Servo

A servo drive is any type of mechanical drive (device, working body) that includes a sensor (position, speed, effort, etc.) and a drive control unit (electronic circuit or mechanical system of rods) that automatically supports the necessary parameters on the sensor (and , respectively, on the device) according to a given external value (the position of the control knob or a numerical value from other systems).

Simply put, the servo drive is an “automatic exact performer” - receiving the value of the control parameter as input (in real time), it “by itself” (based on sensor readings) seeks to create and maintain this value at the output of the actuator.

Servodrives, as a category of drives, include many different regulators and amplifiers with negative feedback, for example, hydraulic, electric, pneumatic amplifiers for manual drive of control elements (in particular, steering and braking system on tractors and automobiles), but the term “ Servo ”is most often (and in this article) used to designate an electric actuator with position feedback used in automatic systems for driving control elements and working bodies .

Servo drives are currently used in high-performance equipment in the following industries: mechanical engineering; automatic production lines: beverages, packaging, building materials, electronics, etc., lifting and transport equipment; polygraphy; woodworking, food industry.

1. Drive - for example, an electric motor with a gearbox , or a pneumatic cylinder ,
2. Feedback sensor - for example, the angle sensor of the output shaft of the gearbox ( encoder ),
3. Power supply and control unit (also known as frequency converter / servo amplifier / inverter / servodrive).
4. Input / converter / sensor control signal / impact (may be part of the control unit).

The simplest control unit of an electric servo can be built on a circuit comparing the values ​​of the feedback sensor and the set value, with a voltage of the appropriate polarity (via a relay) applied to the motor. More complex circuits (on microprocessors) can take into account the inertia of the driven element and implement smooth acceleration and deceleration by the electric motor to reduce dynamic loads and more precise positioning (for example, actuators in modern hard drives).

To control servo drives or servo drive groups, you can use special CNC controllers that can be built on the basis of programmable logic controllers (PLCs).

Engine power: from 0.05 to 15 kW.
Torque (nominal): from 0.15 to 50 N · m.

Another option for precise positioning of driven elements without a feedback sensor is to use a stepper motor . In this case, the control circuit counts the required number of pulses (steps) from the position of the frame (this characteristic is due to the characteristic noise of the stepper motor in 3.5 "drives and CD / DVD when trying to re-read. At the same time, accurate positioning is provided by parametric systems with negative feedback which are formed by the interacting corresponding poles of the stator and the rotor of the stepping motor. The reference signal for the corresponding parametric system forms the stepping control system m motor, activating the corresponding stator pole.

Since the sensor typically controls the driven element, the electric servo has the following advantages over the stepper motor :

• does not impose special requirements on the electric motor and gearbox - they can be of almost any desired type and power (and, as a rule, stepper motors are low-power and slow-moving);
• guarantees maximum accuracy, automatically compensating for:
  • mechanical (backlash in the drive) or electronic drive failures;
  • gradual wear of the drive, while the stepping motor requires periodic adjustment ;
  • thermal expansion of the drive (during operation or seasonal), this was one of the reasons for switching to a servo for positioning heads in hard drives;
  • providing immediate detection of a drive failure (malfunction) (mechanically or electronics);
• the highest possible speed of movement of the element (in a stepper motor, the lowest maximum speed compared to other types of electric motors);
• energy costs are proportional to the resistance of the element (the stepping motor is constantly supplied with a rated voltage with a margin of possible overload);

Disadvantages in comparison with the stepper motor

• the need for an additional element - the sensor;
• the control unit and the logic of its operation are more complicated (the processing of the sensor results and the selection of the control action are required, and the controller of the stepping motor is simply a counter based on the controller);
• fixing problem: usually solved by constant braking of the displaceable element or the motor shaft (which leads to energy loss) or the use of worm / screw gears (design complication) (in a stepper motor each step is fixed by the motor itself).
• servos are usually more expensive stepper.

The servo drive, however, can also be used on the basis of a stepper motor or, in addition to it, by combining their advantages to some extent and eliminating competition between them (the servo drives coarse positioning to the range of the corresponding parametric system of the stepping motor, and the latter performs final positioning with a relatively large moment and fixing the position).

PS:

There are no fixing problems in the servo, as opposed to the stepper. High-precision positioning and retention in a given position is ensured by the operation of an electric machine in valve mode, the essence of which boils down to its work as a source of power. Depending on the position mismatch (and other coordinates of the electric drive) a task for force is formed. In this case, the undoubted advantage of the servo drive is energy efficiency: the current is supplied only to the extent necessary for keeping the working body in a predetermined position. In contrast to step mode, when the maximum current value is applied, which determines the angular characteristic of the machine. The angular characteristic of the machine is similar with small deviations of the mechanical spring, which tries to "pull" the working body to the desired point. In a stepper drive, the greater the position mismatch, the greater the force at a constant current.

1. Servo rotational motion

• Synchronous
• Asynchronous

2. Servo linear motion

• Flat
• Round

Synchronous servo - allows you to accurately set the angle of rotation (up to angular minutes), rotational speed, acceleration. Accelerates faster than asynchronous, but at times more expensive.

Asynchronous servo ( Asynchronous machine with a speed sensor) - allows you to accurately set the speed, even at low speeds.

Linear motors - can develop huge accelerations (up to 70 m / s²).

3. By the principle of action

• Electromechanical
• Electrohydromechanical

In an electromechanical servo, the movement is formed by an electric motor and a gearbox.

In an electrohydromechanical servo, the movement is formed by a piston-cylinder system. In these servo drives, the speed is much higher in comparison with electromechanical ones.

Servo drives are used for precise (sensor) positioning (most often) reducible element in automatic systems:

• control elements of the mechanical system (dampers, valves, rotation angles)
• working bodies and workpieces in machines and tools

Rotary servo drives are used for:

• Industrial robots .
• CNC machine tools .
• Printing machines.
• Industrial sewing machines .
• Packing machines .
• Devices.
• Aircraft modeling .

Linear motion servos are used, for example, in automatic machines for installing electronic components on a printed circuit board.

Servomotor

Servomotor - a servo drive with a motor designed to move the output shaft to the desired position (in accordance with the control signal) and automatically active to hold this position.

Servomotors are used to drive the devices controlled by the rotation of the shaft - like opening and closing valves, switches, and so on.

Important characteristics of the servomotor are the dynamics of the engine, the uniformity of motion, energy efficiency .

Servo motors are widely used in industry , for example, in metallurgy , in CNC machines , press-stamping equipment, automotive industry , traction rolling stock of railways .

Mainly in servo drives, 3-pole collector motors were used, in which a heavy rotor with windings rotates inside the magnets.

The first improvement that was applied was an increase in the number of windings to 5. Thus, the torque and acceleration rate increased. The second improvement is the redesign of the motor. Steel core with windings is very difficult to unwind quickly. Therefore, the design was changed - the windings are outside the magnets and the rotation of the steel core is excluded. Thus, the weight of the engine has decreased, the acceleration time has decreased and the cost has increased.

And finally, the third step - the use of brushless motors. In brushless motors higher efficiency, as there are no brushes and sliding contacts. They are more efficient, provide more power, speed, acceleration, torque.

• Tracking hydraulic drive
• Servo machine
• Mechatronics
• Computer numerical control
• Variable frequency drive - under certain conditions, is an alternative to servo drive.
• Selsin
• Servo control

• Constructions of electrical machines