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What is a servomechanism?

2020-11-25

What is a servomechanism? It’s nothing more but a simple DC motor with integrated servo controllers and gearboxes. The principle of its operation is a feedback system in which an output signal, i.e. data regarding the position, speed, acceleration or offset are entered. The data are then transformed by the correction module and the amplifier to the actuator or electric motor.

In this article you’ll find out:

Servomechanisms are used in almost every branch of light and heavy industries, electronics and electrical engineering, model-making and wherever it is necessary to use precise interpolation motion along a given trajectory between the starting point and the end point.

What is a servomechanism?

Servomechanisms are commonly referred to as servos and are the basic control element in automatic control systems. They are electronic elements which are used to convert control signals into appropriate movement of actuators and receivers. They can be various types of flaps, brakes, levers and other elements of hydraulic and pneumatic systems in machines, equipment or industrial plants.

The construction of the servo mechanism depends on its specificity, parameters and purpose, however, in general, it consists of:

  • a DC motor,
  • a potentiometer,
  • a gearbox,
  • a motor shaft position electronic control system,
  • an enclosure.

DFROBOT SER0001 servomechanism

The position of the shaft is determined by an implemented rotary potentiometer. An example is the DFROBOT SER0001 servomechanism.

How does a servomechanism work?

Small analogue servos are fitted with a generator operating at 50 Hz, which forces the signal sampling every 20 ms. Usually the neutral position of the shaft is determined by a 1.5 ms pulse, and the rotation of the shaft in either direction is forced by the difference between the sampled signal and the neutral position – usually the pulse range oscillates between 1-2 ms.

How does a digital servomechanism work? Modern digital servos provide higher sampling frequencies, which increases the precision of shaft positioning within a range of small deflections, and also accelerates its movement. In addition, it is important that digital servos have up to three times the holding power of analogue servos. An example of a digital servo is the POWER HD MINI DIGITAL SERVO HD-1810MG model.

You should also pay attention to the algorithm implemented in the servomechanism. Currently, the most commonly used are PID and PIV algorithms. For most applications, PID algorithms are sufficient – especially when the performance of the servo does not have to be too high. PIV controllers will work well for more advanced and demanding applications based on a complex drive motion.

POLOLU-1047

POWER HD MINI DIGITAL SERVO HD-1810MG POLOLU servomechanism

What are the parameters of servomechanisms?

The key parameters of servos are:

  • torque and rotational speed,
  • overload operating time,
  • torque to inertia ratio,
  • resolution,
  • frequency response,
  • dimensions,
  • interfaces,
  • network solutions.

The abovementioned parameters that are responsible for how a servomechanism works

Torque and rotational speed

This is one of the most important parameters of servos. For example, the SER0039 DFROBOT model provides a maximum torque of 1,8 kg/cm and the rotational speed of 0,11 s/60° rev.

In general, torque is a parameter that is given separately for continuous operation and operation under momentary load. The torque for continuous operation is the same as the value the motor generates without interruption and risk of overheating, overload and damage. The momentary torque is the maximum torque that a servomechanism can generate in a short time and it usually refers to the braking or acceleration of a mechanism or its response to a momentary disturbance.

When selecting a servomechanism, you should pay attention to the RMS (root mean square) value of torque – it must correspond to the parameters of continuous motor operation. The maximum cycle of the servomechanism must be adjusted to its ability to dissipate heat from the system.

Do not forget about the rotational speed, which determines the maximum and average number of revolutions executed by the servo shaft in a single unit of time. It is the torque and speed that are the power components of the control element.

Inertia ratio

Another important parameter determining the operation of servomechanisms is the ratio of the inertia of the motor rotor and the inertia of the load. The servomechanism is a device whose operation is based on a feedback loop, and the control algorithm regulates the amperage of current drawn by the motor. The current amperage level is calculated by the algorithm on the basis of the relation between measured and set value differences for the current shaft position, torque and speed. The ratio of motor inertia and load has a key influence on precise speed control – if it is too high, the motor will oscillate, which means that it will be controlled incorrectly. Oscillations in the form of vibrations and resonances can lead to damage to mechanisms and components.

The lower the inertia ratio, the lower the risk of oscillation and the greater the precision of servo control. This ratio is also influenced by the choice of the right transmission method to ensure a sufficiently low rotor-to-load inertia ratio.

Resolution

The resolution of the encoder in the servo feedback loop is responsible for the precision of the device shaft control. As a standard, encoders provide a resolution of 20 bits or more. The higher the resolution of the encoder in the servo control system, the faster the servo will detect movement and adjust it and in this way increase the accuracy of motor shaft position control.

Band and frequency response

Another parameter which determines the operation of the servomechanism is the frequency response, i.e. the ability of the system to monitor and adjust to signal corrections. The bandwidth of the servo is a sinusoidal signal in the control loop – the greater the bandwidth, the more precise the control and the higher the rate of adjustment to the input signal changes. Currently, modern servo mechanisms support bandwidths above 1 kHz.

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