Linear Motor Measurement
A test sequence for measuring the performance characteristics of a linear motor for haptic feedback using SoundCheck.
Linear motors (also known as linear vibrators or linear resonant actuators) have become increasingly popular in handheld devices such as phones and tablets for providing haptic feedback to the device’s user. The typical linear motor design uses a fixed voice coil in conjunction with a moving magnetic mass. A spring is placed between the mass and the motor’s housing to provide compliance to the moving system.
The design goal of linear motor is to create a “High-Q” system so that the device has a very strong resonant frequency across a narrow frequency bandwidth. The motor is then operated at its resonant frequency to produce maximum output (vibration) while having minimal power demands on the portable device. The strength of the vibration is controlled by adjusting the magnitude of the AC signal input to the motor.
The purpose of this sequence is to measure the important performance characteristics of a linear motor. Since we are measuring vibration, an accelerometer must be used. The sequence begins by exciting the motor with a sine sweep. Since the operating frequency range of a linear motor is typically 200 Hz ±20%, the sweep range is limited to 150Hz-300Hz. An analysis step calculates the fundamental output of the accelerometer and post processing steps calculate f0 (resonance frequency) and the f1 and f2 (-3dB) values. A second analysis step calculates the electrical impedance of the motor while subsequent steps calculate the electrical f0, Q and current at f0. Lastly, the motor is excited with a single tone at f0 and a Time Envelope of the recorded time waveform is used to calculate the motor’s Rise Time and Fall Time values.
- SoundCheck 18
- Audio Interface – Listen AmpConnect or similar
- Accelerometer (IEPE bias) – Brüel & Kjaer Type 4519-003 or similar
- Test Jig (typical: material=Delrin, dimensions = 105mm x 15mm x 10mm, weight=100g)
- Polyurethane foam pad
Hardware Setup and Calibration
Note: This test sequence is designed for use with Listen’s AmpConnect hardware. With some minor modifications (e.g removal of the AmpConnect Message Step), it can be adapted for use with discrete hardware components.
- Connect the accelerometer to the AmpConnect Reference input and enable the inputs’s IEPE bias
- Calibrate the accelerometer. There are two options:
- If you know the accelerometer’s sensitivity and reference frequency, you can enter these values directly into the appropriate fields in the Accelerometer’s calibration field in System Calibration. When you imported the Accelerometer calibrated device file to your system, it should have pre-configured your calibration with the proper units of force (V/g)
- You can also calibrate your accelerometer using SoundCheck. Choose “Accelerometer Calibration” in the Calibration Sequence drop-down and attach the accelerometer to an accelerometer calibrator such as the B&K Type 4294. Click the Calibrate Device button. This performs a simple Voltmeter reading while the accelerometer is being excited by the calibrator at the reference level. Click OK in the Voltmeter to accept the measurement. You’ll see the sensitivity automatically update in the Sensitivity field.
- Connect the linear motor to AmpConnect Output A
- Mount the linear motor and the accelerometer to the test jig as shown in the following System Diagram
- Place the test jig onto the foam pad