# Automatic door, linear actuator or stepper motor

I need to build a device that has motorized a door with a hinge on top, similar to cat doors:

Between these options, which one makes the most sense: - use an actuator on the side to push the door up - put a pulley on the door hinge and drive it with a stepper motor through a belt - put an anchor above the door, on the same plane, and pull a string on a reel with a stepper motor - or, any other idea :)

What makes the most sense from a mechanical point of view? it's for a small device where the door is roughly 7x7" and weights about 0.6LBS.

The actuator seems the simplest so far, but before I order parts I would like to have opinions from people that understand this stuff :)

• Can you provide more details of your project? We might be able to suggest something better than a top-hinge door. – Carl Witthoft Nov 15 '16 at 14:28
• I'm skeptical that the door panel itself is that massive. Your sheet density is $\frac{ 0.6*2200}{7^2*2.54^2} = 0.86 g/cm^2$ ; if we assume 2mm thickness this implies a density of 4.3. That makes your estimate almost certainly high by a factor of 5 – Carl Witthoft Nov 15 '16 at 14:37
• @CarlWitthoft I think it seems fairly reasonable with a few slightly different assumptions. For example, if it was 1/8" thick (0.3175 cm) that would make your density $\frac{0.86 g/cm^3}{0.3175 cm} = 2.70 g/cm^3$ which is pretty much exactly the density of aluminum. Considering we are just guessing at thickness and material; I don't see why that would be unreasonable. – JMac Nov 15 '16 at 15:26

All of your suggested methods seem workable.

To me it seems that the major differences are:

1. Whether it resists opening the door by external force. A linear actuator would, while a string wouldn't. It depends on your use case which is more desirable, or whether it matters at all.

2. Amount of force produced. A linear actuator is the strongest, but if there is a possibility of someone being stuck in between the door you might actually prefer a pulley that can slip without causing damage.

3. How to find the zero position. After power is cut out and restored, your device must somehow know what is the current position of the door. Typically this would be done with a microswitch at one of the end positions. This would probably be easiest to place in the closed position for the linear actuator or pulley system, and in the open position with the string system.

• and from a reliability standpoint, am I right to assume that the linear actuator is the best choice? – Thomas Nov 15 '16 at 11:57
• There is no inherent reliability difference, it all depends on the implementation. But if you buy an industrially made linear actuator, yes, that should be quite reliable. – jpa Nov 15 '16 at 13:08
• Reliability: a pulley and a winding bobbin for a string are prone to wear, tangles, etc. A belt or "chain" drive is better; a worm drive best. The OP said nothing about cost, free-volume area, etc., so it's hard to say what'll do the job for him. – Carl Witthoft Nov 15 '16 at 14:31