For many years, automatic car washes used either chain-driven or air-operated mechanisms to open and close their doors. Such systems operated fairly well, but car wash owners became almost accustomed to breakdowns and failures. Like any highly mechanical design, the more moving parts, the larger the chance that something can go wrong. And with air-operated designs, sub-freezing temperatures in cold climates can bring a car wash to a halt.
To reduce breakdowns engineers at BayWatch Controls came up with a new electronic design that eliminates chain mechanisms. The door operating system uses a small micro programmable logic controller, two 2-hp winch drives, and two 20:1 gearmotors. Winches attached to each motor at the entrance and exit doors open and close them by winding and unwinding cables.
Electromagnetic sensors count motor shaft rotations to track door position and feed the information to the PLC. Temperature sensors trigger alarms when the ambient temperature in the bay area approaches freezing. The PLC can use this data to activate space heaters. There are also sensors to keep the door from over-travel and from closing on an automobile or a person.
All of the I/O information from the door and the environment feeds into a Siemens Simatic S7-200 micro PLC. The PLC resides in a NEMA 4 fiberglass enclosure mounted on the wall inside the bay. It features a built-in 280-mA, 24-Vdc converter, 14 inputs, 10 outputs, and a microprocessor. There are also pushbuttons for manual operation.
Responsiveness is a big issue for a car-wash door system. To guard against mishaps, the CPU executes a binary instruction in 0.8 msec, and, during an emergency, the instruction set for the CPU allows the use of immediate I/O access as well as interrupts.
Two 6SE31-12 Micromaster drives provide start-stop and electronic braking. The drives also contain parameters to ramp speed up and down for opening and closing the doors.
An operator panel interfaced to the PLC provides visual feedback through an LCD readout, which displays one of 80 messages. Programmers enter parameters for door speed, door position, and alarms as well as door height, which can range from 2 to 32 ft. The LCD panel is mounted inside the “convenience store” where the clerks can continuously monitor the status of the door operating system.
Optional soap and wax barrel monitoring is also available since the PLC has analog capability as well as digital inputs.
What’s more, the wiring scheme of conventional bundled wiring to terminal blocks inside the NEMA 4 enclosure is replaced with an actuator/ sensor interface (AS-I) network system. An AS-I master keeps track of all of the I/O over two wires connected to I/O nodes. The nodes have sensors plugged into them and are located close to the doors. This eliminates having to draw long lengths of wire back to the PLC from every sensor.
Townsend is president of BayWatch Controls Inc. in Northglenn, Colo.