All you need to know about Stepper Motors!
- Introduction
Among electrical machines available in the modern world, motors stand out as being the most frequently used type of electrical machine in a vast number of applications. The world grew exponentially within the past few decades in electrical and technological outbreaks due to the development and focus on factory automation. Because there are several ways to use electrical power to create motion, many different types of electric motors (both AC and DC) are already on the market, each with benefits tailored to specific uses. In this report, we will elaborate details on the Stepper Motor which leads its first roots in the 1970s.
2. Stepper Motor
Stepper motor underlies within the category of Brushless DC motors. Stepper motors or stepping motors stand unique among the other motors since they are well-known for precise movements, which may either be in precise positioning applications or precise speed controlling in automation systems. Stepper motor operation is usually bound with feedback control with fixed angle(step) output utilized in automation system applications.
2.1. Principle of Operation
The Stepper motor operation is quite simple. The energizing of the relevant coils in the stator drives the rotor respectively. Modes of energizing the stator define the operation of the stepper motor. The uniqueness of the stepper motor is its output shaft or the rotor rotates in a series of discrete angular movements for each command pulse.
Figure: Energizing of stator poles consecutively
For a stepper motor with a permanent magnet rotor and a four-pole stator winding, the above figure depicts the basic operating principle. When the stator energizes one pole the permanent magnet rotor attracts the magnetic pole with the energized pole. Subsequently, the next winding is energized and thus the rotor takes another step with a step angle of 90 degrees clockwise. This way by energizing the respective stator winding coils the rotor position can be maneuvered in steps in the principle of attraction and repulsion of magnetic poles.
2.2 Stepper Motor Types
There are three main stepper motor construction types differing from the rotor and stator construction mechanism.
These three types are,
- Permanent Magnet Stepper Motor
- Variable Reluctance Stepper Motor
- Hybrid Stepper Motor
Figure: Types of stepper motors (left-right: Permanent magnet, Variable reluctance, Hybrid)
2.2.1. Permanent Magnet Stepper Motor
The stator windings are wound stator pole type as in most of the DC motors. The rotor is cylindrical in shape and permanently magnetized. The stator is the same as the variable reluctance stepper motor. The stator teeth are aligned with the rotor poles based on the winding excitation. When compared to variable reluctance motors, they offer higher torque capabilities but operate at controlled speeds.
Figure: Permanent magnet stepper motor
2.2.2. Variable Reluctance Stepper Motor
The rotor is made up of ferromagnetic materials and needs to be energized externally to create a magnetization pole. The reluctance of the magnetization circuit of rotor and stator varies with the position, thus it is known as ‘Variable Reluctance Stepper Motor’.
Figure : Variable reluctance stepper motor
The principle of Variable Reluctance Stepper Motor is based on the property of the flux lines which capture the low reluctance path. The stator and the rotor of the motor are aligned in such a way that the magnetic reluctance is minimum. There are two types of the Variable Reluctance Stepper Motor. They are as follows:
- Single-stack
- Multi-stack
Figure: Multi-stack VR Stepper Motor
2.2.3. Hybrid Stepper Motor
The hybrid stepper motor includes both configurations of the variable reluctance motor and permanent magnet motor. The rotor of the hybrid stepper motor is a permanent magnet that is magnetized axially. This means one end is magnetized as the north pole and the other end as a south pole.
Figure: Hybrid Stepper Motor
These consist of two end caps fitted at the ends of the axially magnetized shaft. These end caps contain a fixed number of teeth and respectively possess the North and South pole properties accordingly to the axial permanent magnet.
The stator is constructed as in Variable Reluctance motor, consisting of slots for coil windings.
2.3. Equations and definitions on Stepper Motors
2.3.1. Step angle
The step angle is the angle that the rotor traverses from one signal input to the stator. The smaller the step angle greater the resolution since the number of steps per revolution is increased.
The step angle can be expressed either in terms of rotor and stator poles or in terms of the number of stator phases and the number of rotor teeth.
Ns = Number of stator poles
Nr = Number of rotor poles
2.3.2. Resolution
Resolution refers to the number of steps per revolution in the rotor.
Resolution = Number of Steps / Number of Revolutions of Rotor
Higher the resolution better precision in positioning objects using stepper motors.
2.3.2. Stepping frequency
Stepping frequency is the number of steps the motor shaft rotates within one second. At higher stepping frequencies we can observe that the motor is running continuously. As long as the commanding pulses are in synchronism with the stepping frequency no harm occurs, but when the stepping frequency increases above a certain level where synchronism is lost, then the machine is stalled.
Motor shaft speed = Step angle. Stepping frequency/360 rps
Stalling is not a problem in stepper motors since they are designed in such a manner that although the stator current flow is present in rated current the rotor is at a fixed position.
Detent Torque
Detent torque is the amount of torque produced without the stator windings being energized. Other than for Variable Reluctance motors, permanent magnet, and hybrid stepper motors possess a detent torque due to the presence of permanent magnets in their rotors. These permanent magnet rotors are attracted to the stator poles even if there is no power in the stator windings.
Hybrid stepper motors include teeth on the rotor surface to better regulate magnetic flux between the stator and the rotor, resulting in greater holding, dynamic, and detent torque values than permanent magnet stepper motors.
A motor will have to overcome the detent torque in operation. It reduces the ideal torque that the motor coils have produced. The power that the motor has to introduce to overcome the detent torque is proportional to the speed of the rotor. So if the motor runs at higher speeds the amount of power that the motor has to sacrifice in order to overcome the relevant detent torque is high.
Figure: Torque and Power output vs the Speed (ideal torques in red)
Holding Torque
The torque that is needed to move the rotor from one stepping position to another is called the holding torque. The holding torque is actually the useful torque of a stepper motor that holds the external load in place when the windings are energized but the rotor is stationary.
Holding torque is known to be higher than the running torque and is limited by the maximum current flowing in the motor.
2.4. Driving Techniques and Switching sequences
2.4.1. Stepper motor driving techniques
The stepper motor techniques are divided into several methods with respect to the stator coil energizing sequences.
- Wave mode
- Full-step mode
- Half-step mode
- Microstepping
For simplicity of understanding the concepts of driving techniques, we will consider a permanent magnet rotor stepping motor with a 4-pole bipolar stator.
Wave mode
One phase per time is energized. The motor moves a simple step angle with each pulse of this drive mode. The sequence of the motor stator current in full step drive mode is shown in the diagram below.
Figure: Wave drive mechanism (Energized coils in green)
The current vs time graphs of the stator windings will give us an idea how the coils are energized.
Figure: Current vs time in two phases ‘a’ and ‘b’
Full-step Mode
The stepper motor driver energizes the two coils of the two-phase stepper motor in a pulse/direction instruction during the full-step drive. The motor moves a simple step angle with each pulse of this driving mode. The sequence of the motor stator current in full step drive mode is shown in the diagram below.
Figure: Full-step drive mechanism (Energized coils in green)
The stages are identical to those in the wave mode, with the main difference being that the motor can create greater torque in this mode since more current flows through it and a stronger magnetic field is formed. The step angles are also the same as those in the wave mode.
Half-step Mode
The rotor stops at one point when single-phase excitation is used. The rotor will advance half a step angle and halt in the middle of the two neighboring full-step locations once the driver gets the next pulse and provides two excitations at the same time. The two-phase coil is energized single-phase, then double-phase and the stepping motor rotates at half a step angle for each pulse in this cycle. The half-step approach has the benefit of double accuracy and delivering less vibration at low speeds as compared to the full-step method. A schematic illustration of the motor stator current sequence in half-step drive mode is shown below.
Figure: Half-step drive mechanism (Energized coils in green)
Microstepping
Microstepping is, nevertheless, the most prevalent way of regulating stepper motors presently. In this mode, we use a sin wave to supply variably regulated current to the coils. The rotor will move smoothly, the components will be less stressed, and the stepper motor’s precision will improve. This procedure makes the rotor move smoothly, making it one of the most effective strategies for reducing vibration and noise at moderate speeds.
Figure : Microstepping drive mechanism (Energized coils in green)
2.4.2. Stepper motor switching sequences
To function, stepper motors require certain external electrical components. To establish the step rate, these components generally comprise a power supply, logic sequencer, switching components, and a clock pulse source.
The switching sequences depend on whether the stepper motor is bipolar or unipolar. Most of the time H- bridges are used to control stepper motor stators. The switching devices can be transistors, IGBTs, MOSFETs, etc.
First, we will introduce what is unipolar and bipolar stepper motors.
2.4.2.1 Unipolar Stepper Motor Drives
If the current in the coils can be only flown in one direction we call them as unipolar. The control mechanism for voltage drive is simple, with only one switch, or transistor, per coil. The coil is energized when the transistor is closed. The transistors are alternately closed and opened to commutate the motor.
Let’s focus on the same 2 phase 4 pole permanent magnet stepper motor.
Figure: Two-phase unipolar drive
Phase A comprises of the two coils A+ and A- while phase B comprises of two coils B+ and B-. Here both the transistors Q1 and Q2 cannot be operated together. They should be operated at different times. Only half of the phase is powered at a time with unipolar control, therefore the current only occupies half of the copper volume.
Figure: Current flow in a phase of the unipolar drive(Q1, Q2 conducting separately)
2.4.2.2 Bipolar Stepper Motor Drives
Only one coil winding is required per phase in bipolar motors, and current can flow in both directions through each coil. To control bipolar motors, eight transistors with two H-bridges are required, as illustrated in the figure.
Figure : Two-phase bipolar drive
The operation needs more switching devices. Here all the copper is used in the coil not like in unipolar. Therefore more power is lost as copper losses. Bipolar drive gives better results than the unipolar drive since it provides ~40% more torque value.
Figure: Current flow in a bipolar drive (Two transistors are energized per instance)
Bipolar and Unipolar drives use H-bridges for relevant current flow paths. The driving modes can be achieved using both bipolar and unipolar drives which we discussed in the previous section. However in high speeds, in voltage drive, the unipolar delivers higher torque than the bipolar since current flows faster in the unipolar drive method.
Figure: Bipolar and Unipolar drive Torque vs Speed (Bipolar — Orange)
2.5. Snubbing Resistance
Since these motors are basically inductive and they store the energy we have to consider the flow of stored energy back to the switching devices which may cause harm to them. Snubber circuit mechanisms need to be introduced to ensure the protection of switching devices. They save these switching devices from overvoltage spikes and ensure the safety of the circuits.
Snubber Diode
Figure: Snubber circuit with a diode
The stored energy is allowed to travel through the diode and dissipate energy thus protecting the switching element from voltage spikes.
Snubber diode with a resistor
Figure: Snubber circuit with diode and resistor
The use of a resistor in series with the freewheeling diode ensures better energy dissipation than a single diode-only snubber. The voltage drop across the resistor can be determined by taking into consideration the max Collector-Emitter junction voltage of the transistor.
Snubber circuits with Zener diodes
Figure: Zener diode utilized in the snubber circuit
Since Zener diodes maintain a given voltage across their terminals smoothly. This way it is better than that of using the diode snubber circuits. The Zener diode can be installed across the transistor even. This way a fixed voltage is maintained within the transistor collector-emitter terminals.
Considering all the above-mentioned snubber circuits the following summary is brought up contrasting their energy dissipation action.
Figure: Energy dissipation of various snubber circuits
References
https://www.motioncontroltips.com/faq-whats-the-difference-between-detent-torque-and-holding-torque/
https://www.thomasnet.com/articles/machinery-tools-supplies/stepper-motors/
https://www.techbriefs.com/component/content/article/tb/supplements/md/features/articles/39559
