Sutter Instrument Lambda 10-3, Operation Manual

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LAMBDA 10-3 OPERATION MANUAL – REV. 3.03 (20110829)
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2. Port Bay C
C C
C supports one standalone filter wheel. No shutter support is provided. A filter
wheel with integrated shutter can be connected to Port Bay C, but the shutter will be
inoperative.
1.7 1.7 1.7 1.7 Functional Description Functional Description Functional Description Functional Description
1.7.1 1.7.1 1.7.1 1.7.1 Stepping Motor Operation Stepping Motor Operation Stepping Motor Operation Stepping Motor Operation
Stepping motors are not as familiar to most people as the common DC motor, but there are
some similarities. The DC motor consists of an armature, an electromagnet mounted on a
rotating shaft, which is located inside a permanent magnet. Current is supplied to the
electromagnet through brushes that rub on contacts on the armature. When the
electromagnet is energized, the armature rotates to align the poles of the electromagnet with
the opposite poles of the permanent magnet. Of course, before this can occur, the rotation of
the armature changes the contact plates rubbing on the brushes so that the current is
reversed. This causes the poles of the electromagnet to reverse, establishing a force for
continued rotation. This switching action is called commutation.
In stepping motors, the rotating element, called a rotor, is generally a permanent magnet
while the fixed element, the stator, is the electromagnet. The key difference between
stepping motors and DC motors, however, is the method of commutation. The DC motor
commutates automatically as it rotates. Thus, the timing of the commutation is determined
by the speed of rotation, which may vary with the load or applied power. The commutation of
the stepping motor is set by external electronics, forcing the motor to rotate at a
predetermined rate. If the load is such that the motor does not have the force to produce the
correct rate of rotation, the rotation will become erratic and may even reverse.
The force exerted between two magnet poles is proportional to the square of the distance
between the poles. A motor with a single electromagnet and only two poles would exhibit
considerable loss of power when the distance between the poles of the permanent magnet and
the electromagnet was greatest. It is understandable that, in most practical DC motors, the
armature has more than 2 poles. This allows the commutation to occur over a smaller angle
of rotation, so that the active poles can always be relatively close to the poles of the
permanent magnet.
Stepping motors are also made with multiple poles on both the rotor and stator; the exact
arrangement determines the number of steps per revolution. The motor used in the Lambda
10-3 has 200 steps per revolution (1.8 degrees per step). There are usually two windings in
the stator, and reversing the current on one of the windings produces a single step of
rotation. Reversing the current on the second winding will then produce another step. If the
first winding is then reversed again, returning to its original value, a third step will result.
Finally, reversing the second winding, so that both windings are back to their original state,
will produce a fourth step. This pattern may then be repeated to continue rotation in the
same direction. Reversing the sequence produces steps of rotation in the opposite direction.
The rate and distance of rotation is determined by the rate and number of commutation
steps. As long as the current is held constant in both windings, the rotor will not rotate. This
makes the stepping motor ideal for producing fast start and stop movements. Some
limitations should be considered. Given that there are only 4 states of the control electronics
(2 polarities for each of the 2 windings) but 200 steps per revolution, it follows that, for each
of these 4 states, there are 50 possible rotary positions. In order to establish the absolute