The Case of the Intermittent Vending Machine

By Foye Penn

In the 1980s, I was engineering manager for a company that manufactured snack vending machines. The machine vended various snack foods (chips, candy, cookies, gum, etc). Bag-type products were loaded into wire-wound spirals which were turned by small low-voltage AC motors. A customer inserts coins or bills into a changer which displays credit and pushes a button corresponding to the desired product. A controller receives the signal that sufficient money is deposited and sends a signal to the appropriate motor to vend the product. The machines were sold in the United States and several foreign countries.

This vending machine had been in production for several years with only minor problems. Complaints started coming in from Australia that the machine would start to vend a bag product but stop about half way through the cycle before the product was dropped into the delivery bin. There were no similar complaints from other countries. The first step was to do extensive lab testing of the model of machine sold in Australia. The differences from the U.S. model being the supply voltage and the coin changer settings to accept Australian money. Thousands of bags of U.S. snack products were vended in the company test lab with no vending failures.

The next step was to enlist the aid of the UK company that manufactured the vending machine controller and coin changer. Australian snack products also were vended during tests. With the Australian product, a few random vending failures occurred but were not repeatable. The product bag material looked and felt quite different than the United States or UK bag material.

One tray of the vending machine along with the controller, coin changer and wiring were assembled for operation in a humidity chamber vending Australian snack products. Tests were run at different humidity levels until it was found that the vending failure occurred consistently at low humidity levels. It was discovered that the Australian bag material was generating static electricity at low humidity levels, which are common in Australia.

The static electricity was reaching the controller and stopping the vending motor immediately. There were five trays in the vending machine. Each tray was mounted on four nylon rollers which ran in tracks mounted to the side of the machine. This allowed the trays to be pulled out of the machine for easy loading access. The trays were not properly grounded to the machine, thus allowing the static discharge to shut the controller down.

The solution was to add small leaf springs to the rear of the cabinet so that when each tray was pushed back into the vending machine after loading, that tray was grounded to the machine frame by contact with the leaf spring. A field service technician had to travel all over Australia retrofitting the leaf springs to machines in service. Subsequent production included the tray grounding springs. The machines shipped for many years with no recurring vend stoppage in mid-cycle.

Foye Penn is a retired mechanical engineer, expert witness and CE design consultant in the winch industry.