Sprocket Holding

Sprocket Holding

Implementation of Stepper Motor in Robot

Implementation of Stepper Motor in Robot

Summary

A typical single axis stepper system consists of a stepper controller, a motor drive, a motor (with or without gearbox), and a power supply. A stepper is typically commanded by two digital inputs: a digital pulse train and a direction bit. The stepping drive and motor is used primarily for position control. And unlike all other motor types, stepper motor is moved in "steps" (just one step per one command pulse) and will hold at its present position if no command pulses are received.

Key words

Step angle, rotor angle, torque, speed.

 

1. Introduction

                The frequency of the pulse train controls the velocity of the motor, where the number of pulses determines the length of the move. The direction signal determines in which direction the motor will rotate. For each pulse from the controller, the drive will move the motor "one step" in the direction indicated by the direction command

 

 

The top electromagnet (1) is charged, attracting the topmost four teeth of a sprocket.

 

The top electromagnet (1) is turned off, and the right electromagnet (2) is charged, pulling the nearest four teeth to the right. This results in a rotation of 3.6°.

 

 

The bottom electromagnet (3) is charged; another 3.6° rotation occurs.

 

The left electromagnet (4) is enabled, rotating again by 3.6°. When the top electromagnet (1) is again charged, the teeth in the sprocket will have rotated by one tooth position; since there are 25 teeth, it will take 100 steps to make a full rotation.

A stepper motor is a brushless, syncro electric motor that can divide a full rotation in typically 200 steps.

Of course, this is achieved by increasing the numbers of poles (both on rotor and stator), taking care that they have no common denominator. Additionally, soft magnetic material with many teeth on the rotor and stator cheaply multiplies the number of poles (reluctance motor).Of course, like an AC synchronous motors it is ideally driven by sinusoidal current, allowing a step less operation, but this puts some burden on the controller. When using an 8 bit digital controller 256 micro steps per step are possible. As a analog-to-digital converter produces unwanted ohmic heat in the controller pulse-width modulation is used instead to regulate the mean current.

Simpler models switch voltage only for doing a step needing an extra current limiter. For every step they switch a single cable to the motor. Bipolar controllers can switch between supply voltage, ground, and unconnected. Unipolar controllers can only connect or disconnect a cable, because the voltage is already hard wired. Unipolar controllers need center tapped windings.

Stepper motors are rated by the torque they produce. Syncron electric motors using soft magnetic materials (having a core) have the ability to provide position holding torque while not driven electrically. To achieve full rated torque, the coils in a stepper motor must reach their full rated current during each step. The voltage rating (if there is one) is almost meaningless.

 

 

 

2. Applications

Computer controlled stepper motors are one of the most versatile forms of positioning systems, particularly when digitally controlled as part of a servo system. Stepper motors are used in floppy disk drives, flatbed scanners, printers, plotters and many more devices. Note that hard drives no longer use stepper motors, instead utilizing a voice coil and servo feedback for head positioning.

 

3. Construction and Operation

"Can-stack" stepping motors consist of two stacked sets of toothed stator poles and circular coils and a cylindrical ceramic permanent magnet rotor with radial alternating north and south poles. The number of rotor poles is equal the number of stator teeth in each sets of poles.

The stator pole coils are energized; the rotor will align itself between the two equal stator fields. Typically the number of poles is such that the motors have step angles in the range of 7.5 to 20 degrees. A single step of the rotor is the result of a change of magnetic polarity of one set of stator teeth.

This change in polarity is brought about by reversing the direction of current flow in the coil associated with those teeth. The rotor motion for a single step with no load applied is that of a damped oscillation. The damping characteristics are modified by frictional and inertial loading, the sequence in which windings are energized, and the electronic damping in the drive circuitry.       

 

 

4. Step Angle Accuracy

The average value of the measured step angles of an unloaded stepping motor over 360 degrees will be equal to the nominal step angle. The maximum deviation of the individual steps from the nominal step angle is the error usually specified as a non-cumulative or incremental step angle error. The typical maximum value for this error in a can-stack motor with two phases energized is ± 5%.    

 

5. Stepping Sequences

For continuous rotation a repeating sequence of changing tooth polarity is required. Differences in motor performance characteristics result from different sequences. A commonly used scheme for stepping is to energize both stator coils and to reverse the current in aIternate coils with each successive step. This results in a four step sequence.

Reversing the sequence reverses the direction of rotation. This is called a full step mode with two phases on. It is also possible to step the rotor with the same angular increment by energizing only one phase each step.

This is also a four step sequence and is known as a wave drive. Since only half the copper volume is being used, the efficiency is lower and there is less damping with this scheme than with two phases on. A third sequence alternates between one and two phases energized to produce 1/2 the step angle of the previous sequences. The half step sequence requires eight steps. Angular resolution is improved and the smaller step angel provides an improvement in damping. However, it should also be noted that this scheme produces alternate "weak" steps when only one phase is energized.

 

Resonance All stepping motors exhibit resonance at certain pulse rates. In typical can-stack stepping motor applications resonances are most commonly encountered at low frequencies (less than 100 pulses per second). Although there is no loss of steps at these frequencies, there is an increase in vibration and noise. This becomes even more noticeable when a gear train is coupled to the motor. When operation at resonant frequencies cannot be avoided, some improvement in damping may be obtained with increased frictional damping, reduced input power, modified drive circuitry or half-stepping.         

 

6. Torque Characteristics

The maximum torque developed by the motor is the static or holding torque. It is measured while displacing the rotor one step with one or two phases energized. During continuous stepping with a constant voltage supply the dynamic torque developed decreases with increasing stepping rate. This reflects the relatively large inductance to resistance ration of the motor.

In the typical dynamic torque curve shown (commonly called an L/R cunve) the lower curve represents the maximum torque load which the motor will start and stop without losing steps (pull-in).

The upper curve represents the maximum torque which the motor can develop at a given pulse rate or alternately, the maximum rate to which a given load can be accelerated (pull-out). Motor torque at higher pulse rates can be increased by increasing the input to the motor using a variety of drive techniques.

These include simple schemes such as increasing the voltage directly or decreasing the time constant by adding external series resistance, and more elaborate techniques such as bi-level voltage drives in which winding current is controlled. When overdriving techniques are used to extend motor performance, consideration must be given to the maximum permissible temperature rise of the motor winding based on the insulation rating of the motor.

 

Bifilar and Bipolar Operation The terms bifilar and bipolar refer to two different types of windings that may be used in the stator coils. Bipolar windings contain a single coil in each stator half. The switching circuitry used to reverse the direction of current flow with this coil is typically of the full bridge or dual supply type. Bifflar windings contain two coils in each stator half. When they are connected as show in the figure, the magnetic polarity of the stator teeth can be reversed by switching from one coil to the other of each pair with a unipolar supply. Note that although a bifilar-wound motor does contain four coils or "phases," it is operated as a two phase motor. Bifilar-wound PM steppers are widely used because of the drive circuit simplicity. All stock Hurst stepping motors use this winding configuration. Bifilar and bipolar-wound motors do exhibit some performance differences. Since the winding volume per phase of a bifilar-wound stepper is only half that of a bipolar-wound stepper, the attainable ampere-turns for a given input power will necessarily be lower for the bifilar-wound motor. Therefore, the torque is lower. However, with an L/R drive it is because the bipolar coil with its larger volume will also have a larger time constant. At higher stepping rates the bipolar-wound motor's torque will decrease to approximately the same level as that of the bifilar-wound motor. The choice of winding type will depend upon the application. The holding torque for a bipolar version of a given motor will be 20-30% higher than the bifilar version. Dynamic torque differences will depend upon the drive circuitry. With the simplest drive circuits the bipolar performance exceeds the bifilar performance only at low frequencies. As drive circuit complexity increases the bipolar performance becomes superior

 

7. Brushless DC electric motor

A brushless DC motor (BLDC) is a DC electric motor that uses an electronically-controlled commutation system, instead of a mechanical commutation system. (The rest of this article assumes the reader is familiar with the principles of electrical motors.)

Two subtypes exist:

·           The three-phase AC synchronous motor type has three electrical connections

·           The stepper motor type may have more poles on the stator.

In a conventional (brushed) DC-motor, the brushes make mechanical contact with a set of electrical contacts on the rotor (called the commutator), forming an electrical circuit between the DC electrical source and the armature coil-windings. As the armature rotates on axis, the stationary brushes come into contact with different sections of the rotating commutator. The commutator and brush-system form a set of electrical switches, each firing in sequence, such that electrical-power always flows through the armature-coil closest to the stationary stator (permanent magnet.)

In a BLDC motor, the brush-system/commutator assembly is replaced by an intelligent electronic controller. The controller performs the same power-distribution found in a brushed DC-motor, only without using a commutator/brush system. The controller contains a bank of MOSFET devices to drive high-current DC power, and a microcontroller to precisely orchestrate the rapid-changing current-timings. Because the controller must follow the rotor, the controller needs some means of determining the rotor's orientation/position (relative to the stator coils.) Some designs use Hall effect sensors to directly measure the rotor's position. Others measure the back EMF in the undriven coils to infer the rotor position, eliminating the need for separate Hall Effect sensors. (The BLDC motor has a trapezoidal backemf, while a brushless AC motor has a sinusoidal backemf.)

BLDC motors can be constructed in two different physical configurations: In the 'conventional' configuration, the permanent magnets are mounted on the spinning armature (rotor.) The stator coils surround the rotor. In the 'out runner' configuration, the radial-relationship between the coils and magnets are reversed; the stator coils form the center (core) of the motor, while the permanent magnets spin on an overhanging rotor which surrounds the core. In all BLDC motors, the stator-coils are stationary.

 

8. Comparison with brushed-DC motors

BLDC motors offer several advantages over brushed DC-motors, including higher reliability, longer lifetime (no brush erosion), elimination of ionizing sparks from the commutator, and overall reduction of electromagnetic interference (EMI.) BLDC's main disadvantage is higher cost, which arises from two issues: First, BLDC motors require high-power MOSFET devices in the fabrication of the electronic speed controller. Brushed DC-motors can be regulated by a comparatively trivial variable-resistor (potentiometer or rheostat), which is inefficient but also satisfactory for cost-sensitive applications. BLDC motors need a more expensive integrated circuit, called an electronic speed controller, to offer the same type of variable-control. Second, when comparing manufacturing techniques between BLDC and brushed motors, many BLDC designs require manual-labor, to hand-wind the stator coils. On the other hand, brushed motors use armature coils which can be inexpensively machine-wound.

BLDC motors are considered more efficient than brushed DC-motors. This means for the same input power, a BLDC motor will convert more electrical power into mechanical power than a brushed motor. The enhanced efficiency is greatest in the no-load and low-load region of the motor's performance curve. Under high mechanical loads, BLDC motors and high-quality brushed motors are comparable in efficiency.

 

9. Applications

BLDC motors can potentially be deployed in any field-application currently fulfilled by brushed DC motors. Cost prevents BLDC motors from replacing brushed motors in most common areas of use. Nevertheless, BLDC motors have come to dominate many applications: Consumer devices such as computer hard drives, CD/DVD players, and PC cooling fans use BLDC motors almost exclusively. Low speed, low power brushless DC motors are used in direct-drive turntables. High power BLDC motors are found in electric vehicles and some industrial machinery. These motors are essentially ac synchronous motors with permanent magnet rotors.

 

 

 

About the Author

Assistant professor in lord venkateswara engineering college.I am doing phd in sathyabama university, Tamil Nadu,India.

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