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Stepper Motors
5 questions By Tony R. Kuphaldt
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Question 4 of 5
This shift register circuit drives the four coils of a unipolar stepper motor, one at a time, in a rotating pattern that moves at the pace of the clock. The drive transistor circuitry (Q1, Q2, and resistors R2 through R6) are shown only for one of the four coils. The other three shift register outputs have identical drive circuits connected to the respective motor coils:
Suppose this stepper motor circuit worked just fine for several years, then suddenly stopped working. Explain where you would take your first few measurements to isolate the problem, and why you would measure there.
Reveal answerMy first step would be to check for the presence of adequate DC power to both the shift register IC and the motor (transistor drive circuitry). Then, I would use a voltmeter or logic probe to check for pulsing at any one of the shift register’s Q outputs. That would tell me whether the problem was with the shift register or with the power circuitry.
Notes:This is a good question to discuss with your students, as it helps them understand how to “divide and conquer” a malfunctioning system.
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Question 5 of 5
Stepper motors are often used in low-power servomechanisms such as those found in small robots, computer printers, and other precision electro-mechanical machines. Explain why this type of electric motor is more popular than permanent-magnet DC motors or other motor types. Hint: the answer is closely related to the name of the motor itself (“stepper”).
Reveal answerUnlike other motor types, stepper motors move in discrete “steps,” lending themselves very well to digital modes of position control.
Notes:Make sure students get a chance to feel the motion of a stepper motor in their own hands when you discuss stepper motors in class. If you don’t happen to have any stepper motors available, they are easily obtained by salvaging parts from worn-out computer printers!
