Seismometer and structural vibration meter
Presenting one of the runners-up in the TI LDC1000 inductive sensor design contest.
This Design Idea demonstrates the use of a TI LDC1000 as a sub-micron displacement sensor for vibration measurement and logging in seismic and structural mechanics applications, and also as the feedback sensing element for active suspension applications. The application is illustrated in two modes: passive sensing (assessing the sensitivity and stability of the sensor as a seismometer) and a force balancing application using a PWM DAC and a voice coil actuator as a feedback device.
The accurate long-term logging of low-level structural vibration is an important task when the machinery is installed in the industrial buildings. Excess vibration is dangerous for the structure itself and may be annoying to the personnel. The presence of unexpected vibration is an indication of the wrong installation or wrong architectural design. The usual sources for structures are ventilation equipment, heavy machinery, and elevators. Knowing the frequency and amplitude, as well as the temporal pattern, helps find the root cause of the problem.
A similar problem exists in geophysics, where the long-time logging of surface vibration allows detection of events in the Earth's crust, and prediction of earthquakes and tsunamis. In all cases, the seismic sensors are used to convert the mechanical displacement into an electrical signal which is logged over the desired period of time.
Once vibration displacement is measured and known, it also can be used as a feedback into active suspension systems. While such applications are beyond the scope of this paper, force-compensating systems (Figure 2 in the detailed report) can be used as active suspension if a sufficiently-powered actuator is implemented.
Other application areas
Similar inductive distance sensing can be used not only to measure, but also to provide the feedback in active suspension systems ranging from laboratory and industrial equipment to any other case where sampling rates of 0.01-10kSa/s is sufficient for implementing a versatile tracking loop digitally. One can use a general purpose microcontroller or an embedded DSP to implement the tracking loop.
Another similar application is sensing the level of aggressive or flammable liquids in tanks or pipes. The floating sensor may consist of a ferrite rod floating in a tube in the liquid, and external windings make a variable inductance sensor. Such a sensor may be made very reliable and robust.
This simple prototype of a high-resolution seismic sensor was built with TI’s LDC1000 inductance-to-digital converter. Its sub-micron sensitivity as a displacement measurement device is assessed analytically and demonstrated practically. With some caveats, the prototype appears suitable for seismic wave measurement, and may be also used as a non-contact feedback sensor for precision digital active suspension systems.
Sensing Design Challenge 2013 winners:
- Position sensing of liquid metal antennas for beam scanning antenna arrays
- Inductive scanner maps objects
- Bed levelling for 3D printer