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STEPPER MOTORS
A SURVEY OF PHILIPS/AIRPAX STEPPER MOTORS
A SURVEY OF STEPPER-MOTOR DRIVER CHIPS
Stepper motors are brushless devices whose shafts can be rotated in discrete "steps." Common applications for these motors are: driving the head carriage in many disk drives, as well as positioners in printers and numerically controlled milling machines. "Variable-reluctance" and "hybrid" types are discussed here.
Surplus stores have bins full of stepper motors, some affixed with gears and others with lead screws. You'll know a permanent-magnet stepper motor by the way the shaft feels as you turn it; the shaft wants to move in jumps as the permanent magnets seek pole pieces of the stators. (There is room for some confusion here, since a simple DC motor with a permanent-magnet field can also feel like this, but steppers will usually have more leads than a simple DC motor.)
The main design parameters worth considering are: the "resolution" (the number of degrees per step) and the static torque (will it be powerful enough to stay where you put it?). Many steppers come with gear boxes, making the attainable resolution quite fine. When toying with surplus ones, you can guess the resolution by noting how many times the rotor magnets "home in" on stator poles.
In general, stepper motors have more than one stator. (There is no electrical connection to the rotor.) Each stator is a ring of iron surrounding a coil of wire--the "stator winding." "Pole pairs" are created by attaching L-shaped pieces either side of the ring. This creates interleaving fingers on the inside of the circle; half of these fingers (of one magnetic polarity) point in one direction, and the other set, positioned in between the first set of fingers, points the other way (these being of the opposite magnetic polarity). Thus, when energized, the inner circumference of each stator has alternating magnetic poles--a north followed by a south, then north, and so on. Having multiple stators, we will designate them by letters A, B, C, and so on.
Unlike motors with "brushes," it is the business of driver electronics to be the "commutator." The simplest driving system is to energize one stator (stator A), then energize stator B, then C (if there is a C), etc. Depending on the type, this is not always done, because energizing two stators simultaneously can increase the available torque.
Typically, these motors have three stators. The rotor has "teeth" of soft iron with the same "pitch" (called the "pole pitch") as pole pairs of the stator. When a stator is energized, the rotor moves to the nearest position that minimizes the air gap--the position that establishes the best magnetic circuit.
Possibly to isolate the magnetic circuits, the rotors may have multiple stacks of rotor teeth, one for each stator. Such a motor was described in a textbook as a 3-phase variable-reluctance stepper made by Warner Electric. (Published by the Institute of Electrical Engineering, London and New York, the book is "Stepping Motors" by P.P. Acarnley, c1982.)
We dismantled an old 8-inch disk drive and retrieved the motor used to drive the head; this looks identical to the one in the book by Acarnley. With the brand name being Applied Motion Products, this is a 3-stator 24-volt device. One lead of all three stators goes to a common (black) lead; the free ends of the stators come out as brown, red and orange leads (colors representing 1, 2, and 3, respectively). The winding resistance is listed on the motor as being 19 ohms (per winding).
Two driving schemes are discussed in Acarnley: The simplest is the "full-step" system; first A is energized, then A is turned off and B is energized, then C (by itself) is energized, then A again. With our cannibalized motor, this worked very well, and it was observed that 24 steps complete a revolution. However, as described in Acarnley's book, there is a "half-step" driving scheme which goes as follows:
First A is energized by itself; this causes poles of the rotor to "home in" on poles of A. Then B is added--causing the rotor to make a "half step" forward (in the direction of B). Then A is turned off; this causes the rotor to home in on B. Then C is added, causing the rotor to advance a half step toward C. When B is turned off, the rotor advances a half step to home in on C. When A is next energized, the rotor advances (in the direction it has been going) a half step away from C--teeth in the rotor are about to align with poles of A, but shifted by one period of the pole pitch. When C is turned off, poles in the rotor home in on A, except that the shaft has made one-eighth of a revolution; 48 steps will be required for each complete revolution. Of course, any such sequence can be run in reverse to change the direction of rotation.
For some reason, our liberated disk-head motor does not make the half steps properly. This may be due to alignment of stacks in the rotor, or this motor may just not be made for it.
The driving circuit for a "whole-step" process can be very simple: A "one-of-three" up-down counter could drive power transistors in sequence--the counter advancing in accordance with a clock signal.
So-called "hybrid stepper motors" have a "single stack" of permanent magnets on their rotors. These often have two sets of stators with "N" number of pole pairs. Their rotors have a corresponding number of radially mounted permanent magnets with alternating polarity--a south pole followed by a space, then a north pole followed by a space, then another south pole, and so on. The amount of rotation for each step is one-fourth the "pole pitch"; the rotor can be moved to any of four positions for any stator pole pair.
The two sets of stators are displaced from one another; the poles of one set are aligned with spaces between poles of the other. For analysis, let us call these stators "A" and "B."
The traditional driving scheme is a four-step switching process. A graph of the drive signals indicates that two flip-flops generate square waves which are out of phase by 90 degrees; these flip-flops are driving the two stators. We can go through the process "step by step" as follows:
When a stator is energized, its pole pieces take on alternating magnetic polarity--north followed by a space, south followed by a space, another north pole, etc. Suppose we energize both stators with the same polarity; there will be two north poles adjacent to one another, followed by two adjacent south poles, and so on. (By adjacent, I mean "across the aisle," from one stator to the other.) Let us consider the current north pole of stator A, the starting place of our analysis.
Magnets of the rotor will align so that each permanent-magnet south pole is attracted to adjacent stator north poles. Thus, the rotor "starts" by being displaced from the pole pieces of either stator.
If we change the polarity of the drive to stator A (and proceed around the stator from where we started), we have a south pole, followed by two adjacent north poles, followed by adjacent south poles; i.e., coincident poles of the same polarity have been shifted in the direction of stator B. Now, by changing the polarity of stator B and noting polarity from our starting position we have: two adjacent south poles, followed by adjacent north poles, followed by adjacent south poles, etc.
Changing A again, we polarize stator A as we started with. We now have north, followed by adjacent south poles, then two norths, etc. Finally, we change B back to the polarity with which we began; establishing two norths, two souths, which is the order we started with.
Each time we change one polarity, we shift the location of "like poles" of the two stators, and magnets of the rotor are attracted to these. Of course, this sequence can be reversed, giving us the ability to run the motor in either direction. Note that skipping steps can get you into trouble; for example, if you change polarity of both stators, the rotor will be in a quandary as to which way to go and the torque is theoretically zero.
If each stator has twelve pole pairs, the rotor will move four steps per pole pair, making 48 steps per revolution (rotating 7.5 degrees per step).
Two winding configurations are available: The stator windings in "bipolar" stepper motors have two leads per stator. Bipolar stepper motors with two stators have four leads; the winding of one stator is fed by one pair of leads, while the winding of the other is fed by the other pair. The "unipolar" version has center-tapped windings; the connections come out as three leads per stator--a motor with two stators has six leads in all.
Bipolar motors require more complicated switching than unipolar ones, since polarity of the voltages applied to the windings must be reversed; this requires eight driver transistors. In driving unipolar motors, the polarity of the electromagnets is changed by using one-half of the winding or the other; twice as much magnet wire is required to get the same number of ampere-turns in the motor, but only four driver transistors are needed.
The literature discusses timing issues of driver circuits; these discussions have been put to rest by the emergence of driver chips which do the work for us. The simplest driver circuit, used for the unipolar style, is as follows: The center taps of both stator windings go to the positive supply; the ends of the windings go to open-collector outputs of flip-flops, a flip-flop for each stator. The bipolar circuit requires polarity reversals: Two flip-flops, one for each stator, drive complementary transistor pairs whose outputs can either pull to the positive supply or to ground. The stators, which only have two leads each, can thus be powered in either direction. (One of the timing issues is that without safeguards, both transistors in a complementary pair might turn on at the same time and burn themselves out.)
Comparing performance of the two winding styles, choosing motors of identical physical size, unipolar motors have about 30% less torque at low-to-moderate speeds. This is due to the unavoidable requirement of reducing wire size in order to fit as many turns each side of the center tap as can fit on the whole winding of a bipolar unit. Torque of the two types is equivalent at high speeds.
Inductance of the windings causes a decrease in torque as speed increases. You can compensate for this by increasing the power supply voltage by a factor of four and placing a resistor in series with each stator winding. They recommend a resistor value of three times the DC resistance of the winding. (This is called the "L/4R circuit," counting the winding resistance.) Naturally, this will increase power drain, and the power rating of the resistors will have to be 2 times I squared times R, using carbon power resistors.
In the unipolar circuit, a resistor is placed in series with the center tap; since only half the winding is powered at any given time, resistors on each end are not required. The winding resistances listed in specifications for unipolar motors are for half the winding--measured either side of center tap.
We cannibalized a Seagate hard drive and found two stepper motors. Oddly enough, the rotor that spins the two hard disks is a very crude stepper capable of four steps per turn. It has two 3-ohm stators; three leads connect to these, making them look like a center-tapped winding.
Of much more interest is the stepper that drives the head carriage; this is capable of a resolution too high to count by hand. It has two bipolar stators of 27.5 ohms each.
Lying about the place, we found a head motor of exactly the same physical dimensions--also a Seagate--which has five stators. This appears to have a similar resolution, although I don't relish the task of driving it. The winding resistance, all identical, is 19 ohms for each stator.
Now that IC's to drive stepper motors can be gotten, they become an attractive electromagnetic device. The signals required for the driver are pulses, which can come from a variable-frequency clock, and a direction-determining signal. If you couple this with a counter, you can tell the system how far to go and when to stop the motor.
I just came back from our major electronics show of the West Coast, WESCON. I saw some stepper motors of the types described here, all right, but there are new kinds.
It seems that they are able to get about 20 times the position resolution using lasers to cut pole pieces of the stators. A company called Semix had a unit capable of 800 steps per turn in the whole-step mode--1600 steps per revolution in the half-step mode. Another exhibitor, Motion Science, Inc., has an eighteen-inch long table full of permanent-magnet poles, with a shuttle containing stators riding atop an air bearing (air escaping from holes underneath the shuttle to make it nearly frictionless). He was claiming a movement of only 2 microns per step.
Well, it looks like we can look forward to some very fine control with these new kinds. They were all bipolar types using permanent magnets.
Once known as Airpax and now a division of Philips, the products listed here are but a few representative samples--all of the "hybrid" type (using permanent-magnet rotors). The Philips motors all come in 5-volt and 12-volt models, as well as bipolar and unipolar. Often, there are related models with different resolutions, 24 versus 48 steps per turn.
These models have gear boxes with 1/4th-inch output shafts. Their motor bodies are 2.187 inches in diameter. The over-all length--up to the mounting surface from which the shaft emerges--is 2.48 inches. The shaft protrudes one inch, but for some of this distance (a bit less than half an inch), there is a bushing around the shaft that serves as a bearing. (There is a "flat" on the shaft for a setscrew to rest against.) On opposite corners of the gear boxes are 8-32 mounting studs; mounting holes on the remaining corners presumably accept 8-32 bolts.
All in this series are unipolar; they have six leads and the stator windings are center-tapped. The K82600 units have 24 steps per revolution (one step moves the rotor 15 degrees); the K82800 ones have 48 (one step moves the rotor 7.5 degrees). Speaking for the basic motors (before their gear boxes), the holding torque of the K82800 units is 13 inch-ounces, while the K82600 ones have a holding torque of 10 inch-ounces.
They carry a hyphenated suffix; -P1 means 5 volts, while -P2 means 12 volts. The winding resistance of 5V units is approximately 7.5 ohms, and that of 12V units is about 45 ohms. The inductances (for 5V and 12V units, respectively) are 7mH and 32mH for 82600 ones; for the 82800 series, inductances are 8.5mH and 44mH, respectively.
Gear boxes are specified by the last two digits; for example, K82616 and K82816 each have gear reductions of 5 to 1. Available gear ratios are as follows:
The K82601 and K82801 units have no gear boxes; the ratio is listed as 1 to 1. Remembering that a 6 or 8 specifies 24 or 48 steps: 11 means 2 to 1, 16 is 5 to 1, 21 is 10 to 1, 24 is 15 to 1, 27 is 20 to 1, 30 is 25 to 1, 31 is 30 to 1, 36 is 50 to 1, 37 is 60 to 1, 39 is 75 to 1, and 45 means 150 to 1.
Now, remember that the gear reduction affects both the torque and the angle of rotation per step. For example, a 10 to 1 gear box--such as that of the K82821--creates a holding torque of 130 inch-ounces, and the rotation per step is 0.75 degrees. The K82845-P2 is a 12V unit whose steps rotate the shaft 0.05dg.
B Series--These also have gear boxes. They are 1.86 inches deep--including the gear box. The gear box is egg shaped (2.44 inches in its longest direction). A 1/8-inch diameter shaft emerges nearest the narrow end. Ears either side of the gear box are provided for mounting.
They are available in unipolar and bipolar configurations--either available as 5- or 12-volt units. Three step angles are available--18dg (20 steps per revolution), 15dg (24 steps), and 7.5dg (48 steps). (Remember, torque and resolution specs are for the motor before the gear box.)
For the unipolar ones, the 18dg, 15dg, and 7.5dg units have holding torques of 1.9, 2.4, and 2.6 ounce-inches, respectively. As you would expect, the holding torques for bipolar ones are higher--2.3, 2.8, and 3.5 ounce-inches, respectively.
The 35M020B has 20 steps per revolution of the rotor, the 35M024B has 24 steps per revolution, and the 35M048B has 48 steps. A 1 or a 2 following the B means 5V or 12V, respectively. Following that designation, a U or B specifies unipolar or bipolar. Thus, a 35020B1B unit is a 5-volt bipolar 20-step motor.
Gear ratios are another suffix--a hyphen X, followed by a number: 24 is 15 to 1, 27 is 20 to 1, 31 is 30 to 1, 37 is 60 to 1, 39 is 75 to 1, 45 is 150 to 1, 52 is 300 to 1, and 64 is 1350 to 1. Thus, a 35M020B1B-x31 is a 5V bipolar 20-step motor carrying a 30 to 1 gear box. (Note that this would give you 600 steps per revolution. Do you sense a clock in the works?)
For both winding styles, 5V units have 12.5-ohm resistances, while the 12V units are 72 ohms. Inductances vary through the list of combinations. Rounding off, unipolar 5V motors in this series present 6 or 7 millihenries, while 12V unipolar ones range from 30 to 36 millihenries. Bipolar ones have about twice the inductance--14mH for 5V motors and about 75mH for 12V ones.
This has a 1.6-inch diameter mounting flange at the output end. It is about 0.92 inches deep and the motor body is 1.654 inches in diameter. The shaft diameter is 1/8 inch. The holding torque of the unipolar version is 10.4 ounce-inches; for the bipolar version, this torque is 12.4. It has 48 steps per revolution.
The L82401 is unipolar; the L82402 is bipolar. The suffix -P1 means 5 volts; -P2 means 12 volts.
The winding resistances for both bipolar and unipolar ones are 9.1 ohms for the 5V one and 52.4 ohms for the 12V one. Inductances for unipolar ones are 7.5mH and 46.6mH for 5- and 12-volt units, respectively. For bipolar, inductances are 14.3mH and 77.9mH.
Slightly more than an inch deep, this model has two mounting holes on a roughly diamond-shaped flange. The diameter of the body is 2.28 inches. Its shaft diameter is 1/4 inch.
This motor has 48 steps per revolution. It only comes in a uni-polar version. The holding torque is 15 inch-ounces.
For 5V and 12V versions (specified by -P1 and -P2 suffixes), the winding resistances are 6.3 ohms and 36 ohms, respectively. The corresponding inductances are 7mH and 38mH.
These are the cutest little darlings! They look like a small can with mounting ears at one end. Running through the center is a leadscrew--a threaded shaft with a rather coarse pitch to the thread. The rotor has no shaft; it is tapped to match the thread of the leadscrew.
The leadscrew is secured to something which is to be moved to and fro; i.e., the idea is to keep it from turning. When the rotor inside the device turns, it operates the leadscrew so as to make it move in or out.
The diameter of these devices is 1.06 inches, and the body is 1.05 inches deep. The body has a bushing which protrudes beyond the mounting face by about 0.6 inches. The output end of the leadscrew has a 4-40 thread--terminated by a shoulder--so that it can be screwed to the piece being moved.
The part number is preceded either by K or L; the K has a maximum travel of 1/2 inch, while the L model (provided with a longer leadscrew) has a maximum travel of 1-7/8 inch.
Available units are unipolar; this is shown by the last digit in the number, which is always 1. Then, the third digit in the number determines the distance traveled per step: 4 means 0.004 inches per step, 2 means 0.002 inches, and 1 means 0.001 inches per step. Thus, an L9211 is a unipolar linear actuator whose range is 1-7/8 inches (specified by the L), and whose resolution is one-thousandth of an inch per step. A K9241 is a short-range (1/2 inch) unit whose resolution is four-thousandths per step.
Finally, 5V and 12V actuators are available, specified by the suffix -P1 and -P2, respectively.
The coil resistances are 15 ohms for the 5V version and 84 ohms for the 12V one. Winding inductances are 5mH and 29mH, respectively.
The catalogue lists two addresses as follows:
Three Motorola chips, as well as a couple of experimenters' kits, are presented here. It seems that currently available devices are designed to drive 2-phase bipolar hybrid motors, examples of which are listed in the previous article.
For about $4, this is available from at least two sources. The Jameco Cat. No. is 25216. It can also be gotten from JDR Micro Devices under the catalog number of its own part designation--MC3479P.
This is said to be appropriate for driving 6- or 12-volt motors. Actually, by associating a resistor with a bias pin, the outputs act like current sources, so they can drive 5-volt stators of units listed in the previous article. With this particular chip, the maximum drive current available is 350mA.
This is a 16-pin chip. Pins 4, 5, 12 and 13 are connected to the substrate; a heat sink can be soldered to these, or soldering these pins to a ground plane on the board can aid in dissipating the heat. These pins are the negative supply terminal on the chip and can be considered as ground.
Provision is made so that the outputs can be diode protected; the four outputs go to diodes internal to the chip (the anodes of these diodes being at the output pins). The cathodes of these four clamping diodes go to a common pin (pin 1), and pin 1 is connected to the VCC (pin 16) through a zener diode, a regular diode, or even a jumper. If a 3-volt zener is used (such as a 1N5221), its cathode goes to pin 1 and its anode goes to pin 16.
Outputs 2 and 3 go to one stator--the leads called L2 and L1, respectively. Outputs 14 and 15 go to the other stator--these leads are called L4 and L3, respectively.
This chip is capable of supplying two kinds of drive signals. The conventional scheme is to always supply both stators simultaneously. The Motorola literature calls this the "whole-step" drive. The advantage is that the "holding torque" is consistent for each step. This is true because magnets of the rotor are always attracted to like poles of both stators. Four steps per pole pair are made (see the previous article).
The so-called "half-step" drive alternates energizing both stators with powering one or the other singly. This causes the rotor to move eight steps per pole pair of a stator. The disadvantage is that, when only one stator is energized, the holding torque is not as high as when both are powered.
There is an open-collector output, pin 11, that when the output drivers are in the "phase A" state, the collector brings this pin low. Phase A is defined as follows:
Connections L1 and L3, pins 3 and 15, are high. L2 and L4, pins 2 and 14, are low. In other words, the coil between pins 2 and 3 will see negative on pin 2 and positive on pin 3. The coil between pins 14 and 15 will see negative on pin 14 and positive on pin 15.
Motorola defines "clockwise" as first changing the polarity of the L1-L2 stator, then reversing the current through the L3-L4 stator. Thus, in the whole-step mode, the first clock pulse causes pin 2 (L2) to go high and pin 3 to go low. In the half-step mode, this clock pulse would de-energize the L1-L2 stator, and a subsequent pulse would cause its reversal.
There is a pin (pin 6) which has three functions:
First, it is a "bias pin." This pin determines the base currents of the NPN transistors which sink current at the negative ends of the coils; these transistors are current sources. Normally, this pin is taken to ground through a resistor, the value of which determines the drive current.
Pin 6 is also called the "set pin"; if less than 5 microamps is drawn from it--if it is open-circuited--the internal logic presets to the "Phase A" condition. When pin 6 is open, pin 11 (the "phase-A output detection pin) goes low, and all other inputs are ignored until current is drawn from pin 6 again.
Finally, opening pin 6 causes the outputs to assume a high-impedance condition (simultaneously setting the chip for the "phase A" state).
Unlike the other inputs, pin 6 is not to be operated at the 5V logic level. Unless it is permanently connected to ground through a resistor, pin 6 should go through that resistor to an open-collector output. This could be an open-collector gate or just an NPN transistor as follows:
Pin 6 goes through a resistor to the collector of a 2N2222 whose emitter is grounded. The base of the 2222 is then operated through a 10K resistor. (A small capacitor in parallel with this 10K resistor will make the transistor turn off faster.)
The output sink current is determined by: IBS (the current in microamps drawn from pin 6, the "bias/set pin") equals 0.86 times the output current in milliamps. Pin 6 wants to see a current sink of greater than 5uA. In normal operation, the voltage on pin 6 will be VM (the main supply) minus 0.7V. Thus, the appropriate RB (the resistor from pin 6 to ground) will be the quantity VM minus 0.7V, divided by IBS.
Suppose I wish to drive a 2-phase bipolar stepper motor with drive currents of 200mA, and suppose I supply the circuit with a healthy 9V battery. This 200mA times 0.86 equals 172uA. Next, I want the value of a resistor--from pin 6 to ground--which will draw 172 microamps: The quantity 9 minus 0.7 volts, or 8.3 volts, divided by 1.72 times 10 to the minus fourth, equals 43K.
One suggested use of pin 6 is to reduce current drain when the motor is not being operated. If the stepper is positioning something which does not require the full holding torque to keep the mechanical assembly in place, the current drawn from pin 6 can be reduced while the system is dormant.
Besides the "bias/set" pin, this chip has four inputs. They are intended to be operated with 5-volt logic; the maximum allowable voltage is 5.5V. They are CMOS compatible, however, with the maximum hysteresis being 0.4V.
The "clock input" (pin 7) advances the logic that controls the output drivers, and it does so on positive transitions. When the "set pin" is open-circuited, pulses of the clock--as well as all other input signals--have no effect.
The direction of the logic sequence is determined by pin 10. When high, driver outputs operate in a sequence defined as "clockwise"; when pin 10 is low, the sequence is defined as "counterclockwise." (The actual direction of rotation also depends on how the leads to the stators are wired to the chip.)
Pin 9 selects the whole-step vs. half-step mode. When low, the motor will move four steps per pole pair. When pin 9 is high, the motor can make twice as many steps because there are times when only one stator is energized. (In the half-step mode, the holding torque will not be uniform, since the rotor, during these "half-steps," will be attracted to one stator only.)
Pin 8 is called the "output impedance control." Although it always should be committed to 5V or ground, the setting of pin 8 is only relevant when driving motors in the half-step mode. Its operation gives you a choice:
In the half-step mode, one or the other of the stators will not be energized one-fourth of the time. When pin 8 is low, the two outputs of a non-energized coil will be in a high-impedance state. When pin 8 is at logic 1, the PNP transistors will be turned on; the outputs will have a low impedance with respect to VM. (The importance of this selection has eluded your editor thus far.)
These are different from the above 3479 in two respects. First, they have mounting ears by which they can be attached to heat sinks. Second, they are not quite as versatile; they have fewer features. However, since they are capable of supplying 500mA, they are worth listing here.
The SAA1042 is for 6V and 12V motors; the SAA1042A can run 24V motors. Running 5V motors is no problem, since they, too, have current-sourcing outputs. Their pin assignments are the same; the 1042A just has higher ratings.
Their pin arrangements are slightly different from the 3479: they have a separate supply pin for the logic. Coil-output drivers are on pins 1 and 3, 14 and 16. This means that the external jumper or diode for clamping the output voltages goes between pins 2 and 15, a small inconvenience.
Pin 6, the "bias/set" pin is the same, except that the relationship between bias current and drive current is not such a nice linear one as it is with the 3479. Some sample values for RB off pin 6 were gotten from a graph in the literature, and the corresponding drive currents are listed as follows:
For over-voltage clamping of the outputs, they recommend a 3.9V zener between pins 2 and 15 (cathode at pin 2). Appropriate part numbers would be: 1N5520, 1N4622, 1N4730, or 1N748.
As with the MC3479, pin 8 determines half-step versus whole-step mode; bringing it high puts the chip in the "half-step" mode--eight steps per pole pair. Pin 10 is the clockwise versus counterclockwise pin; bringing it high means that the outputs work in the "clockwise" direction in response to the clock. ("Clockwise" means that the L1-L2, pins 1 and 3, change first; they reverse polarity in the "whole-step" mode, or de-energize the coil in the "half-step" mode.)
If pin 6 is open-circuited, the chips (both 1042 and 1042A) are set to "state A"; pin 3 will be high and pin 1 will be low, while pin 16 will be high and pin 14 will be low. (Note that pin 3 is L1, pin 1 is L2; pin 16 is L3 and pin 14 is L4.)
An advantage in these chips is that the logic need not be restricted to 5V. The logic VCC pin, pin 11, can be up to 20 volts. (This restriction of 20V applies to the "logic supply" of both chips.) The SAA1042A allows a "VM" (on pin 15) of 30V, while the SAA1042 allows an absolute maximum VM of 20V.
As with any driver system, power dissipation is an issue. The SAA chips list a maximum power dissipation of 2 watts; it is the editor's impression that this is without a heat sink.
The literature for the MC3479P does not give a maximum package dissipation directly. They say that dissipation should be such as not to exceed junction temperatures of 150 degrees C. They then say that "thermal conductivity," junction to ambient, is 45 degrees per watt without a heat sink (hmmmmmmm).
Calculating the power dissipated by the chip is a bit of a trick, since output transistors needn't be saturated. However, if you know the motor coil resistances and the currents you intend to run them with, you know the voltages across the windings; subtracting a winding's voltage from the full supply and multiplying the difference by the winding current, you will know how much each driver output pair will dissipate while that winding is energized. In the "whole-step" mode, both coil drivers are always on simultaneously, so the power dissipated by the whole package will be twice the winding current times voltage dropped across the driver transistors. We are only partly finished; power dissipation must now be linked with junction temperature.
Suppose our power calculations lead us to a figure of 2 watts of dissipation. Multiplying 2 watts by 45dg per watt, we get 90dg C. They just add an arbitrary 25dg C. to this--an assumed figure for the ambient temperature--thus obtaining a figure of 115dg C. With the 3479 in mind, this is less than the 150dg C. which they allow without a heat sink.
These should get you started. We cannibalized a Seagate hard drive, and we were able to run its head to and fro across the disk just for fun.
The basic driver here makes it just a variable speed motor with selectable direction. An embellishment is then added to make the shaft oscillate (rotate to and fro). This is just the sort of hookup you could use to make a mechanical monkey beat a drum and tap his foot.
For those who remember the fabulous window displays set up by F.A.O. Schwartz at Christmas time, compare how much easier it is to build this monkey without gears and a reciprocating mechanism. With that in mind, why not design a mobile window display for your business or something.
Just a clock and the driver chip are required to run the motor, as long as the direction terminal, pin 10 of the 3479, is committed to VCC or ground.
The oscillating shaft requires a primitive scheme for starting the mechanism at an initial position. That way, the monkey won't poke himself in the eye with his drumstick or put his foot through your work bench.
A supply of 9V or 12V is used. As you might expect, VCC must be bypassed heavily because of hash from the motor--parallel combinations of 220uF and 0.1uF at either ends of the rails will do (negatives of the electrolytics at ground). This supply does not have to be well regulated, however.
Pin 1 of a 555 is grounded; pins 4 and 8 go to VCC. Located close to the chip is 0.1uF between pins 1 and 8. Also, pin 5, the voltage-control pin, is bypassed to ground by 0.1uF.
Pins 2 and 6 are tied together and go through 0.01uF to ground. Pins 2 and 6 also go through 100K to pin 7. Pin 7 goes through 47K, in series with a 500K rheostat to VCC.
The 555's output, pin 3, goes to the clock input, pin 7, of the MC3479P motor driver chip. This clock input is positive-edge triggered.
On the MC3479P, pin 4 is grounded. Pin 16 goes to VCC--heavily bypassed nearby. Pin 6 goes through 47K to ground. Pin 8 goes to VCC (although in "full-step mode," pin 8 can be committed anywhere). Pin 9 is grounded; this selects the "whole-step mode." Finally, committing pin 10 to ground, or to VCC, selects direction of rotation (which is also affected by hookup of the stator leads).
Pin 1 goes to the anode of a 1N4001 diode; the cathode goes to pin 16. One stator goes between pins 2 and 3, while the other stator goes between pins 14 and 15. Experimentation will determine which way the motor runs when the chip runs "clockwise" (with pin 10 committed to VCC).
The motor is made to run in one direction until a switch is tripped; thereafter, the excursions are determined by selecting outputs from a counter. A flip-flop sets the initial direction by bringing pin 10 of the driver high; closure of the switch when the mechanism arrives changes the direction and enables a 4040 12-bit counter. (The switch can be a simple spring leaf, or an opto-isolator.)
A 4011 quad NAND gate is used to create a NOT-R/NOT-S flip-flop. Pin 7 of the 4011 is grounded, while pin 14 goes to VCC. Pin 3, the output of one gate, goes to pin 5, an inpinput of the second. Pin 4, the output of the second gate, goes to pin 2, an input of the first. Pin 1 is NOT-Reset and pin 6 is NOT-Set.
One side of the switch is grounded. The other side goes to pin 1 of the 4011. Pin 1 also goes through 100K to VCC. Pin 6 of the 4011 goes through 910K to VCC, as well as going through 0.1uF to ground.
The clear terminal of the 4040 counter, pin 11, goes to pin 4 of the 4011. Pin 3 of the 4011 goes to its own pin 12, another gate input. Pin 11, the output of this gate, goes to pin 10 of the 3479 driver chip.
The pin 13 input of this gate receives inverted squarewave pulses from the desired 4040 output. Thus, pin 13 of the 4011 goes to its own pin 10. Its pins 8 and 9 are tied together and are taken to an output of the 4040--pin 13 or pin 12 being likely choices.
Pin 3 of the 555 goes to pin 10, the clock input, of the 4040 counter (positive-edge triggering for this pin). Pin 8 of the 4040 is grounded; pin 16 goes to VCC. Located close to the chip, pin 16 is bypassed to pin 8 by 0.1uF.
The amount of rotation back and forth is determined by the choice of output on the 4040 12-bit counter. Pin 12 causes the shaft of our disk-drive motor to go approximately one full turn. The following are the divider outputs of the 4040 in order: Pin 9 divides the clock by two--the shaft would go two steps in either direction. Next come: 7, 6, 5, 3, 2, 4, 13, 12, 14, 15 and 1.
A company called All Electronics, an outfit that sells various oddments, has a stepper-motor kit. Called the SMKIT-2, it comes with a motor, a driver (which must be assembled on a PC board), and a power supply--all of this selling for about $25. The driver allows you to run the motor step-by-step or at variable speed.
We have not purchased this kit, so we have no verbal description for its assembly. But, it sure sounds like a screamin' deal.