Basics of motors

In today's post we will be learning about the different types of motors and their principles of working .

Let us start by defining what a motor is.

"An electric motor is an electrical machine that converts electrical energy into mechanical energy."

Some of the motors that we commonly use for  robotic purposes are

1. Servo motor
2. Stepper motor
3. DC motors

Let us discuss the working of these motors in detail.

Servo Motors :-

A servomotor is a rotary actuator that allows for precise control of angular position, velocity and acceleration .It consists of a suitable motor coupled to a sensor for position feedback. It also requires a relatively sophisticated controller, often a dedicated module designed specifically for use with servomotors.
Servomotors are not a specific class of motor although the term servomotor is often used to refer to a motor suitable for use in a closed-loop control system.
Servomotors are used in applications such as robotics, CNC machinery or automated manufacturing.

1. Types of Servomotors

A servo motor

Unlike large industrial motors, servomotors are not used for continuous energy conversion . The basic operating principle is same as other electromagnetic motors . Design, construction and mode of operation are different. The rotors of this kind of motor are designed with long rotor lengths and smaller diameters . They have large size than that of conventional motors of same power ratings . There are various types of servomotors which are – series motors, split series motors, shunt control motor and permanent magnet shunt motor. We will now discuss these types of dc servomotors in brief.

• Series motors: The series motor has a high starting torque and draws large current .Speed regulation of this kind of motor is poor . Reversal can be obtained by reversing the polarity of field voltage with split series field winding (i.e one winding for direction of rotation). This method reduces motor efficiency to some extent .

• Split series motors: Split series motor are the dc series motor with split-field rated with some fractional kilowatt . This type of motor can operate as a separately excited field-controlled motor. The armature is supplied with a constant current source. Split series motor has a typical torque-speed curve . This curve denotes high stall torque and a rapid reduction in torque with increase in speed. This results in good damping.

• Shunt control motor : DC shunt type servomotor is not different from any other dc shunt motor . It has two separate windings – field windings placed on stator and armature winding placed on the rotor of the machine . Both windings are connected to a dc supply source. In a conventional dc shunt motor , the two windings are connected inparallel across the dc supply . In case of a servomotor , the windings are supplied with separate dc source.

•  Permanent magnet shunt motor: Permanent magnet shunt motor is a fixed excitation motor where the field is actually supplied by a permanent magnet . Performane is similar to armature controlled fixed field motor that we are going to know in the next section

2. Mechanism of working

As the name suggests, a servomotor is a servomechanism. More specifically, it is a closed-loop servo mechanism that uses position feedback to control its motion and final position. The input to its control is some signal, either analogue or digital, representing the position commanded for the output shaft.

There are two fundamental characteristics of any servo motor. These are:

•   The motor output torque is proportional to the voltage applied to it ( i.e     control voltage developed by amplifier in response to an error signal ) .

•   The instantaneous polarity of control voltage governs the direction of         torque developed by servomotors.

A servo motor is basically a DC motor(in some special cases it is AC motor) along with some other special purpose components that make a DC motor a servo. In a servo unit, you will find a small DC motor, a potentiometer, gear arrangement and an intelligent circuitry. The intelligent circuitry along with the potentiometer makes the servo to rotate according to our wishes.

As we know, a small DC motor will rotate with high speed but the torque generated by its rotation will not be enough to move even a light load. This is where the gear system inside a servomechanism comes into picture. The gear mechanism will take high input speed of the motor (fast) and at the output, we will get a output speed which is slower than original input speed but more practical and widely applicable.

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. This output port of the potentiometer is connected with one of the input terminals of the error detector amplifier. 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 external source, will be amplified in the error detector amplifier and feeds the DC motor. This amplified error signal acts as the input power of the dc motor and the motor starts rotating in desired direction. As the motor shaft progresses the potentiometer knob also rotates as it is coupled with motor shaft with help of gear arrangement. As the position of the potentiometer knob changes there will be an electrical signal produced at the potentiometer port. As the angular position of the potentiometer knob progresses the output or feedback signal increases. After desired angular position of motor shaft the potentiometer knob is reaches at such position the electrical signal generated in the potentiometer becomes same as of external electrical signal given to amplifier. 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. As the input signal to the motor is nil at that position, the motor stops rotating. This is how a simple conceptual servo motor works.

Stepper motor :-

Dynamics of a stepper motor

A stepper motor is an electromechanical device which converts electrical pulses into discrete mechanical movements. The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence. The motors rotation has several direct relationships to these appliedinput pulses. The sequence of the appliedpulses is directly related to the directiionmotor shafts rotation. The speed of themotor shafts rotation is directly related to the frequency of the input pulses and thelength of rotation is directly related to the number of input pulses applied.

Types of Stepper Motors :

A stepper motor is an electromechanical device which converts electrical pulses into discrete mechanical movements. The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence. The motors rotation has several direct relationships to these appliedinput pulses. The sequence of the appliedpulses is directly related to the directiionmotor shafts rotation. The speed of themotor shafts rotation is directly related to the frequency of the input pulses and thelength of rotation is directly related to the number of input pulses applied.

There are three basic types of step motors: variable reluctance, permanent magnet, and hybrid. This discussion will concentrate on the hybrid motor, since these step motors combine the best characteristics of the variable reluctance and permanent magnet motors. They are constructed with multi-toothed stator poles and a permanent magnet rotor. Standard hybrid motors have 200 rotor teeth and rotate at 1.8º step angles. Because they exhibit high static and dynamic torque and run at very high step rates, hybrid step motors are used in a wide variety of commercial applications including computer disk drives, printers/plotters, and CD players. Some industrial and scientific applications of stepper motors include robotics, machine tools, pick and place machines, automated wire cutting and wire bonding machines, and even precise fluid control devices. 

Principle of working :

There are many kind of stepper motors. Unipolar type, Bipolar type, Single-phase type, Multi-phase type... Single-phase stepper motor is often used for quartz watch.
On this page, I will explain the operation principle of the 2-phase unipolar PM type stepper motor.

In the PM type stepper motor, a permanent magnet is used for rotor and coils are put on stator. The stepper motor model which has 4-poles is shown in the figure on the left. In case of this motor, step angle of the rotor is 90 degrees.

As for four poles, the top and the bottom and either side are a pair.  coil,  coil and  coil,  coil correspond respectively. For example,  coil and  coil are put to the upper and lower pole.  coil and coil are rolled up for the direction of the pole to become opposite when applying an electric current to the coil and applying an electric current to the  coil. It is similar about  and , too.
The turn of the motor is controlled by the electric current which pours into , ,  and . The rotor rotational speed and the direction of the turn can be controlled by this control.

Clockwise control , ,  and  are controlled in the following order. Step angle 0 1 0 1 0° 1 0 0 1 90° 1 0 1 0 180° 0 1 1 0 270° "0" means grounding.  Counterclockwise control , ,  and  are controlled in the following order. Step angle 0 1 0 1 0° 0 1 1 0 -90° 1 0 1 0 -180° 1 0 0 1 -270° "0" means grounding. You can find by the figure, the rotor is stable in the middle of 2 poles of stator. When one side of the stator polarity is changed, the bounce by the magnetism occurs. As a result, the direction of rotor's turn is fixed. The characteristic of stepper motor is the angle can be correctly controlled and to be stable rotates ( It is due to the reliability of the control signal ). Moreover, because the rotor is fixed by the magnetism in the stationary condition as shown in the principle, the stationary power(Stationary torque) is large. It suits the use to make stop at some angle. DC Motors : Electrical motors are everywhere around us. Almost all the electro-mechanical movements we see around us are caused either by an A.C. or a DC motor. Here we will be exploring this kind of motors. This is a device that converts DC electrical energy to a mechanical energy. Principle of DC Motor This DC or direct current motor works on the principal, when a current carrying conductor is placed in a magnetic field, it experiences a torque and has a tendency to move. This is known as motoring action. If the direction of electric current in the wire is reversed, the direction of rotation also reverses. When magnetic field and electric field interact they produce a mechanical force, and based on that the working principle of dc motorestablished. The direction of rotation of a this motor is given by Fleming’s left hand rule, which states that if the index finger, middle finger and thumb of your left hand are extended mutually perpendicular to each other and if the index finger represents the direction of magnetic field, middle finger indicates the direction of electric current, then the thumb represents the direction in which force is experienced by the shaft of thedc motor. Structurally and construction wise a direct current motor is exactly similar to a DC generator, but electrically it is just the opposite. Here we unlike a generator we supply electrical energy to the input port and derive mechanical energy from the output port. We can represent it by the block diagram shown below. Here in a DC motor, the supply voltage E and electric current I is given to the electrical port or the input port and we derive the mechanical output i.e. torque T and speed ω from the mechanical port or output port. The input and output port variables of the direct current motor are related by the parameter K. So from the picture above we can well understand that motor is just the opposite phenomena of a DC generator, and we can derive both motoring and generating operation from the same machine by simply reversing the ports. In our next post we will discuss about the H-bridge and its operation.