stepper motor drives
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STEPPER MOTOR DRIVES
SEMINAR REPORTSubmitted To: Presented By:
Dr. H.K.Verma Dr. Shailendra Sharma
Amit Kumar ShuklaM.E. Electrical Engg.(P.E.) 801EE11ME26
1.AbstractToday Stepper motor is highly use in : Dot Matrix printers Disk Drives, Floppy Drive CD-Drives Robotics Precise Industrial Controls Low speed High Torque Applications Since the motor has full torque at standstill & Excellent response to starting/stopping/reversing.
2.IntroductionA 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 applied input pulses. The sequence of the applied pulses is directly related to the direction of motor shafts rotation. The speed of the motor shafts rotation is directly related to the frequency of the input pulses and the length of rotation is directly related to the number of input pulses applied.
A stepper motor (or step motor) is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor's position can then be commanded to move and hold at one of these steps without any feedback sensor (an open-loop controller), as long as the motor is carefully sized to the application. The stepper motor can only take one step at a time and each step is the same size. As the digital pulses increase in frequency, the step movement changes into continuous rotation, with the speed of rotation directly proportional to the frequency of the pulses. Step motors are used every day in both industrial and commercial applications because of their low cost, high reliability, high torque at low speeds and a simple, rugged construction that operates in almost any environment.
3.ClassificationThere are three basic stepper motor types. They are : Variable-reluctance Permanent-magnet Hybrid Lavet type stepping motor
3.1Variable-reluctanceThis type of stepper motor has been around for a long time. It is probably the easiest to understand from a structural point of view. Figure 3.1 shows a cross section of a typical V.R. stepper motor. This type of motor consists of a soft iron multi-toothed rotor and a wound stator. When the stator windings are energized with DC current the poles become magnetized. Rotation occurs when the rotor teeth are attracted to the energized stator poles.
3.2Permanent-magnetOften referred to as a tin can or canstock motor the permanent magnet step motor is a low cost and low resolution type motor with typical step angles of 7.5 to 15. (48 24steps/revolution) PM motors as the name implies have permanent magnets added to the motor structure. The rotor no longer has teeth as with the VR motor. Instead the rotor is magnetized with alternating north and south poles situated in a straight line parallel to the rotor shaft. These magnetized rotor poles provide an increased magnetic flux intensity and because of this the PM motor exhibits improved torque characteristics when compared with the VR type.
3.3HybridThe hybrid stepper motor is more expensive than the PM stepper motor but provides better performance with respect to step resolution, torque and speed. Typical step angles for the HB stepper motor range from 3.6 to 0.9 (100 400 steps per revolution). The hybrid stepper motor combines the best features of both the PM and VR type stepper motors. The rotor is multi-toothed like the VR motor and contains an axially magnetized concentric magnet around its shaft. The teeth on the rotor provide an even better path which helps guide the magnetic flux to preferred locations in the airgap. This further increases the detent, holding and dynamic torque characteristics of the motor when compared with both the VR and PM types.
3.4Lavet type stepping motorThe lavet type stepping motor has widespread use as a drive in electro-mechanical clocks and is a special kind of single-phase stepping motor. Through miniaturization it can be used in wristwatches and requires very little power, making a battery last for many years. The French engineer Marius Lavet is known as the inventor for this kind of drives and described it 1936. Like other single-phase motors the lavet motor is only able to turn in one direction, which depends on the geometry of its stator. The rotor is a permanent magnet. The motor can be built with a strong magnet and large stator to deliver high torque, but it is mostly built small, to drive the load through a low gear ratio.
4.OperationThe principle on which stepping motors are based is very simple. When a bar of iron or steel is suspended so that it is free to rotate in a magnetic field, it will align itself with the field. If the direction of the field is changed, the bar will turn until it is again aligned, by the action of the so-called reluctance torque. DC brush motors rotate continuously when voltage is applied to their terminals. The stepper motor uses the theory of operation for magnets to make the motor shaft turn a precise distance when a pulse of electricity is provided.
The rotor will require 24 pulses of electricity to move the 24 steps to make one complete revolution. Another way to say this is that the rotor will move precisely 15 for each pulse of electricity that the motor receives. The number of degrees the rotor will turn when a pulse of electricity is delivered to the motor can be calculated by dividing the number of degrees in one revolution of the shaft (360) by the number of poles (north and south) in the rotor. In this stepper motor 360 is divided by 24 to get 15.
Figure 4.1 shows a typical cross-sectional view of the rotor and stator of a stepper motor. From this diagram you can see that the stator has eight poles, and the rotor has six poles.
When no power is applied to the motor, the residual magnetism in the rotor magnets will cause the rotor to detent or align one set of its magnetic poles with the magnetic poles of one of the stator magnets. This means that the rotor will have 24 possible detent positions. When the rotor is in a detent position, it will have enough magnetic force to keep the shaft from moving to the next position. This is what makes the rotor feel like it is clicking from one position to the next as you rotate the rotor by hand with no power applied. When power is applied, it is directed to only one of the stator pairs of windings, which will cause that winding pair to become a magnet. One of the coils for the pair will become the North Pole, and the other will become the South Pole.
When this occurs, the stator coil that is the North Pole will attract the closest rotor tooth that has the opposite polarity, and the stator coil that is the South Pole will attract the closest rotor tooth that has the opposite polarity. When current is flowing through these poles, the rotor will now have a much stronger attraction to the stator winding, and the increased torque is called holding torque. By changing the current flow to the next stator winding, the magnetic field will be changed 45. The rotor will only move 15 before its magnetic fields will again align with the change in the stator field. The magnetic field in the stator is continually changed as the rotor moves through the 24 steps to move a total of 360. Figure 4.2 shows the position of the rotor changing as the current supplied to the stator changes.
Winding Status(a) Current is applied to the A and A windings, so the A winding is north, (b) (b) Current is applied to B and B windings, so the B winding is north, (c) (c) Current is applied to the C and C windings, so the C winding is north, (d) (d) Current is applied to the D and D windings so the D winding is north. (e) (e) Current is applied to the A and A windings, so the A winding is north.
In Fig. 4.2 (a) you can see that when current is applied to the A and A stator windings, they will become a magnet with the top part of the winding being the North Pole, and the bottom part of the winding being the South Pole. You should notice that this will cause the rotor to move a small amount so that one of its south poles is aligned with the north stator pole (at A), and the opposite end of the rotor pole, which is the north pole, will align with the south pole of the stator (at A). A line is placed on the south-pole piece so that you can follow its movement as current is moved from one stator winding to the next.
In Fig. 4.2 (b) current has been turned off to the A and A windings, and current is now applied to the stator windings shown at the B and B sides of the motor. When this occurs, the stator winding at the B position will have the polarity for the south pole of the stator magnet, and the winding at the B position will have the north-pole polarity. In this condition, the next rotor pole that will be able to align with the stator magnets is the next pole in the clockwise position to the previous pole. This means that the rotor will only need to rotate 15 in the clockwise position for this set of poles to align itself so that it attracts the stator poles.
In Fig. 4.2 (c) you can see that the C and C stator windings are again energized, but this time the C winding is the north pole of the magnetic field and the C winding is the south pole. This change in magnetic field will cause the rotor to again move 15 in the clockwise position until its poles will align with the C and C stator poles. You should notice that the original rotor pole that was labeled 1 now moved three steps in the clockwise position.
In Fig. 4.2 (d) you can see that the D and D stator windings are energized, the winding at D position is the north pole. This change in polarity will cause the rotor to move another 15 in the clockwise direction. You should notice that the rotor has moved four steps of 15 each, which means the rotor has moved a total of 60 from its original position. This can be verified by the position of the rotor pole that has the line on it, which is now pointing at the stator winding that is located in the 2 o'clock position.
In Fig. 4.2 (e) you can see that the A and A stator windings are energized, the winding at A position is the south pole. This change in polarity will cause the rotor to move another 15 in the clockwise direction. You should notice that the rotor has moved four steps of 15 each, which means the rotor has moved a total of 75 from its original position. Thus the sequence of energizing ABCDA will move the rotor in the clockwise direction. It can be easily verified that for the counter clockwise direction the sequence should be ADCBA.
5.CharacteristicsA torque-speed curve is a graphical chart representing the relation between a motors torque and RPM. This information is very helpful in judging the motors performance. Many sellers will quote a specific torque which is the maximum torque. With a torque curve you can estimate the torque at a given RPM. This is very helpful when designing your own machine as you can choose you gear ratio to get the best performance from your motor.
Stepper motors are constant power devices. As motor speed increases, torque decreases. Most motors exhibit maximum torque when stationary, however the torque of a motor when stationary (holding torque) defines the ability of the motor to maintain a desired position while under external load. The torque curve may be extended by using current limiting drivers and increasing the driving voltage (sometimes referred to as a 'chopper' circuit; there are several off the shelf driver chips capable of doing this in a simple manner). Holding Torque: Amount of torque that the motor produces when it has rated current flowing through the windings but the motor is at rest.
Detent Torque: Amount of torque that the motor produces when it is not energized. No current is flowing through the windings. Pull-in Torque Curve: Shows the maximum value of torque at given speeds that the motor can start, stop or reverse in synchronism with the input pulses. The motor cannot start at a speed that is beyond this curve. It also cannot instantly reverse or stop with any accuracy at a point beyond this curve. Stop / Start Region: area on and underneath the pull-in curve. For any load value in this region, the motor can start, stop, or reverse "instantly" (no ramping required) at the corresponding speed value.
Pull-out Torque Curve: Shows the maximum value of torque at given speeds that the motor can generate while running in synchronism. If the motor is run outside of this curve, it will stall. Slew Range: the area between the pull-in and the pull-out curves, where to maintain synchronism, the motor speed must be ramped (adjusted gradually).
From the Figure 5.1, it is apparent that torque is greatest at zero steps per second and decreases as the number of steps increases.
6. StabilityThe single-step response characteristics of a stepper motor is shown in figure 6.1.
When one step pulse is applied to a stepper motor the rotor behaves in a manner as defined by the above curve. The step time t is the time it takes the motor shaft to rotate one step angle once the first step pulse is applied. This step time is highly dependent on the ratio of torque to inertia (load) as well as the type of driver used. Since the torque is a function of the displacement it follows that the acceleration will also be. Therefore, when moving in large step increments a high torque is developed and consequently a high acceleration. This can cause overshoots and ringing as shown. The settling time T is the time it takes these oscillations or ringing to cease. In certain applications this phenomena can be undesirable. It is possible to reduce or eliminate this behaviour by micro stepping the stepper motor. For more information on micro stepping please consult the micro stepping note.
Stepper motors can often exhibit a phenomena referred to as resonance at certain step rates. This can be seen as a sudden loss or drop in torque at certain speeds which can result in missed steps or loss of synchronism. It occurs when the input step pulse rate coincides with the natural oscillation frequency of the rotor. Often there is a resonance area around the 100 200 pps region and also one in the high step pulse rate region. The resonance phenomena of a stepper motor comes from its basic construction and therefore it is not possible to eliminate it completely. It is also dependent upon the load conditions. It can be reduced by driving the motor in half or micro stepping modes.
7.Cloded Loop Control Of Stepper Motor DriveStepping motors are normally operated without feedback and may suffer from loss of synchronization. This can be prevented by the use of positional feedback. A simple control algorithm is developed which allows a stepping motor to operate effectively in open-loop mode as long as it remains synchronized, but which recovers from loss of synchronization following a disturbance. The control algorithm can be implemented as four interconnected state machines, an up/down counter and a rate- meter.
Most closed-loop stepping motor controllers have used positional feedback simply to control the time at which the winding excitation changes [1,2,3,4,5,7]. This approach can be used to obtain the maximum torque at any given speed, but it does not allow the motor position or speed to be controlled as is required in many traditional applications of stepping motors. The purpose of this paper is to describe a control algorithm which provides the advantages of closed-loop control while employing standard open-loop command signals and using a standard open-loop sequencer. As a result the closed-loop controller can be incorporated in an existing open-loop stepping motor system with the minimum of disruption.
A typical open-loop multi-axis mechanical positioning system using stepping motors is shown in figure 8.1. The coordinator controls the motion of several stepping motors generating, for each motor or axis, two signals: step and dir. Conventionally the step signal is active-low and a 1-to-0 transition causes the motor to perform a single step in the direction determined by the value of dir. A typical open-loop multi-axis mechanical positioning system using stepping motors is shown in figure 8.1. The coordinator controls the motion of several stepping motors generating, for each motor or axis, two signals: step and dir. Conventionally the step signal is active-low and a 1-to-0 transition causes the motor to perform a single step in the direction determined by the value of dir.
A typical open-loop multi-axis mechanical positioning system using stepping motors is shown in figure 8.1. The coordinator controls the motion of several stepping motors generating, for each motor or axis, two signals: step and dir. Conventionally the step signal is active-low and a 1-to-0 transition causes the motor to perform a single step in the direction determined by the value of dir. If feedback is to be employed then the additional logic, which will be termed the closed-loop controller or CLC, should ideally be situated between the coordinator and the sequencer, as shown in figure 8.1. The CLC accepts the step and dir signals from the coordinator, together with the feedback signals A and B from the encoder, and generates two new step and dir signals to feed to the sequencer.
Electric Machinery 6th Edition by fitzgerald,
kingsley & uman. Internet Electric Motor Drives - Modeling Analysis and Control by
9.ConclusionStepper Motors are good for precision. You will always know how many steps you've taken with a stepper motor. DC motors in comparison are unreliable and difficult to control. One drawback to stepper motors is that they are inefficient. They typically sink more current than geared dc motors do. The problem of loss of synchronization in open-loop stepping motor systems can be overcome by the use of positional feedback. An important application of stepping motors is in mechanical positioning systems where, typically, a single coordinator generates step and dir signals for a number of motors.It is convenient to place the CLC between coordinator and sequencers, with feedback derived from incremental encoders. This simplifies retro-fitting of a CLC to an existing open-loop system.