Instrument drives and transducers

Both the TMA and DMA apply force and measure the displacement that results from this stimulus. Most TMAs and DMAs currently in production use linear variable displacement transducers (LVDTs) to measure displacement. There is one exception, which is the TA Instruments 2980, or later variant Q800, which uses an optical displacement transducer instead. 

Older TMAs are not fitted with motors, but rely upon a top-loading scalepan for the application of force. In TMA the force is constant for all classic experiments, therefore the manual arrangement is quite satisfactory for this work. In DMAs and more modern TMAs a voice coil type motor is employed. This comprises a light coil and former, connected via fine wires, often made from Litz wire. This assembly is either supported by springs or the drive arrangement is supported by an air bearing. Conceptually it is similar to the design of a loudspeaker. 

Whilst all DMAs utilise a voice coil type motor, some are fitted with a force transducer to measure the force transmitted through the sample. This requires a different design of instrument. It is frequently and incorrectly thought that the omission of a force transducer leads to poor modulus accuracy. Provided that the voice coil motor is correctly calibrated and its own contribution to the total stiffness measurement is removed, it does not make any difference, since the force measured by the transducer and the one taken from the motor calibration are both accurate to within better than 1%, which is adequate. Far larger errors occur 

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