Motor

The stepper motor is responsible for the precise movement of the moving mirror in the Michelson interferometer. The quality of the spectrum directly depends on the precision and stability of this displacement.

Principle

The Michelson interferometer requires linear displacement of the moving mirror over a few centimeters, with sub-micrometric regularity. The motor drives a translation stage via a lead screw.

Stepper Motor → Lead screw (0.5 mm pitch) → Linear stage → Moving mirror
       ↓
   A4988 Driver → Microcontroller (STM32)

Specifications

Parameter	Value
Type	NEMA 17 bipolar stepper
Step angle	1.8° (200 steps/rev)
Micro-stepping	1/16 (via A4988)
Travel range	25 mm
Linear resolution	0.156 µm (theoretical)
Max speed	10 mm/s
Holding torque	0.45 N·m
Supply voltage	12 V DC
Current per phase	1.2 A

Electronic Control

A4988 Driver

The A4988 driver is configured in 1/16 micro-stepping mode (MS1, MS2, MS3 pins high). The effective angular resolution is 3200 micro-steps per revolution, giving a theoretical linear displacement of:

0.5 mm/rev ÷ 3200 micro-steps/rev = 0.156 µm/micro-step

Control Signals

STM32 Pin	A4988 Signal	Function
PA0	STEP	Step pulse
PA1	DIR	Direction
PA2	ENABLE	Driver enable
PA3	MS1, MS2, MS3	Micro-step configuration

Acquisition Sequence

A typical FTIR acquisition follows this sequence:

1. Homing: the mirror moves back to the limit switch.
2. Acceleration: speed ramp over 2 mm (4000 micro-steps).
3. Constant scan: 20 mm at steady speed (128000 micro-steps).
4. Synchronous acquisition: ADC samples on each STEP rising edge.
5. Deceleration: braking ramp over 2 mm.
6. Fast return: maximum speed in reverse.

# Acquisition sequence pseudo-code
def acquisition_scan():
    homing()                        # Return to zero
    ramp_up(steps=4000, v_max=10)   # Acceleration
    for step in range(128000):      # Constant scan
        pulse_step()                # One micro-step
        sample = read_adc()         # Synchronous ADC acquisition
        interferogram.append(sample)
    ramp_down(steps=4000)           # Deceleration
    return interferogram

Displacement Calibration

Calibration is performed with a He-Ne laser (λ = 632.8 nm):

1. Replace the IR source with the He-Ne laser.
2. Run a complete scan.
3. Count the laser interference fringes.
4. Number of fringes × λ/2 gives the actual travel distance.
5. Adjust the micro-step/pixel factor in firmware.

Maintenance

• Lubrication: lithium grease on the lead screw, every 6 months.
• Backlash check: measure backlash with a dial indicator (< 5 µm acceptable).
• Thermal control: add a heatsink on the A4988 driver if T > 60 °C.
• Limit switch: test the limit switch before each session.

Common Issues

Symptom	Likely Cause	Solution
Missed steps	Insufficient current	Increase A4988 Vref to 0.9 V
Excessive vibration	Mechanical resonance	Add a damper on the shaft
Shifted spectrum	Uncompensated backlash	Enable firmware compensation
Periodic noise in spectrum	Irregular micro-steps	Check 12 V power supply

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