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Mechanical Testing

In cases where certain mechanical properties of materials are to be determined, such as elasticity or force-displacement curves, the lock-in amplifier can be a useful basis for the measurement system. The instrument's oscillator is connected to some form of sample probe drive system, for example a power amplifier and solenoid, with the probe being positioned on the sample under test. A detection system, for example using a laser beam and photodetector, is used to generate a signal that is proportional to the actual movement of the probe, with this signal being measured by the lock-in amplifier. By changing the amplitude of the drive signal and recording the resulting actual movement of the probe, the elasticity of the sample can be measured.

Other applications include those using scanning probe techniques, such as Atomic Force Microscopy (AFM). In this, a lock-in amplifier is used to maintain the resonance of a cantilever that has a fine tip positioned close to the sample. As the sample is scanned by an X-Y stage under the tip, interactions with it cause changes in oscillation amplitude, frequency, and phase, all of which the lock-in amplifier can measure, which give information about the sample. By scanning over an area it is possible to build up a virtual image of the sample surface.

Scanned applications often need to use lock-in amplifiers that can operate at high frequencies and with short output filter time constants, in order to allow images with sufficient resolution to be acquired in reasonable periods of time. The Signal Recovery model 7280 lock-in amplifier has proved especially suited to such work, although other units from our range can also be used. Typically, users in mechanical testing buy: