The "smart" current-signature sensor is an instrument that noninvasively measures and analyzes steady-state and transient components of the magnetic field of (and, thus, indirectly, the electric current in) a solenoid valve during normal operation. The instrument is being developed to enable continuous monitoring of integrity and operational status of solenoid valves without need for interrupting operation to conduct frequent inspections. The instrument is expected to be capable of warning of imminent solenoid-valve failures so that preventive repairs can be performed. The basic instrument concept should also be adaptable to similar monitoring of electromechanical devices, other than solenoid valves, that are required to be highly reliable.

Characteristic Peaks and Valleys can be seen in the current and in the rate of change of the current in a solenoid at turn-on and turnoff.
This current-signature sensor exploits the fact that unique characteristics (signatures) of the solenoid current — especially of the turn-on and turn-off current transitions — are affected by electrical and mechanical deterioration of the solenoid and valve parts. Current signatures include characteristic peaks and valleys (see figure) that repeat at well defined times during every operating cycle and have well defined magnitudes and shapes. As electrical and/or mechanical deterioration occurs, the peaks and valleys change both in time and magnitude; these changes can serve as indications of potential trouble.

The hardware portion of this current-signature sensor comprises a signal-acquisition assembly and a signal-conditioner/controller assembly. The signal-acquisition assembly contains a linear Hall-effect sensor for measuring the magnetic field generated by the current in the solenoid, plus a flux concentrator to maximize the response of the sensor and a shielding cage to prevent unwanted external magnetic fields from reaching the sensor. A temperature sensor is included to enable compensation for the temperature dependence of the response of the Hall-effect sensor.

The signal-conditioner/controller assembly includes an analog module, a microprocessor controller module, and a power-supply module. A real-time calibration module was being designed at the time of reporting the information for this article. The analog module conditions the low-level signal coming from the signal-acquisition assembly. The preamplification and final amplification stages in the analog module contain digitally controlled potentiometers that are used to compensate in real time for variations, with temperature, of the offset and gain components of the sensor and the signal-processing circuitry.

The settings for the digitally controlled potentiometers are provided by the microprocessor controller module: Prior to operation, calibration measurements with known inputs are taken at various temperatures to characterize the temperature dependence of the sensor and the signal-processing circuitry. A compensation curve is then calculated and programmed in the microprocessor controller module for use in the real-time temperature compensation as described above.

The real-time calibration module is envisioned to be connected to a calibration coil that would be part of the signal-acquisition assembly. Upon command by either the microprocessor controller module or a technician, the real-time calibration module would perform a complete sequence of calibration measurements to determine whether the sensor or other parts of the instrument had deteriorated.

In addition to temperature compensation, the microprocessor controller module is responsible for both real-time and trend analysis of the current signature. In operation, every valve cycle would be monitored and "health" parameters would be calculated to determine whether the monitored solenoid valve is performing within the nominal parameters. The "health" analysis and prediction of failure would be performed by software residing in the microprocessor controller module.