Motors and Stepper Motors

Electrical Motors

Electrical Motors are continuous actuators that convert electrical energy into a rotational type movement, although linear motors are also available. There are basically three types of conventional electrical motor available: AC type Motors, DC type Motors and Stepper Motors. AC Motors are generally used in high power single or multi-phase industrial applications were a constant rotational torque and speed is required to control large loads. In this tutorial on motors we will look only at simple light duty DC Motors and Stepper Motors which are used in many electronics, positional control, microprocessor, PIC and robotic circuits and systems.

The DC Motor

The DC Motor or Direct Current Motor is the most commonly used actuator for producing continuous movement and whose speed of rotation can easily be controlled, making them ideal for use in applications where speed control, servo type control, and/or positioning is required. There are basically 3 types of DC Motor:

• Brushed Motor - This type of motor produces a magnetic field in a wound rotor by passing an electrical current through a commutator and carbon brush assembly, hence the term "Brushed". The stators magnetic field is produced by using either a wound stator field winding or by permanent magnets. Generally brushed DC motors are cheap, small and easily controlled.
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• Brushless Motor - This type of motor produce a magnetic field in the rotor by using permanent magnets attached to it and commutation is achieved electronically. They are generally smaller but more expensive than conventional brushed type DC motors because they use "Hall effect" switches in the stator to produce the required stator field rotational sequence but they have better torque/speed characteristics, are more efficient and have a longer operating life than equivalent brushed types.
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• Servo Motor - This type of motor is basically a brushed DC motor with some form of positional feedback control connected to the rotor shaft. They are connected to and controlled by a PWM type controller and are mainly used in positional control systems and radio controlled models.

DC motors have almost linear characteristics with their speed of rotation being determined by the applied DC voltage and their output torque being determined by the current flowing through the motor windings. The speed of rotation of any DC motor can be varied from a few revolutions per minute (rpm) to many thousands of revolutions per minute making them suitable for electronic, automotive or robotic applications. By connecting them to gearboxes or gear-trains their output speed can be decreased while at the same time increasing the torque output of the motor.

The "Brushed" DC Motor

A conventional DC Brushed Motor consist basically of two parts, the stationary body of the motor called the "Stator" and the inner part which rotates producing the movement called the "Rotor" or "Armature". The stator consists of electrical coils connected together in a circular configuration to produce a North-Pole then a South-Pole then a North-Pole etc, type stationary field system (as opposed to AC machines whose stator field rotates) with the current flowing within these field coils being known as the motor field current. In permanent magnet DC (PMDC) motors these field coils are replaced with strong Rare Earth (i.e. Samarium Cobolt, or Neodymium Iron Boron) type magnets which have very high magnetic energy fields.

The rotor or armature of a DC machine consists of current carrying conductors connected together at one end to electrically isolated copper segments called the "commutator". The commutator allows an electrical connection to be made via carbon brushes (hence the name "Brushed" motor) to an external power supply as the armature rotates. The magnetic field setup by the rotor tries to align itself with the stationary stator field causing the rotor to rotate on its axis, but can not align itself due to commutation delays. The rotational speed of the motor is dependent on the strength of the rotors magnetic field and the more voltage that is applied to the motor the faster the rotor will rotate. By varying this applied DC voltage the rotational speed of the motor can also be varied. Problems associated with this type of motor is that sparking occurs under heavy load conditions between the two surfaces of the commutator and carbon brushes resulting in self generating heat and short life span.

Conventional (Brushed) DC Motor

Permanent magnet (PMDC) brushed motors are generally much smaller and cheaper than their equivalent wound type d.c. motor cousins as they have no field winding. They also have much better linear speed/torque characteristics than equivalent wound motors making them more suitable for use in models, robotics and servos. There are two basic types of stator field windings in DC motors, Series wound and Shunt wound. These motors also use a similar armature with brushes and a commutator. A series wound d.c. motor has the stator field windings connected in Series with the armature while a shunt wound DC motor has the stator field windings connected in Parallel with the armature. The series wound motor is more common.

The DC Servo Motor

DC Servo motors are used in closed loop type applications were the position of the output motor shaft is fed back to the motor control circuit. Typical positional "Feedback" devices include Resolvers, Encoders and Potentiometers as used in radio control models such as airplanes and boats etc. A servo motor generally includes a built-in gearbox for speed reduction and is capable of delivering high torques directly. The output shaft of a servo motor does not rotate freely as do the shafts of DC motors because of the gearbox and feedback devices attached.

DC Servo Motor Block Diagram

A servo motor consists of a DC motor, reduction gearbox, positional feedback device and some form of error correction. The speed or position is controlled in relation to a positional input signal or reference signal applied to the device. The error detection amplifier looks at this input signal and compares it with the feedback signal from the motors output shaft and determines if the motor output shaft is in an error condition and, if so, the controller makes appropriate corrections either speeding up the motor or slowing it down. This response to the positional feedback device means that the servo motor operates within a "Closed Loop System".

Servo motors are also used in remote control models with most servo motors being able to rotate up to about 180 degrees in both directions making them ideal for accurate angular positioning. However, these RC type servos are unable to continually rotate at high speed like conventional DC motors unless specially modified. A servo motor consist of several devices in one package, motor, gearbox, feedback device and error correction for controlling position, direction or speed. They are controlled using just three wires, Power, Ground and Signal Control.

DC Motor Switching and Control

Small DC motors can be switched "On" or "Off" by means of relays, transistors or mosfet circuits. The simplest form of motor control is "Linear" control. This type of circuit uses a bipolar Transistor as a Switch (A Darlington transistor may also be used were a higher current rating is required) to control the motor from a single power supply. By varying the amount of base current flowing into the transistor the speed of the motor can be controlled for example, if the transistor is turned on "half way", then only half of the supply voltage goes to the motor. If the transistor is turned "fully ON", then all of the supply voltage goes to the motor and it rotates faster. Then for the linear type of control, power is delivered constantly as shown below.

This circuit shows the connections for a Uni-directional (one direction only) motor control circuit. A continuous logic "1" or logic "0" is applied to the input of the circuit to turn the motor "ON" or "OFF" respectively and a flywheel diode is connected across the motor terminals to protect the transistor from any back emf generated by the motor when the transistor turns "OFF". As well as the basic "ON/OFF" control the same circuit can also be used to control the motors rotational speed. By repeatedly switching the motor current "ON" and "OFF" the speed of the motor can be varied between stand still (0 rpm) and full speed (100%). This is achieved by varying the proportion of "ON" time (ton) to the "OFF" time (toff) and this is called "Pulse Width Speed Control".

Pulse Width Speed Control

The rotational speed of a DC motor is directly proportional to the mean (average) value of its supply voltage and the higher this value, up to maximum allowed motor volts, the faster the motor will rotate. In other words more voltage more speed. By varying the ratio between the "ON" (t_on) time and the "OFF" (t_off) time durations, called the "Duty Ratio", "Mark/Space Ratio" or "Duty Cycle", the average value of the motor voltage and hence its rotational speed can be varied. For simple unipolar drives the duty ratio β is given as:

and the mean DC output voltage fed to the motor is given as: Vm = β x Vs. Then by varying the widths of the pulses the motor voltage and hence the power applied to the motor can be controlled and this type of control is called Pulse Width Modulation or PWM.

Another way of controlling the rotational speed of the motor is to vary the frequency (and hence the time period of the controlling voltage) while the "ON" and "OFF" duty ratio times are kept constant. This type of control is called Pulse Frequency Modulation or PFM. With pulse frequency modulation, the motor voltage is controlled by applying pulses of variable frequency for example, at low frequency or with very few pulses the average voltage applied to the motor is low, and therefore the motor speed is slow. At a higher frequency or many pulses, the average motor terminal voltage is increased and the motor speed will increase.

Then, Transistors can be used to control the amount of power applied to a d.c. motor with the mode of operation being either "Linear" (varying motor voltage), "Pulse Width Modulation" (varying width of pulse) or "Pulse Frequency Modulation" (varying frequency of pulse).

H-bridge Motor Control

While controlling the speed of a DC motor with a single transistor has many advantages it also has one main disadvantage, the direction of rotation is always the same, its a "Uni-directional" circuit. In many applications we need to operate the motor in both directions forward and back. One very good way of achieving this is to connect the motor into a "Transistor H-bridge" circuit arrangement and this type of circuit will give us "Bi-directional" DC motor control as shown below.

Basic Bi-directional H-bridge Circuit

The "H-bridge" circuit is so named because the basic configuration of the four switches, either electro-mechanical relays or transistors resembles that of the letter "H" with the motor positioned on the centre bar. The Transistor or MOSFET H-bridge is probably one of the most commonly used type of Bi-directional motor control circuits which uses "complementary transistor pairs" both NPN and PNP in each branch with the transistors being switched together in pairs to control the motor. Control input A operates the motor in one direction ie, Forward rotation and input B operates the motor in the other direction ie, Reverse rotation. Then by switching the transistors "ON" or "OFF" in their "diagonal pairs" results in directional control of the motor.

For example, when transistor TR1 is "ON" and transistor TR2 is "OFF", point A is connected to the supply voltage (+Vcc) and if transistor TR3 is "OFF" and transistor TR4 is "ON" point B is connected to 0 volts (GND). Then the motor will rotate in one direction corresponding to motor terminal A being positive and motor terminal B being negative. If the switching states are reversed so that TR1 is "OFF", TR2 is "ON", TR3 is "ON" and TR4 is "OFF", the motor current will now flow in the opposite direction causing the motor to rotate in the opposite direction.

Then, by applying opposite logic levels "1" or "0" to the inputs A and B the motors rotational direction can be controlled as follows.

H-bridge Truth Table

It is important that no other combination of inputs are allowed as this may cause the power supply to be shorted out, ie both transistors, TR1 and TR2 switched "ON" at the same time, (fuse = bang!).

As with Uni-directional motor control as seen above, the rotational speed of the motor can also be controlled using Pulse Width Modulation or PWM. Then by combining H-bridge switching with PWM control, both the direction and the speed of the motor can be accurately controlled. Commercial off the shelf decoder IC's such as the SN754410 Quad Half H-Bridge IC or the L298N which has 2 H-Bridges are available with all the necessary control and safety logic built in are specially designed for H-bridge bi-directional motor control circuits.

The Stepper Motor

Like the DC motor above, Stepper Motors are also electromechanical actuators that convert a pulsed digital input signal into a discrete (incremental) mechanical movement are used widely in industrial control applications. A stepper motor is a type of synchronous brushless motor in that it does not have an armature with a commutator and carbon brushes but has a rotor made up of many, some types have hundreds of permanent magnetic teeth and a stator with individual windings. As it name implies, a stepper motor does not rotate in a continuous fashion like a conventional DC motor but moves in discrete "Steps" or "Increments", with the angle of each rotational movement or step for example, 3.6, 7.5 degrees dependant upon the number of stator poles and rotor teeth each stepper motor has. For example, assume a stepper motor completes one full revolution in 100 steps. Then the step angle for the motor is given as 360 degrees/100 steps = 3.6 degrees per step. This is commonly known as the motors "Step Angle".

There are three basic types of stepper motor, Variable Reluctance, Permanent Magnet and Hybrid (a sort of combination of both). A Stepper Motor is particularly well suited to applications that require accurate positioning and repeatability with a fast response to starting, stopping, reversing and speed control.

Modern multi-pole, multi-teeth stepper motors are capable of accuracies of less than 0.9 degs per step (400 Pulses per Revolution) and are mainly used for highly accurate positioning systems like those used for magnetic-heads in floppy/hard disc drives, printers/plotters or robotic applications. The most commonly used stepper motor being the 200 step per revolution stepper motor. It has a 50 teeth rotor, 4-phase stator and a step angle of 1.8 degrees (360 degs/(50x4)).

Example of a Stepper Motor and Control Circuit.

In our simple example of a variable reluctance stepper motor above, the motor consists of a central rotor surrounded by 4 field coils labelled A, B, C and D. All coils with the same letter are connected together so that energising, say coils marked A will cause the rotor to align itself with that set of coils. By applying power to each set of coils in turn the rotor can be made to rotate or "step" from one position to the next by an angle determined by its step angle construction, and by energising the coils in sequence the rotor will produce a rotary motion. By energising the coils in a set sequence of "ABCD, ABCD, ABCD, A..." etc, the rotor will rotate in one direction and by reversing the sequence to "DCBA, DCBA, DCBA, D..." etc, the rotor will rotate in the opposite direction.

It is also possible to control the speed of rotation of a stepper motor by altering the time delay between the digital pulses applied to the coils (the frequency), the longer the delay the slower the speed for one complete revolution. By applying a fixed number of pulses to the motor, the motor shaft will rotate through a given angle and so there would be no need for any form of additional feedback because by counting the number of pulses given to the motor the final position of the rotor will be exactly known. This response to a set number of digital input pulses allows the stepper motor to operate in an "Open Loop System" making it both easier and cheaper to control.

For example, assume our stepper motor above has a step angle of 3.6 degs per step. To rotate the motor through an angle of say 216 degrees and then stop would only require 216 degrees/(3.6 degs/step) = 80 pulses applied to the stator coils.

Stepper motor controller IC's are available such as the SAA1027 which have all the necessary counter and code conversion built-in, and automatically drives the 4 fully controlled bridge outputs to the motor in the correct sequence. The direction of rotation can also be selected along with single step mode or continuous (stepless) rotation in the selected direction, but this puts some burden on the controller. When using an 8-bit digital controller, 256 microsteps per step are also possible