Stepper Motor

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Analysis of Operating Mechanism and Stepping Sequence of Stepper Motor Drive

For some applications, there is a choice between using servomotors and stepping motors. Both types of motors offer similar opportunities for precise positioning, but they differ in a number of ways. Servomotors require analog feedback control systems of some type. Typically, this involves a potentiometer to provide feedback about the rotor position, and some mix of circuitry to drive a current through the motor inversely proportional to the difference between the desired position and the current position.

In making a choice between steppers and servos, a number of issues must be considered; which of these will matter depends on the application. For example, the repeatability of positioning done with a stepping motor depends on the geometry of the motor rotor, while the repeatability of positioning done with a servomotor generally depends on the stability of the potentiometer and other analog components in the feedback circuit.

Stepping motors can be used in simple open-loop control systems; these are generally adequate for systems that operate at low accelerations with static loads, but closed loop control may be essential for high accelerations, particularly if they involve variable loads. If a stepper in an open-loop control system is overtorqued, all knowledge of rotor position is lost and the system must be reinitialized; servomotors are not subject to this problem.

Introduction

Stepping motors come in two varieties, permanent magnet and variable reluctance (there are also hybrid motors, which are indistinguishable from permanent magnet motors from the controller's point of view). Lacking a label on the motor, you can generally tell the two apart by feel when no power is applied. Permanent magnet motors tend to "cog" as you twist the rotor with your fingers, while variable reluctance motors almost spin freely (although they may cog slightly because of residual magnetization in the rotor). You can also distinguish between the two varieties with an ohmmeter. Variable reluctance motors usually have three (sometimes four) windings, with a common return, while permanent magnet motors usually have two independent windings, with or without center taps. Center-tapped windings are used in unipolar permanent magnet motors.

Stepping motors come in a wide range of angular resolution. The coarsest motors typically turn 90 degrees per step, while high resolution permanent magnet motors are commonly able to handle 1.8 or even 0.72 degrees per step. With an appropriate controller, most permanent magnet and hybrid motors can be run in half-steps, and some controllers can handle smaller fractional steps or microsteps.

For both permanent magnet and variable reluctance stepping motors, if just one winding of the motor is energized, the rotor (under no load) will snap to a fixed angle and then hold that angle until the torque exceeds the holding torque of the motor, at which point, the rotor will turn, trying to hold at each successive equilibrium point.

Variable Reluctance Motors

Figure 1.1

If your motor has three windings, typically connected as shown in the schematic diagram in Figure 1.1, with one terminal common to all windings, it is most likely a variable reluctance stepping motor. In use, the common wire typically goes to the positive supply and the windings are energized in sequence.

The cross section shown in Figure 1.1 is of 30 degree per step variable reluctance motor. The rotor in this motor has 4 teeth and the stator has 6 poles, with each winding wrapped around two opposite poles. With winding number 1 energized, the rotor teeth marked X are attracted to this winding's poles. If the current through winding 1 is turned off and winding 2 is turned on, the rotor will rotate 30 degrees clockwise so that the poles marked Y line up with the poles marked 2. An animated GIF of figure 1.1 is available.

To rotate this motor continuously, we just apply power to the 3 windings in sequence. Assuming positive logic, where a 1 means turning on the current through a motor winding, the following control sequence will spin the motor illustrated in Figure 1.1 clockwise 24 steps or 2 revolutions:

Winding 1 1001001001001001001001001

Winding 2 0100100100100100100100100

Winding 3 0010010010010010010010010

time --->

The section of this tutorial on Mid-Level Control provides details on methods for generating such sequences of control signals, while the section on Control Circuits discusses the power switching circuitry needed to drive the motor windings from such control sequences.

There are also variable reluctance stepping motors with 4 and 5 windings, requiring 5 or 6 wires. The principle for driving these motors is the same as that for the three winding variety, but it becomes important to work out the correct order to energise the windings to make the motor step nicely.

The motor geometry illustrated in Figure 1.1, giving 30 degrees per step, uses the fewest number of rotor teeth and stator poles that performs satisfactorily. Using more motor poles and more rotor teeth allows construction of motors with smaller step angle. Toothed faces on each pole and a correspondingly finely toothed rotor allows for step angles as small as a few degrees.

Unipolar Motors

Figure 1.2

Unipolar stepping motors, both Permanent magnet and hybrid stepping motors with 5 or 6 wires are usually wired as shown in the schematic in Figure 1.2, with a center tap on each of two windings. In use, the center taps of the windings are typically wired to the positive supply, and the two ends of each winding are alternately grounded to reverse the direction of the field provided by that winding. An animated GIF of figure 1.2 is available.

The motor cross section shown in Figure 1.2 is of a 30 degree per step permanent magnet or hybrid motor -- the difference between these two motor types is not relevant at this level of abstraction. Motor winding number 1 is distributed between the top and bottom stator pole, while motor winding number 2 is distributed between the left and right motor poles. The rotor is a permanent magnet with 6 poles, 3 south and 3 north, arranged around its circumference.

For higher angular resolutions, the rotor must have proportionally more poles. The 30 degree per step motor in the figure is one of the most common permanent magnet motor designs, although 15 and 7.5 degree per step motors are widely available. Permanent magnet motors with resolutions as good as 1.8 degrees per step are made, and hybrid motors are routinely built with 3.6 and 1.8 degrees per step, with resolutions as fine as 0.72 degrees per step available.

As shown in the figure, the current flowing from the center tap of winding 1 to terminal a causes the top stator pole to be a north pole while the bottom stator pole is a south pole. This attracts the rotor into the position shown. If the power to winding 1 is removed and winding 2 is energised, the rotor will turn 30 degrees, or one step.

To rotate the motor continuously, we just apply power to the two windings in sequence. Assuming positive logic, where a 1 means turning on the current through a motor winding, the following two control sequences will spin the motor illustrated in Figure 1.2 clockwise 24 steps or 2 revolutions:

Winding 1a 1000100010001000100010001

Winding 1b 0010001000100010001000100

Winding 2a 0100010001000100010001000

Winding 2b 0001000100010001000100010

time --->

Winding 1a 1100110011001100110011001

Winding 1b 0011001100110011001100110

Winding 2a 0110011001100110011001100

Winding 2b 1001100110011001100110011

time --->

Note that the two halves of each winding are never energized at the same time. Both sequences shown above will rotate a permanent magnet one step at a time. The top sequence only powers one winding at a time, as illustrated in the figure above; thus, it uses less power. The bottom sequence involves powering two windings at a time and generally produces a torque about 1.4 times greater than the top sequence while using twice as much power.

The section of this tutorial on Mid-Level Control provides details on methods for generating such sequences of control signals, while the section on Control Circuits discusses the power switching circuitry needed to drive the motor windings from such control sequences.

The step positions produced by the two sequences above are not the same; as a result, combining the two sequences allows half stepping, with the motor stopping alternately at the positions indicated by one or the other sequence. The combined sequence is as follows:

Winding 1a 11000001110000011100000111

Winding 1b 00011100000111000001110000

Winding 2a 01110000011100000111000001

Winding 2b 00000111000001110000011100

time --->

Bipolar Motors

Figure 1.3

Bipolar permanent magnet and hybrid motors are constructed with exactly the same mechanism as is used on unipolar motors, but the two windings are wired more simply, with no center taps. Thus, the motor itself is simpler but the drive circuitry needed to reverse the polarity of each pair of motor poles is more complex. The schematic in Figure 1.3 shows how such a motor is wired, while the motor cross section shown here is exactly the same as the cross section shown in Figure 1.2.

The drive circuitry for such a motor requires an H-bridge control circuit for each winding; these are discussed in more detail in the section on Control Circuits. Briefly, an H-bridge allows the polarity of the power applied to each end of each winding to be controlled independently. The control sequences for single stepping such a motor are shown below, using + and - symbols to indicate the polarity of the power applied to each motor terminal:

Terminal 1a +---+---+---+--- ++--++--++--++--

Terminal 1b --+---+---+---+- --++--++--++--++

Terminal 2a -+---+---+---+-- -++--++--++--++-

Terminal 2b ---+---+---+---+ +--++--++--++--+

time --->

Note that these sequences are identical to those for a unipolar permanent magnet motor, at an abstract level, and that above the level of the H-bridge power switching electronics, the control systems for the two types of motor can be identical.

Note that many full H-bridge driver chips have one control input to enable the output and another to control the direction. Given two such bridge chips, one per winding, the following control sequences will spin the motor identically to the control sequences given above:

Enable 1 1010101010101010 1111111111111111

Direction 1 1x0x1x0x1x0x1x0x 1100110011001100

Enable 2 0101010101010101 1111111111111111

Direction 2 x1x0x1x0x1x0x1x0 0110011001100110

time --->

To distinguish a bipolar permanent magnet motor from other 4 wire motors, measure the resistances between the different terminals. It is worth noting that some permanent magnet stepping motors have 4 independent windings, organized as two sets of two. Within each set, if the two windings are wired in series, the result can be used as a high voltage bipolar motor. If they are wired in parallel, the result can be used as a low voltage bipolar motor. If they are wired in series with a center tap, the result can be used as a low voltage unipolar motor.

Bifilar Motors

Bifilar windings on a stepping motor are applied to the same rotor and stator geometry as a bipolar motor, but instead of winding each coil in the stator with a single wire, two wires are wound in parallel with each other. As a result, the motor has 8 wires, not four.

In practice, motors with bifilar windings are always powered as either unipolar or bipolar motors. Figure 1.4 shows the alternative connections to the windings of such a motor.

Figure 1.4

To use a bifilar motor as a unipolar motor, the two wires of each winding are connected in series and the point of connection is used as a center-tap. Winding 1 in Figure 1.4 is shown connected this way.

To use a bifilar motor as a bipolar motor, the two wires of each winding are connected either in parallel or in series. Winding 2 in Figure 1.4 is shown with a parallel connection; this allows low voltage high-current operation. Winding 1 in Figure 1.4 is shown with a series connection; if the center tap is ignored, this allows operation at a higher voltage and lower current than would be used with the windings in parallel.

It should be noted that essentially all 6-wire motors sold for bipolar use are actually wound using bifilar windings, so that the external connection that serves as a center tap is actually connected as shown for winding 1 in Figure 1.4. Naturally, therefore, any unipolar motor may be used as a bipolar motor at twice the rated voltage and half the rated current as is given on the nameplate.

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

Source: http://www.articlesbase.com/electronics-articles/analysis-of-operating-mechanism-and-stepping-sequence-of-stepper-motor-drive-590067.html

Frequently Asked Questions

  1. QUESTION:
    Will the cost of stepper motor reduce if the pull out torque is decreased?
    In an existing stepper motor with step angle 7.5deg, the pull-out torque based on load calculations is more than 120gfcm. I have recalculated and found that it is enough if the pull-out torque is more than 110gfcm. Can this lower pull-out torque specification reduce the cost of the stepper motor? If so, what would be the percentage reduction in cost?

    • ANSWER:
      In general yes, motors with less torque will cost less. However, there are exceptions to that rule:

      1. If the motor is mass produced in large quantities and commonly available, very large, very high torque motors can be quite low cost. For example, there are some motors commonly found in printers and photo copiers which can be had for the labor of removing them from old equipment. Check office equipment repair businesses and offer to remove old, dead, units if they have steppers inside. For a list including some models of printers or copiers known to have good stepper motors, see:
      http://techref.massmind.org/techref/io/stepper/linistep/motors.htm

      2. If the motor coils are wound with high impedance (more turns, thinner wire), the torque can be quite high even though the motor is smaller, uses less material, and is therefor less expensive. The catch? They are slower, because the higher impedance means it takes longer to build up the magnetic field between steps.

      Keep in mind, motor specifications are really best case: Be sure to give yourself some "wiggle room" at get a motor that has a bit more torque than you think you need.

  2. QUESTION:
    What is the difference betwwen a 3-phase stepper motor with encoder and a dc-brushless motor?
    What is the difference between a 3 phase 16step/rev stepper motor in comparision to a DC-brushless motor with encoder but without hall effect sensors? Please help?

    • ANSWER:
      In some configurations there is very little difference. Reasons follow:

      Stepper motors by continuous encodings or a stopped (addressing) code provide a rotating field or even a stopped field by converting digital BCD counts into a code that by a planned coincidence causes the fields' electromagnetic poles to N and S positions to attract the S and N armature poles to a desired orientation, whether still or rotating in real time. When the computer causes the armature to rotate, the field normally does not slip, because stepmotors are chiefly used for precise positioning, in which the number of revolutions will determine how far a mail or female thread will travel.

      Stepper motors can be four-pole, 8-pole ... even 2 exp x pole. Stepper motors can be 6 pole, 12 pole, ... even 3 exp x pole. More poles make the positioning more focused in final addressed position. The fact that 6-pole 3-pole motors can run as a 3-phase motor, in, like Y connection to 3 phase and neutral AC power may have an interesting crossover as an application of stepping motor engineered system. Most 3-phase motors are used for grunt labor applications, e.g., assembly line belt drives. If such a three-phase motor were driving an elevator by a cogs and chain, it would be possible to drive the elevator and stop and start the elevator at exact positions with no brakes. (But there would be no precision whatever if the armature were a typical squirrel cage armature. Precision motoring requires an armature that has steady-state N & S poles--so that it would remain synchronous with each movement of each change in field. (Squirrel cage motors armatures receive their field strength by magnetic induction from the rotating control field (a rotating magnetic field caused by the peripheral electromagnets as the magnetic fields of each electromagnet vector-sum in the empty hollow cylindrical space between the armatur and the peripheral electromagnet poles, and armatures like that inherently slip a little bit all the time, which doesn't hurt much in a refrigerator motor of a washer motor or in a 3-phase coal conveyor belt drive. Who cares if the coal is delayed 3% if the motor doesn't have to have a DC field supply and sliprings to make the armature synchronous? Hence the 3-phase elevator motor running from a computer-controlled stepmotor power supply would also need a synchronous 3-phase motor, which would either have permanent magnets to energize the armature field in steady state; or it would have to have 3 pairs of 2 electromagnets forming a six points of an asterisk crosssection of the armature. The steady state armature field also would help a stopped motor field grab the armature while the elevator is at a station. Typical 3-phase synchronous motors have slip rings to bridge DC current to the rotating armature. Stepmotors for many precision robot jobs need dozens of poles to obtain more shaft angle addressing precision. However, stepmotors can also have more addressing precision than a simple count of poles can provide, because there is a way to have some of the extra poles reversed, so that the motor can stop with the armature having some of its fields distorted as the armature. This compromises the resulting field. This arrangement can be happening in real time travel, which reduces the steps that a milling machine might leave on a finished product.

      A DC brushless motor operates like a DC motor with brushes, except that the DC power routes current to the armature poles according to semiconductors that receive their steering signals from photocells that tell the semiconductors in series with each opposite electromagnet when to turn on and when to turn off. Nearly all garden-variety dc motors have a 2-pole distributed d-c energized N-S field on the outside, the stator windings.

      (irrelevant but interesting, these motors need to have a way to deal with back emf voltage produces as any given field collapses. A varactor is one way to protect the switching transistors from these high voltage. The varactor's resistance goes up sharply with voltage that is above the normal operating voltage that the armature receives.)

      It is also possible to put all the stuff that is in the outside of the dc motor on the inside, and put all the stuff that is on the inside onto the outside. Because after all, the business end of the poles positions are all relative, right?

      Dc motors change their speed when under load. They normally draw more current when they slow down. For some applications, you could just energize the DC motor which has a permanent-magnet armature by means of encoded stepped fields on the stator winding area providing on the outside tha same number of N-S field pairs that would have been on the inside, the rotor.

      If the load increased beyond the torque that would hold the armature, the windings would draw a lot of extra power and the armature would probably almost stall. To mitagate against stall, then all currents need to run at or near peak required currents yet no winding should be allowed to burn out. This doesn't sound efficient.

      So if you install a digital position code reader on on the outside and install digital position code positions on the armature, any time that the armature position slips backwards beyond tolerable specifications, the motor can receive more current in all outside rotating field coils. If the field slips regardless, the motor can be put in reset by computer control to protect the power supply and the field coils.

      When a traditional dc motor has been engineered with "reverse inside-out configuraton," digital encoding, and these measures of field control, there is very little difference between a stepmotor and a brushless dc motor.

      Only old timers and engineering geeks care about things like this. Such old timers often have arthritis, and probably the most effective and most natural formula to treat that is Flex Protex, available at www.itvventures.com/104134. Such old timers deserve the best. Invite your fingers and joints to this site and see the 30-minute video to substantiate my assertion.

  3. QUESTION:
    Can an unipolar stepper motor common wire be connected to ground instead of V+?
    I want to control a unipolar stepper, but my controller cannot pull the outputs to ground, I can only supply V+. Can I connect the common lead on the motor to GND and then power each coil through the controller?

    • ANSWER:
      Not sure what type of controller you have as its always easy to pull the output to ground by using a simple transistor or MOSFET !!
      It make no difference even if you connect the common to Ground and drive other end to V+.

  4. QUESTION:
    What's the difference in a stepper motor and a regular DC motor?
    I am an electrical engineering student so feel free to get a little technical on me... but try to keep it simple.

    By regular DC motor I mean a plain jane commutated DC motor - you hook up a DC source and it turns. Is the only difference that a stepper motor is not commutated... and you turn it by changing the input waveform?

    • ANSWER:
      A stepper motor has multiple pole pairs, with the rotor locked to an energize pair. Stepper motors don't slip regardless of applied load for any load within the motor's torque capability. The shaft position is precisely known by counting the number of pulses applied. The "pulses" are decoded by a stepper drive circuit that sequentially energizes and deenergizes pole pairs, either in a forward or reverse direction as directed. In some applications, the motor is advanced only one or a few steps at a time. In others, the motor runs continuously with its speed proportional to the applied frequency of pulses. A pulse interface is easy for a microcontroller to utilize. Or you can roll your own stepper driver, which might include a microcontroller.

      A plane-jane DC motor slips with increasing load. It does not have the ability to go to a precise angular rotation without the addition of feedback (such as a rotary encoder with feedback controller). But they are easy to use; just apply power.

      A variable-frequency drive is usually used with an AC 3-phase motor, not a DC motor. It adjusts the voltage and frequency of the power waveform applied to the motor. These drives normally aren't used at really-low speeds or for step control because the AC motor will overheat. The normal minimum speeds are typically 10% of base speed if external forced-air cooling is provided and about 30% if external cooling is not provided.


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