control system for automatic hammock ?

I'm looking for a simple principal to build an 'automatic hammock rocker'. I would imagine this is a classic control system problem.
The system should be able to pull the hammock to one side, up to a certain force, then release the force and start pulling at the opposite side. By using forces (and not distance or speed) it should be possible to start from a non-moving situation and then generate very tiny movements, which gradually get larger, until a certain maximum swing-radius, were the force needed for additional swing equals the maximum force delivered by the control system.
This system would require some form of 'force-measuring' device. This could be done by using springs. The only design I could come up with is a system of 2 electromotors continously running in opposite directions, and a cogwheel, that 'feeds' of motor1 (forward), motor2 (backward) or none at all (neutral). This 'gear selection' could be done by a set of springs, that measure which force is needed. The cogwheel could pull the hammock ropes
(Motor1) O s (spring) s (cogwheel) o--------* (pivot) s (Motor2) O s (spring)
Any thoughts ?
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andreas gammel wrote:

1. Why a hammock?
2. The reason you don't find many purely mechanical control systems out there is because they're difficult to keep working correctly - this is why most controllers these days are implemented in software and driven electronically to mechanical actuators. This example could be realized with an appropriate driver and a motor, for instance.
3. Driving your hammock with a force is going to give you a response that's fairly unbounded -- the hammock will form a fairly good pendulum, and a perfect one would have an ever-growing response. There's a good chance that your output wouldn't be limited until the ropes started going slack and the jerking absorbed the energy being input.
4. I'm going to assume that this isn't a joke and point out that any kind of feedback controller that you could envision for a hammock would be too expensive to be marketed.
If you seriously want to pursue this for the resort trade or whatever I suggest that you use a single motor, geared down to the approximate speed of the hammock's period with an average-weight tourist, and use it to turn a crank that torques one of the ends of the hammock. This will give you a fairly limited response, and it'll be cheap and reliable.
--

Tim Wescott
Wescott Design Services
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No, it's no joke. I have an actual hammock in my attic and often lie in there thinking if this problem could be solved. I also have a Lego MindStorms robot set lying around and wondered if I could use it to build such a system. The problem is that traditional electromotors deliver a constant force and torque, whereas the hammock/pendulum system has constantly varying force and speed. I have no commercial intent whatsoever, just a crackpot inventors mind.
I know there are 'cradle rockers' for sale but I think they are all based a constantly rotating crank. Starting up a clock pendulum would be the same problem.
I want to know how to mechanically translate a 'constant speed rotation system' into a 'variable force linear system'

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snipped-for-privacy@yahoo.com (andreas gammel) wrote:

Use a crank and a rubber band (bungee).
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Sounds like a very novel and well-thought out solution. A couple of comments: 1. Might it be cheaper to use just one bi-directional motor, and use the spring/cogwheel arrangement to toggle a switch that reverses current to the motor ? 2. A fancy way of controlling the current might be to use some sort of linear potentiometer device to regulate current to the motor. You could vary the pot setting based on the hammock travel, and this would in turn regulate the torque from the motor. It's just an idea; will probably need thinking out. 3. The quality of feedback that you get (from your springs) will depend on where you mount the device on the hammock. The few hammocks that I have seen are secured to a post (say a tree) via a single piece of rope. Perhaps placement should be an issue to get best feedback. Also, hammocks may tend to shake and vibrate, which should be considered during design.
Regards, Manu *******************
snipped-for-privacy@yahoo.com (andreas gammel) wrote in message

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Drink too much alcohol
The hammock will swing by itself - no moving parts to go wrong

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