Ever had an ATM spit out an extra twenty? Me neither. Thanks to some fairly sophisticated technology including a variety of sensors, mistakes rarely happen. One such sensor comes from TE Connectivity of Pennsauken, N.J. The company’s PR 750 Series of 3/4 in.-diameter ac-operated Linear Variable Differential Transformers (LVDTs) serve as measurement feedback devices in the paper bill detector of automated teller machines to ensure proper currency dispensing.
In an ATM, these linear position sensors are used to measure the separation of a pair of rollers, where bills passing between the rollers displace the LVDT core. As bills move between rollers, the LVDT output changes according to their thickness. A typical bill measures 0.0003 in. in thickness, and the LVDT detects any derivation should two or more bills get stuck together.
Because the ATM may be used hundreds of times throughout the day, repeatability and reliability of the linear position sensor are of key importance. And since there is no contact between the LVDT core and coil of the PR Series, there is no friction to cause inaccuracy or part of the LVDT to wear out during the ATM’s service life. To account for any dirt accumulation on the rollers, a sample readout is made for one complete revolution. Any zero-offset value is stored and then compared against the LVDT output as the customer’s currency is measured. This eliminates the risk of passing an unexpected number of bills, while at the same time compensating for any variation of the roller surfaces.