Sensors & Transducers

Application Note - 
Coin Verification Using Piezoelectric Materials

Operating Principle

Automatic dispensing of goods and services relies on payment beforehand with coins and due allowance for many possible combinations to achieve the target fee must be made. Piezoelectric materials such as PZT provide a cost-effective means of verifying individual combinations offered. 

A PZT plate measuring a few millimetres in length and width will provide an adequate electrical signal that can be processed electronically for validation purposes when impacted by a coin.

With reference to the diagram, a coin is rolled down a guide and allowed to strike an isolated metal anvil. The PZT plate, attached to the anvil, produces an electrical signature for the unique modes of vibration of the anvil when struck by each coin type.

The advantage of using a PZT sensor is that it only responds to mechanical vibrations in the anvil. Dimensions, material density, elastic coefficient and mass distribution in the coin all determine how the anvil vibrates making it very unlikely that two different coins will produce identical signatures. 

Practical considerations

As the PZT sensor senses vibration, it is important that stray vibrations are minimised. The use of plastic in manufacture helps in this respect due to its inherent vibration damping. To ensure the sensor does not respond while the coin rolls down the guide, the anvil is generally located on another part of the structure.

The anvil may be attached by screws, adhesive or moulded-in during processing of the plastic mounting components.

The most efficient way of attaching the PZT plate to the anvil is by adhesive – epoxy resin being most often used.

Typical Signatures

The above oscillograms show signatures for £0.05 and £0.02 coins from a typical experiment. They both have 0.2V/div vertical and 0.5ms/div horizontal sensitivity.

Many forms of analysis are possible depending on the degree of uncertainty acceptable for the verification process. In this case, a simple RMS calculation over the time period between the vertical cursors provides a good measure of the difference between the signatures. Signatures may be interrogated over several time periods throughout the transient timespan to improve discrimination in the case where the differences are more subtle.

As the above signatures show, validation is possible within a few milliseconds of the coin striking the anvil making the process suitable for high speed operation if necessary.

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