I had a cat who didn't want to use the catflap, he felt threatened with the flap on his head. A rubber band to hold the flap open fixed that. Not a good solution in the winter.
Then I made a motorized flap for him, to look if that would help. Infrared sensors on both sides, AVR processor ,rotary hall encoder chip from AMS and a PID controlled DC motor. Workes like a charm, including for the reluctant cat. Unfortunaly he passed away last year.
I also have trouble with cats who like dining out and I have the best food. So next plan was RFID (chip recognition), timer lock, data logger and a traffic display. This proved difficult, it's radio and its analog. But I was nearing the assembly time. Then my vet implanted my last cat with an 8*1.5 chip instead of 12*2. I couldn't read the chip. The vet is right and I was passed by advancing technology. Not enough range.
Thus time to look at the Sureflap patents. Black analog voodoo with misused MOSFETs. Works. 200 mm range with 12*2 chips, 145 mm for the 8.5 * 1. Measured mid chip-mid coil. Almost time for new China PCB's. Now that's a blessing for humanity!