"Conventional" Stepper Motor

"CONVENTIONAL" STEPPER MOTOR







An extremely simple stepper motor circuit is shown to the right; it consists of a bar shaped permanent magnet which can rotate about its center, a "C" shaped piece of soft iron, a length of insulated copper wire (wrapped around the piece of soft iron), a switch, and a battery.

With the switch open, the bar magnet will rotate to position itself with the soft iron as shown and it will stay there.







When the switch is closed, current flows through the wire creating a magnetic field. With the wire wrapped around the soft iron in the direction shown here, the magnetic field North pole will be located at the top.

Since "like" poles repel and opposites attract, the permanent magnet rotor will turn 180 degrees to align itself with the new magnetic field and then it will stop there.


NOTE: In this simple motor, the rotor can turn in either direction to reach its new position.


Opening the switch will cause the magnetic field to collapse, but the rotor will remain in its new position.







By reversing battery polarity, though, and closing the switch, a new magnetic field is created. One that has its North pole located at the bottom of the "C" The rotor moves again, this time back to its original position.







Well, the orientation of the magnetic field can also be changed by leaving the battery in its original position and changing the direction that the wire is wound around the soft iron core.

Notice that the winding wire now first goes behind the core. This may seem like an insignificant change but, electrically speaking, it's critically important.







Using this information, we can again redraw the parts of the circuit in a slightly different way to get a stepper motor that is closer to the ones found on BMW vehicles.

All we've done is wrap two coils around the soft iron core (one wound in each direction to give opposite polarities), and use two switches so each coil can be energized independently.







Rearranging some parts of this circuit just one more time puts the beginning of both coils at the middle of the "C", (for manufacturing reasons) so we have to wind the top coil in the other direction to have its polarity stay the same.

The bottom coil is unchanged.







Now, momentarily closing switch "B" energizes the bottom coil, a magnetic field forms with its North pole at the top, and the rotor turns 180 degrees to align with the field.

The rotor then stops there.







Momentarily closing switch "A" energizes the top coil, a magnetic field forms with its North pole at the bottom, and the rotor turns 180 degrees again to align with the new field.

Alternately closing switches "A" and "B" toggles the magnetic field polarity, causing the rotor to rotate in 180 degree steps







To take this concept one step further; add another identical iron core with coils and switches to our 180 degree stepper motor.

And, have the new assembly share the same battery.

To keep the components straight we should label the coils ("P," "Q," "R, " and "S") and the switches ("S1," "S2, "S3", and "S4").







To get our new 90 degree stepper motor to turn one step clockwise, we need only close switch 53 to energize coil "R".

We now know that the rotor will turn clockwise because it will always take the shortest path to its new position.

So 4 coils give us directional control!







To move the rotor another 90 degrees, energize coil "Q" (by closing switch "S2").

Current flows through coil "Q" in such a way as to create a magnetic field having its North pole at the top and the rotor turns clockwise 90 degrees to align with the field.







To get the rotor to back up 90 degrees (turn counter-clockwise) we need only close switch "S4" to energize coil "S."

Current flows through coil "S" creating a magnetic field having its North pole at the bottom. The rotor turns counter-clockwise 90 degrees to align with the field.

By closing and opening the 4 switches in certain sequences, we can get the rotor to turn in the direction we want, and we can get it to turn as many times as we want.






Both
- have 4 coils arranged around a permanent magnet rotor
- have the 4 coils receiving power directly from the battery
- energize the coils individually using ground switches (transistors inside the control module for the ETM version)







The stepper motors actually used on BMW climate control systems have a few important differences, though. For one, the rotor is turned in 15 degree increments using just 4 coils. This allows very precise motor positioning and is made possible by using more complex iron cores having multiple poles and by energizing up to two coils at the same time.

The multiple coil energizing strategy is easily illustrated using our familiar 90 degree stepper motor example. If we dose switches "S2" and "S3" at the same time to energize coils "Q", and "R," two magnetic fields of equal strength are formed. Since both North poles are at the bottom, the rotor will turn just 45 degrees and stop, balanced between the two fields.

Our 90 degree stepper motor has become a 45 degree stepper motor by changing just one thing: which switches are closed at any one time. The rotor can be "stepped" around in one complete rotation (360 degrees) in 8 separate 45 degree increments.

By comparison, the more sophisticated BMW stepper motor, whose rotor moves in 15 degree steps, takes 24 steps to complete a revolution.







Another important difference for the actual BMW part is that it is mounted on a high reduction gearbox. In fact, on the typical stepper motor, the stepper motor rotor must rotate about 300 revolutions (that's 7200 separate rotor steps) for the gearbox output shaft to rotate through lust a single revolution.

This high-reduction gear box allows a relatively low power motor to operate the flaps against the substantial suction and pressure forces generated by the blower motor.


NOTE: The output shaft never actually rotates through a full revolution; it doesn't need to do this to fully open and close flaps. In most applications, output shaft rotation is 180 degrees or less.


Because so many separate motor coil activations are required to cause perceptible output shaft movement, the climate control system control module is programmed to open and close the right switch(es), in the correct sequence, very quickly. (The average time for a flap to go from fully closed to fully opened is about 12 seconds. That translates to about 3600 pulses in 12 seconds, or 300 pulses per second.) The drawing below shows an actual sequence of switch openings and closings for a stepper motor to partially open a flap.







In addition to operating the stepper motor, the control module also remembers where the motor is positioned and, therefore, exactly where the flap it opens and closes is positioned.

When the module first receives power it automatically runs all the stepper motors to fully close all the various flaps. It continues to keep pulsing the motors for a few seconds after they stall, just to make sure that the flaps are really closed (in case a motor is sluggish or linkage is binding). Since the module is programmed to "know" exactly how many pulses it takes to fully open the motor, It then counts and keeps track of the number of pulses it provides as it runs the motor to open the flap. So it knows where the flap is positioned, by counting and storing the number of pulses it applied to the motor coils.







Taking another look at an "actual" BMW stepper motor notice that it requires 5 connections:
- power (to all 4 coils)
- ground (control) to coil "S1"
- ground (control) to coil "S2"
- ground (control) to coil "S3"
- ground (control) to coil "S4"


NOTE: Pre-1991 E34 and E34 stepper motors had two power circuit connections -- one to each pair of coils.


These connections are important for diagnostic purposes, since it is impractical to run a stepper motor once it has been disconnected from the control module. You would have to ground the correct motor terminals, in the correct sequence, hundreds of times to notice any output shaft movement.







So instead, one way to test for a faulty stepper motor is to measure coil resistance.

On E31, E32, and E34 stepper motors except the fresh air flap motor, each of the motor's 4 field coils has a resistance of about 85 ohms.

For the fresh air flap motor only on E31, E32, and E34 vehicles, each of the 4 field coils has a resistance of about 26 ohms.







Why is the fresh air flap motor specification different? Because unlike the other stepper motors, the fresh air flap motor is designed to operate faster in the "close flap" direction than in the "open flap" direction, so it has different characteristics. It's easy to distinguish the fresh air flap stepper motor from the other motors because it has a different color identification label; typically red or blue, depending on production date.