A few mornings ago, I drove to the office, bleary-eyed and still waiting for my first liter of coffee to kick in.  I parked, exited my car, and started walking to the entrance.  Someone a few meters ahead of me held the door open, but let go while I was still about a meter away.  Judging the closing speed of the door, I thought I had enough time to sneak in.  However, during the latter stages of its closing sweep, it suddenly sped up, and slammed shut. Not yet being suitably caffeinated, I uttered a small curse, damning the door and all its close mechanical relatives, and reached for my key fob.

The culprit was the door closer, pictured here.

They’re designed to speed up during the last one-third of its sweep to engage the latching mechanism, and it’s this sudden burst of energy that caught me out.

Once I sat down at my desk, I modeled the door and the door closer in MapleSim.  It’s just a collection of rigid bodies and frames, revolute joints, and a custom component generated from Maple equations (to add some torque during the latter stages of its sweep, and to prevent it swinging inwards at the end of its sweep). 

I experimented with the physical parameters to match the dynamics of the door in question.  Here’s the model, and the animation generated by MapleSim’s visualization blocks. (The red blocks are purely used to generate a better looking animation than the default ball-and-stick format.)

I then spent five vicarious minutes (about one-third the time it took me to develop the model) investigating how I would set the dynamics of the door so that I wouldn’t be caught unawares in the future.

After realising that caffeine had finally started to bind to the appropriate receptors in my brain, I concluded I really needed to leave my apartment twenty minutes earlier to give my morning coffee time to kick in.  Or start brewing stronger coffee.