converts the torque and serves to reduce the engine speed, since it is often so large that it is not at all suitable for practical use.

Together with turning the electric motor on and off, the output shaft also rotates, to which the rocking chair is attached - it, in turn, is attached to the steering wheel of the model. It is the rocker that will set the movement of our model, and for this, a potentiometer is provided in the servo device - a sensor that can turn the angle of rotation back into an electric signal.

However, one of the main elements is the control board, which is electronic circuit. It is she who receives an electrical impulse, analyzes the received signal with the data of the potentiometer and turns on / off the electric motor. Here is how the servo is arranged and how its elements work.

By the way, collector, collector Coreless and brushless motors can be used as a motor in a servo device.

Servo control. Principle of operation.

The servo receives pulse signals that pass through a special wire from the receiver. The frequency of such signals is 20ms, and their duration can vary within 0.8-2.2ms. In order for you to have a clear idea of ​​how the signal is nevertheless transformed into the movement of the rocker, you need to analyze standard scheme servos.

where, GOP is an opon pulse generator (a potentiometer is connected to it), K is a compatator, UVH is a sample-and-hold device, M is an electric motor, which is covered by a power bridge diagonal.

Now let's take a closer look at how a servo works. So, the pulse signal comes from the receiver to the compatator and at the same time activates the GOP. The duration of the reference pulse is related to the position of the potentiometer, which is physically connected to the output shaft. When the rocker is in the middle position, the signal length is 1.5ms, if the position is extreme - 0.8 or 2.2ms. The control signal and the reference pulse are analyzed by the compatator, which calculates their difference value (the calculation is based on the pulse duration). It is the length of the difference pulse that determines how much the “expected” and “actual” state of the steering wheel coincides. The resulting indicator is stored as a potential in the UVH. Difficult?

The principle of operation of the servo drive in different conditions

The position of the servo rocker corresponds to the state of the control stick. The duration of the reference and control pulses is the same. All compatator outputs are set to "0". The motor is de-energized and the rocker holds its original position.

The pilot changes the position of the stick, thereby increasing the control impulse. At one output of the compatator, a difference pulse will be output, which will be stored in the SHA memory. At this moment, voltage will be applied to the motor, it will begin to rotate, and with it the gearbox will begin to move, turning the rocker and potentiometer in such a way that the duration of the reference pulse increases. Such conditions will continue until the lengths of both pulses reach the same values. The motor will then stop spinning.

The pilot moves the remote control stick in the opposite direction, while reducing the length of the control pulse. Servo control at this stage is similar to the process described above. At the lower output of the compatator, a difference pulse is formed, which is stored by the UVH and supplies voltage to the motor. The motor starts to rotate, but in the opposite direction, and continues to work until the pulse lengths again take the same values.

The pilot does not interact with the control panel. The rudder of the model begins to turn the servo arm, as it takes into account the load during the course. Now the duration of the reference pulse is changing, due to which the difference pulse, through the compatator and SHA, acts on the motor and the torque is applied to the gearbox, which prevents the rocker from turning. Those. the rocker is held in one position.

We have analyzed the operation of the servo in a simplified version. In fact, there are many nuances for setting up and using the device, knowing which you can avoid breakdowns and unpleasant situations.

Now, knowing how the servo is arranged, the principle of its operation, you can go and choose a device for your model. To do this, you need to go to the site "Planeta Hobby". If you don’t know how to choose the right servo for your aircraft or car, ask our consultant for advice or read this useful article.