Stepper Motors

The stepper motor is a device used to convert electrical pulses into discrete mechanical rotational movements.

Most common are 2-phase permanent magnet (PM) motors which provide discrete angular movement every time the polarity of a winding is changed.

CONSTRUCTION
In a typical motor, electrical power is applied to two coils. Two stator cups formed around each of these coils, with pole pairs mechanically displaced by 1/2 a pole pitch, become alternately energized North and South magnetic poles. Between the two stator-coil pairs, the displacement is 1/4 of a pole pitch. The permanent magnet rotor is magnetized with the same number of pole pairs as contained by the stator-coil section.

Interaction between the rotor and stator (opposite poles attracting and likes repelling) causes the rotor to move 1/4 of a pole pitch per winding polarity change. A 2-phase motor with 12 pole pairs per stator-coil section would thus move 48 steps per revolution or 7.5° per step.

STEP ANGLE
Step angles for steppers are available in a range from .72° to 90°.
Standard step angles are:

3.6º — 100 steps per revolution.

7.5° — 48 steps per revolution.

15° — 24 steps per revolution.

18° — 20 steps per revolution.

A movement of any multiple of these angles is possible. For example, six steps of a 15° stepper motor would give a movement of 90°.

ACCURACY
A 7.5° stepper motor, either under a load or no load condition, will have a step-to-step accuracy of 6.6% or 0.5º. This error is noncumulative so that even after making a full revolution, the position of the rotor shaft will be 360º ± 0.5º.

The step error is noncumulative. It averages out to zero within a 4-step sequence which corresponds to 360 electrical degrees. A particular step characteristic of the 4-step is to sequence repeatedly using the same coil, magnetic polarity and flux path. Thus, the most accurate movement would be to step in multiples of four, since electrical and magnetic imbalances are eliminated. Increased accuracy also results from movements which are multiples of two steps.

Keeping this in mind, positioning applications should use 2 or 4 steps (or multiples thereof) for each desired measured increment, wherever possible.

BIPOLAR AND UNIPOLAR OPERATION
Stepper motors are available with either 2-coil Bipolar, or 4-coil Unipolar windings. The stator flux with a Bipolar winding is reversed by reversing the current in the winding. It requires a push-pull Bipolar drive.

Care must be taken to design the circuit so that the drive transistors, which are in series, do not short the power supply by coming on at the same time. Properly operated, the Bipolar winding gives the optimum motor performance at low-to-medium step rates.

A Unipolar winding has two coils wound on the same bobbin (one bobbin resides in each stator half) per stator half. Flux is reversed in each coil bobbin assembly by sequentially grounding ends of each half of the coil winding. The use of a Unipolar winding, some times called a bifilar winding, allows the drive circuit to be simplified. Not only are half as many power switches required (4 vs. 8), but the timing is not as critical to prevent a current short through two transistors as is possible with a Bipolar drive.

For a Unipolar motor to have the same number of turns per winding as a Bipolar motor, the wire diameter must be decreased and the resistance increased. As a result, Unipolar motors have 30% less torque at low step rates. However, at higher rates the torque outputs are equivalent.

Information Courtesy: Thomson Industries, Inc.

 

 

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MSP430FR2433 Internal Oscillator

Configure MSP430FR2433 MCLK for 8 Mhz or 16 Mhz internal oscillator.

MSP430FR2433 – 8MHZ INTERNAL OSCILLATOR

MSP430FR2433 – 16MHZ INTERNAL OSCILLATOR

MSP430FR2433 – ROUTE CLOCK TO PORT PINS

 

8 MHZ INTERNAL OSCILLATOR

 

16 MHZ INTERNAL OSCILLATOR

 

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MSP430FR2433 External Oscillator

This code block can be used to add an external clock oscillator to the MSP430FR2433 Launchpad. The code should be located at the top of the MAIN function. Note that it is not necessary to set the direction of P2.0 (xout) and P2.1 (xin) as the input/output direction is automatically set with P2SEL0 and P2SEL1.

An oscilloscope can be used to check clock output on P1.3.

Crystal Oscillator

 

MSP430FR2433 LAUNCHPAD EXTERNAL OSCILLATOR CODE

MSP430FR2433 LAUNCHPAD CLOCK OUTPUT TO PORT PINS CODE

 

MSP430FR2433 LAUNCHPAD WITH EXTERNAL OSCILLATOR

 

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MSP430G Servo Tester Program

This program can be used with a Texas Instruments MSP-EXP430G2 Launchpad to create an encoder adjustable servo output that ranges from 1.0 ms to 2.5 ms and can be used to test hobby servos. The servo output is on port pin P1.2 and is adjustable in 0.020 ms (20 us) increments. Port pin P1.5 goes high when the servo output is at 1.5 ms. This allows an LED indicator to be connected as a visual aid.

Encoder connections are similar to this: Encoder Connections.

MAIN PROGRAM

PROGRAM INTERRUPTS

OUTPUT WAVEFORMS

1.0 ms SERVO WAVEFORM

 

1.5 ms SERVO WAVEFORM

 

2.5 ms SERVO WAVEFORM

 

SERVO TESTER

 

SERVO TESTER

 

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Decorative Sun Project

This is a decorative Sun made to hang on a door or wall. The center wood face is made from 0.5 inch Baltic Birch plywood and is 10 inches in diameter.

The center face part has several coats of white primer and the finish coat was several coats of Rust-Oleum bright copper.

The outer flame part is made from 0.050 inch white styrene plastic that has a faceted surface. This material was designed primarily for fluorescent light fixtures.

A flame pattern was created on cardboard and was traced onto a stack of four pieces of the styrene material that was then taped together and cut out at the same time. The flames were solvent welded together using Methyl Ethyl Ketone (MEK).

The outer flame part was also finished with several coats of Rust-Oleum metallic bright copper.

 

 

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