Servo Motor

A servo motor is an electrical device which can push or rotate an object with great precision. It is used to rotate an object at some specific angles or distance. It is just made up of simple motor which run through servo mechanism. If motor is used is DC powered then it is called DC servo motor, and if it is AC powered motor then it is called AC servo motor. It is available with very high torque in a small and light weight packages. These features they are being used in many applications like toy car, RC helicopters and planes, Robotics, Machine etc.

Servo motors are rated in kg/cm (kilogram per centimeter) most hobby servo motors are rated at 3kg/cm or 6kg/cm or 12kg/cm. This kg/cm tells you how much weight your servo motor can lift at a particular distance. For example: A 6kg/cm Servo motor should be able to lift 6kg if the load is suspended 1cm away from the motors shaft, the greater the distance the lesser the weight carrying capacity. 

The position of a servo motor is decided by electrical pulse and its circuitry is placed beside the motor.

Servo Mechanism

It consists of three parts:

1. Controlled device
2. Output sensor
3. Feedback system

It is a closed loop system where it uses positive feedback system to control motion and final position of the shaft. The device is controlled by a feedback signal generated by comparing output signal and reference input signal.

Here reference input signal is compared to reference output signal and the third signal is produces by feedback system and this third signal acts as input signal to control device. This signal is present as long as feedback signal is generated or there is difference between reference input signal and reference output signal. So the main task of servomechanism is to maintain output of a system at desired value at presence of noises.

 

Working principle of Servo Motors   

A servo consists of a Motor (DC or AC), a potentiometer, gear assembly and a controlling circuit. First of all we use gear assembly to reduce RPM and to increase torque of motor. Say at initial position of servo motor shaft, the position of the potentiometer knob is such that there is no electrical signal generated at the output port of the potentiometer. Now an electrical signal is given to another input terminal of the error detector amplifier. Now difference between these two signals, one comes from potentiometer and another comes from other source, will be processed in feedback mechanism and output will be provided in term of error signal. This error signal acts as the input for motor and motor starts rotating. Now motor shaft is connected with potentiometer and as motor rotates so the potentiometer and it will generate a signal. So as the potentiometer’s angular position changes, its output feedback signal changes. After sometime the position of potentiometer reaches at a position that the output of potentiometer is same as external signal provided. At this condition, there will be no output signal from the amplifier to the motor input as there is no difference between external applied signal and the signal generated at potentiometer, and in this situation motor stops rotating.

 

Controlling Servo Motor

All motors have three wires coming out of them. Out of which two will be used for Supply (positive and negative) and one will be used for the signal that is to be sent from the MCU.

Servo motor is controlled by PWM (Pulse with Modulation) which is provided by the control wires. There is a minimum pulse, a maximum pulse and a repetition rate. Servo motor can turn 90 degree from either direction form its neutral position. The servo motor expects to see a pulse every 20 milliseconds (ms) and the length of the pulse will determine how far the motor turns. For example, a 1.5ms pulse will make the motor turn to the 90° position, such as if pulse is shorter than 1.5ms shaft moves to 0° and if it is longer than 1.5ms than it will turn the servo to 180°.

Servo motor works on PWM (Pulse width modulation) principle means its angle of rotation is controlled by the duration of applied pulse to its Control PIN. Basically servo motor is made up of DC motor which is controlled by a variable resistor (potentiometer) and some gears. High speed force of DC motor is converted into torque by Gears.

We know that, WORK= FORCE X DISTANCE, in DC motor Force is less and distance (speed) is high and in Servo, force is High and distance is less. Potentiometer is connected to the output shaft of the Servo, to calculate the angle and stop the DC motor on required angle.

Servo motor can be rotated from 0 to 180 degree, but it can go up to 210 degree, depending on the manufacturing. This degree of rotation can be controlled by applying the Electrical Pulse of proper width, to its Control pin. Servo checks the pulse in every 20 milliseconds. Pulse of 1 ms (1 millisecond) width can rotate servo to 0 degree, 1.5ms can rotate to 90 degree (neutral position) and 2 ms pulse can rotate it to 180 degree. 

BLDC motor

The BLDC motor is widely used in applications including appliances, automotive, aerospace, consumer, medical, automated industrial equipment and instrumentation.

The BLDC motor is electrically commutated by power switches instead of brushes.  Compared with a brushed DC motor or an induction motor, the BLDC motor has many advantages:

• Higher efficiency and reliability
• Lower acoustic noise 
•  Smaller and lighter 
•  Greater dynamic response
• Better speed versus torque characteristics 
•  Higher speed range
• Longer life

a. Motor Principles

Motors convert electrical energy into mechanical energy using electromagnetic principles. The energy conversion method is fundamentally the same in all electric motors.

b. Stator

A single-phase motor has one stator winding wound either clockwise or counter-clockwise along each arm of the stator to produce four magnetic poles as shown in figure.

Figure 1. Stator

Each phase turns on sequentially to make the rotor revolve. There are two types of stator windings: trapezoidal and sinusoidal, which refers to the shape of the back electromotive force (BEMF) signal. The shape of the BEMF is determined by different coil interconnections and the distance of the air gap. In addition to the BEMF, the phase current also follows a trapezoidal and sinusoidal shape. A sinusoidal motor produces smoother electromagnetic torque than a trapezoidal motor, though at a higher cost due to their use of extra copper windings. A BLDC motor uses a simplified structure with trapezoidal stator windings.

c. Rotor

A rotor consists of a shaft and a hub with permanent magnets arranged to form between two to eight pole pairs that alternate between north and south poles. Figure shows cross sections of three kinds of magnets arrangements in a rotor.

Figure 2. Rotor

There are multiple magnet materials, such as ferrous mixtures and rare-earth alloys. Ferrite magnets are traditional and relatively inexpensive, though rare-earth alloy magnets are becoming increasingly popular because of their high magnetic density. The higher density helps to shrink rotors while maintaining high relative torque when compared to similar ferrite magnets.

d. Operational Motor Theory

Motor operation is based on the attraction or repulsion between magnetic poles. 

Figure 3. 3 Phase motor

Using the three-phase motor shown in figure, the process starts when current flows through one of the three stator windings and generates a magnetic pole that attracts the closest permanent magnet of the opposite pole. The rotor will move if the current shifts to an adjacent winding. Sequentially charging each winding will cause the rotor to follow in a rotating field. The torque in this example depends on the current amplitude and the number of turns on the stator windings, the strength and the size of the permanent magnets, the air gap between the rotor and the windings, and the length of the rotating arm.

Types of Motor

a. Brushed DC Motor

A brushed DC motor consists of a commutator and brushes that convert a DC current in an armature coil to an AC current, as shown in figure. 

As current flows through the commutator through the armature windings, the electromagnetic field repels the nearby magnets with the same polarity, and causes the winging to turn to the attracting magnets of opposite polarity. As the armature turns, the commutator reverses the current in the armature coil to repel the nearby magnets, thus causing the motor to continuously turn. The fact that this motor can be driven by DC voltages and currents makes it very attractive for low cost applications. However, the arcing produced by the armature coils on the brush-commutator surface generates heat, wear, and EMI, and is a major drawback.

b. Brushless DC (BLDC) Motor

A BLDC motor accomplishes commutation electronically using rotor position feedback to determine when to switch the current. The structure is shown in figure. 

Feedback usually entails an attached Hall sensor or a rotary encoder. The stator windings work in conjunction with permanent magnets on the rotor to generate a nearly uniform flux density in the air gap. This permits the stator coils to be driven by a constant DC voltage (hence the name brushless DC), which simply switches from one stator coil to the next to generate an AC voltage waveform with a trapezoidal shape.

c. AC Induction Motor (ACIM)

A sinusoidal AC current runs through the stator to create a rotating variable magnetic field that induces a current in the rotor (typically made of non-ferrous materials). This induced current circulates in the bars of the rotor to generate a magnetic field. These two magnetic fields run at different frequencies (usually ω_-s>ω_-r for the motor) and to generate torque. Figure shows the motor structure.

d. Permanent Magnet Synchronous Motor (PMSM)

The PMSM motor shares some similarities with the BLDC motor, but is driven by a sinusoidal signal to achieve lower torque ripple. The sinusoidal distribution of the multi-phase stator windings generates a sinusoidal flux density in the air gap that is different from BLDC motor’s trapezoidal flux density. However, newer designs can achieve this sinusoidal flux density with concentrated stator windings and a modified rotor structure. Rotor magnet position can significantly alter the electrical properties of a PMSM; Mounting the rotor magnets on the surface as shown in figure results in lower torque ripple, while burying the magnets inside the rotor structure as shown in figure increases saliency, which increases the reluctance torque of the motor. The structure of PMSM is shown in figure.

e. Stepper Motor & Switched Reluctance (SR) Motor

Both stepper motors and SR motors have similar physical structures; The stator consists of concentrated winding coils while the rotor is made of soft iron laminates without coils. It has a doubly salient structure (teeth on both the rotor and stator) as shown in figure.

Stepper motors are designed to replace more expensive servo motors. When the current switches from one set of stator coils to the next, the magnetic attraction between rotor and stator teeth induces enough torque to rotate the rotor to the next stable position, or "step." The rotation speed is determined by the frequency of the current pulse, and the rotational distance is determined by the number of pulses. Since each step results in a small displacement, a stepper motor is typically limited to low-speed position-control applications.

There is no reactive torque (magnet to magnet) in an SR motor because the rotor cannot generate its own magnetic field. Instead, both rotor and stator poles have protrusions so that the flux length is a function of angular position, which gives rise to saliency torque. This is the only torque-producing mechanism in an SR motor, which tends to result in high torque ripple. However, due to their simple design, SR motor is very economical to build, and is perhaps the most robust motor available.

Brush-less DC Motor Control

a. Switch Configuration and PWM

Brush-less DC motors use electric switches to realize current commutation, and thus continuously rotate the motor. These electric switches are usually connected in an H-bridge structure for a single-phase BLDC motor. Usually the high-side switches are controlled using pulse-width modulation (PWM), which converts a DC voltage into a modulated voltage, which easily and efficiently limits the startup current, control speed and torque. Generally, raising the switching frequency increases PWM losses, though lowering the switching frequency limits the system’s bandwidth and can raise the ripple current pulses to the points where they become destructive or shut down the BLDC motor driver.

b. Electronics Commutation Principle of Single-Phase BLDC Motor

BLDC commutation relies on feedback on the rotor position to decide when to energize the corresponding switches to generate the biggest torque. The easiest way to accurately detect position is to use a position sensor. The most popular position sensor device is Hall sensor. Most BLDC motors have Hall sensors embedded into the stator on the non-driving end of the motor. figure shows the commutation sequence of a single-phase BLDC motor driver circuit.

The permanent magnets form the rotor and are located inside the stator. A Hall position sensor (“a”) is mounted to the outside stator, which induces an output voltage proportional to the magnetic intensity (assume the sensor goes HIGH when the rotor’s North Pole passes by, and goes LOW when the rotor’s South Pole passes by). SW1 and SW4 turn on when Hall sensor output is HIGH, as shown in figure. 

At this stage, armature current flows through the stator windings from OUT1 to OUT2 and induces the alternate stator electromagnetic poles accordingly. The magnetic force generated by rotor magnetic field and stator electromagnetic field causes the rotor to rotate. After the rotor signal reaches 180°, the Hall output voltage reverses due to its proximity to a South Pole. SW2 and SW3 then turn on with current reversing from OUT2 to OUT1, as shown in figure. The opposite stator magnetic poles induce the rotor to continue rotating in the same direction. 

Figure shows an example of Hall sensor signals with respect to switch drive signals and armature current. The armature current exhibits a saw tooth waveform due to PWM control. The applied voltage, switching frequency, and the PWM duty cycle are three key parameters to determine the speed and the torque of the motor.

Basic DC Voltmeter

The basic d.c. voltmeter is nothing but a PMMC D'Arsonoval galvanometer. The resistance is required to be connected in series with the basic meter to use it as a voltmeter.

This series resistance (Rs) is called a multiplier. The main function of the multiplier is to limit the current through the basic meter so that the meter current does not exceed the full scale deflection (Ifsd) value. The voltmeter measures the voltage across the two points of a circuit or a voltage across circuit component.

The multiplier resistance can be calculated as:

                                 From Figure,

                                  V = I_m  (R_m + R_s )

                                  V  =  I_mR_m + I_m   R_s 

                                              I_mR_s = V -  I_m  R_m

       Where,   R_m = Internal resistance of coil i.e. meter

                      R_s  = series multiplier resistance 

                      I_m  = full scale deflection current

                      V   = full range voltage to be measured

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