control system for automatic hammock ?

[andreas gammel]

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 ?

[Tim Wescott]

Post by andreas gammel
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 ?

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.

[andreas gammel]

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'

Post by Tim Wescott
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.

[nospam]

Post by andreas gammel
I want to know how to mechanically translate a 'constant speed
rotation system' into a 'variable force linear system'

Use a crank and a rubber band (bungee).

[Manu Murthy]

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
*******************

Post by andreas gammel
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'

Post by Tim Wescott
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.

[Wild Turkey]

Drink too much alcohol

The hammock will swing by itself - no moving parts to go wrong

Post by andreas gammel
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 ?