Inventors of the Automated Doggie Door, Joe Ambrose and Burt Siegal, use an inexpensive gearmotor to lift the door’s sliding panel but replaced the cable that typically would pull a load with a constant-tension spring, a patent-pending design. This provides a simple mechanical way to increase the motor’s torque, allowing the use of smaller and less-expensive motors, while giving the small gearmotors the power to lift the Doggie Door’s hefty sliding panel. The panel is made from thick, laminated ABS, necessary to withstand a burglar’s kick or block flying debris from hurricanes.

The constant-tension spring itself sits on a relatively large 4-in.-diameter hub to ensure its fatigue life will last over hundreds of thousands of operations without failure.

The end of the inner coil of the spring attaches to a plastic hub on the motor shaft. The other end of the coil hooks to a shoulder screw on a hinged toggle linkage (also patent-pending).

Pet dogs in a household with an Automated Doggie Door wear a collar embedded with a low-frequency RFID chip. When the collared dog walks within 18 in. of either side of the Doggie Door, a rectangular antenna inside the door sends a signal that energizes the RFID chip.

It returns the correct-coded RFID signal to the unit and the microprocessor in the Doggie Door sends current to the motor, causing the plastic hub on the shaft to rotate the spring. The spring first pulls the toggle linkage into a folded position, which withdraws the dead bolt from the door’s frame, and then draws the panel and the attached toggle linkage upward. The panel moves upwards until a little magnet senses the top of the panel and stops the motor. After about 5 sec, a timer in the microprocessor causes the panel to lower.

However, let’s say the dog happens to stop and lie down with his tail under the panel. The constant-tension spring also acts as a counterweight, which let the inventors adjust its force to balance-out the weight of the door to only about 6 oz. This weight is enough to let the panel slide down but not with enough force to harm the dog. The pressure just reminds him to get going. The panel then bottoms-out but the constant-tension spring wants to go down a little more. This movement causes the toggle linkage to extend and push the deadbolt back into the frame. A limit switch senses this and stops the motor.

Ambrose and Siegal used a low-frequency RFID chip because conventional RFID chips use a high-frequency carrier to get the maximum range possible — about 10 ft — making it easier, for instance, to inventory merchandise on store shelves. Such chips might cause the door to operate every time the dog entered the room or ran past it outdoors. The inventors redesigned the antenna several times for the optimal range so different-size dogs could activate the door. For more information contact Joe Ambrose at (941) 448-7024 or e-mail ambrose@ambroseclan.com.

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