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.