The BME Systems Inc. Vibratactile Stimulator is a linear motor for delivering precise, single-point vibratory stimuli. Two modes of operation are possible. Position mode permits control of the motor's probe tip to within 1 micron over a 2 mm range. Force mode enables the application of up to 200 grams to a specimen with sinusoidal modulation flat (* 1 dB) to 200 Hz. The motor features completely new design principles that extend its performance and operating envelope well beyond that of previous models. It is characterized by its flat response ( < 1 dB deviation from flat, 1 to 300 microns peak-to-peak travel from 0 to 300 Hz) and minimal distortion through its extended range.

Principles of operation

The stimulator is centered around a cylindrical, moving-coil, type of linear actuator governed by the Lorentz Force Principle. Its permanent magnet has been developed with a flux-focused design to increase the flux density within the air gap in which the coil moves. The BME Systems Vibratactile Stimulator motor design incorporates no suspension. Instead, the coil and its attached moving components are guided by a connected shaft, captured by a low-friction bushing which extends through the center of the motor magnet body. The end of the shaft opposite the coil form is attached to a solid cylinder, the upper surface of which is used as a target for an inductive position sensor with 0.2 micron static resolution. The shaft extends through the coil form and is joined to a load cell through a temperature-insulating coupling. A light-weight Delrin* probe is attached to the active end of the load cell.

Advantages of the BME Linear Motor design are:

  Nonlinearities associated with large movements in spring suspension designs are eliminated. This permits a high degree of spectral purity for large dynamic displacements. Only a small amount of viscous friction (linearly related to velocity) impedes free movement of the motor's parts.

  The relatively light components used in the moving-coil design increase the force/mass ratio which permits an extended frequency response.

  The increased efficiency of the stator/coil combination (force sensitivity, 1.6 lb/A) contributes to a low mechanical time constant which enables higher gain for critically damped control at high frequencies. In addition, integral compensation is implemented to increase stiffness and virtually eliminate static position error.

  The force required to overcome the acceleration and deceleration of the moving mass is related to the square of the frequency of oscillation. The high force capacity (8.0 lb. for 10 sec.) of the motor also extends the usable frequency range.

  The stimulator uses a primary method for sensing force very close to the specimen. This yields highly accurate control of both static and dynamic force stimulation. The motor is powered by a unique controller. Command inputs are given as analog voltages through rear panel BNCs. The power supply and motor drive components are specifically designed to provide power that is free of harmonic and crossover distortion with minimal phase lag.  The controller monitors its RMS current output and gives indication to the user (via a front panel LED) when the maximum continuous value for the motor has been reached. The controller will supply up to 8 amps peak-to-peak (current is automatically limited) while operating above the maximum continuous RMS value for up to 10 seconds. This enables the user to produce intense stimuli for short periods. If operation continues for more than 10 seconds above the maximum continuous value, the controller will automatically shut-down the motor for 20 seconds. Indication of system shut-down is given via a front panel LED and a rear panel logic output. The mode in which the motor operates (Position or Force) is normally controlled via a rear panel BNC. However, this can be overridden with a front panel switch to enable static control through front panel potentiometers. Both force (0 to 199.0 grams) and position (0 to 1.999 mm) are displayed through two front-panel LED indicators. In Position mode, Proportional-Integral-Lead compensation is implemented to provide damping and improved steady state accuracy.  Force mode is centered around the application of a highly sensitive load cell. The load cell is separated from the specimen by a probe tip weighing less than 0.25 grams. Primary measurement of force is, therefore, achieved with minimal error from both gravity and mass acceleration/deceleration forces of the probe itself. Force commands are applied via a rear panel BNC (20 grams/volt). If the probe should extend to within 5% of its maximum travel, the closed force loop is broken and between 40 and 160 grams of force (depending on the orientation of the motor) is applied to its mechanical stop. The loop is restored by retracting the probe either by command or by physically countering the default force for a distance of one millimeter. A similar default force is applied, in the opposite direction, to a retraction stop for any negative force command. Contact us for more information.