Question about using a motor to do this.
metron9
Posts: 1,100
I am working on a little project to rock my hammock while I ponder thoughts of how the world came to be.
Doing some rudimentary work on how much power·I would need to start with, I put my wife in the hammock and attached a string to the edge and fed that string through a pully attached to a ladder. I then used a 10 poiund weight· at the other end and lifted the weight and let it fall back thus rocking the hammock. Ten pounds was also a good weight with both of in the hammock.
16 inches was the full side to side swing and the swing is about 30 full swings per minute.
The question I have is·if I use·a motor like this E-150 http://www.battlekits.com/robot_motors.htm·and a pulley with a string wound around it.
What I plan to do is monitor the current with an IC shunt like the zxct1009fct-nd and connecting a pully with the string simply power up the motor using a pwm ramp up to a specific current and then allow the hammock to swing the other way by dropping the voltage to a low level near zero. Monitoring the motors shaft (perhaps the feedback of the motors electrical generation on the counter rotation in the case of a permanent magnet motor) to see when the counter rotation stops and then ramping the motor again to a higher current eventually reaching the preset maximum swing set by the user (me).
·I tested the idea using a 5.9 to 1 15V gearmotor I had and it worked quite well with the exception the motor was a bit too small. Not sure how much power it has but i will test its power to see if it can lift 10 pounds on a string. It only pulls 180mA no load. The ratio is ok for the reverse rotation but I would expect damage would be a problem with continious use (I think alot in the hammock)
I assume that would not damage the motor even though it never reaches anything near its maximum rotation specs.
Does that sound ok?
Other Ideas are to use a gear motor like a car window motor like the AME 210 series http://www.robotmarketplace.com/products/ame_motors.html
The problem I think though with a gear motor is I would not want to reverse the motor back and forth (I don't think anyway as it could be a timing issue and other wear and tear potential problems.
In the case of a gear motor though, a ratchet gear could be used to allow the reverse swing but noise could be a problem and I have not been able to find any such off the shelf ratchet gears.
Additional costs of course using a magnetic break could be used and just keep a motor spinning.
Using an eccentric and a continious rotation to move a bar could also work but the natural swing movement I think would be hard to monitor so timing would be more difficult.
I plan to order both motors mentioned above (wish I could find that window motor I had hanging in my garage, I can't remember what I did with it).
Any Ideas for using the gear motor setup and getting away from the reverse rotation problem?
Thanks.
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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!
Doing some rudimentary work on how much power·I would need to start with, I put my wife in the hammock and attached a string to the edge and fed that string through a pully attached to a ladder. I then used a 10 poiund weight· at the other end and lifted the weight and let it fall back thus rocking the hammock. Ten pounds was also a good weight with both of in the hammock.
16 inches was the full side to side swing and the swing is about 30 full swings per minute.
The question I have is·if I use·a motor like this E-150 http://www.battlekits.com/robot_motors.htm·and a pulley with a string wound around it.
What I plan to do is monitor the current with an IC shunt like the zxct1009fct-nd and connecting a pully with the string simply power up the motor using a pwm ramp up to a specific current and then allow the hammock to swing the other way by dropping the voltage to a low level near zero. Monitoring the motors shaft (perhaps the feedback of the motors electrical generation on the counter rotation in the case of a permanent magnet motor) to see when the counter rotation stops and then ramping the motor again to a higher current eventually reaching the preset maximum swing set by the user (me).
·I tested the idea using a 5.9 to 1 15V gearmotor I had and it worked quite well with the exception the motor was a bit too small. Not sure how much power it has but i will test its power to see if it can lift 10 pounds on a string. It only pulls 180mA no load. The ratio is ok for the reverse rotation but I would expect damage would be a problem with continious use (I think alot in the hammock)
I assume that would not damage the motor even though it never reaches anything near its maximum rotation specs.
Does that sound ok?
Other Ideas are to use a gear motor like a car window motor like the AME 210 series http://www.robotmarketplace.com/products/ame_motors.html
The problem I think though with a gear motor is I would not want to reverse the motor back and forth (I don't think anyway as it could be a timing issue and other wear and tear potential problems.
In the case of a gear motor though, a ratchet gear could be used to allow the reverse swing but noise could be a problem and I have not been able to find any such off the shelf ratchet gears.
Additional costs of course using a magnetic break could be used and just keep a motor spinning.
Using an eccentric and a continious rotation to move a bar could also work but the natural swing movement I think would be hard to monitor so timing would be more difficult.
I plan to order both motors mentioned above (wish I could find that window motor I had hanging in my garage, I can't remember what I did with it).
Any Ideas for using the gear motor setup and getting away from the reverse rotation problem?
Thanks.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!
Comments
As kwinn has pointed out, getting synchronized with the natural swing frequency of the hammock is important.
You can measure it with ease by putting yourself in the hammock and getting an assistant to start the motion. As the hammock swings, time how long it takes to go from side to side. For a given weight, the time will be the same. It may take several hours of swinging to obtain a result you have confidence in.
Once you know the frequency required, you can research how to control the stroke length to facilitate start-up and slow down. I would recommend a book by Chironis and Sclater, "Mechanisms and Mechanical Devices Sourcebook". It is definitely a hammock-type thinking and problem solving book!
Cheers,
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Tom Sisk
http://www.siskconsult.com
·
A math and physics major could simply put himself or herself in front of a blackboard and find the frequency for a given mass in the hammock.
Since I do not have the background in math and physics I still am able to apply a common sense approach by testing as stamptrol pointed out.
Kwinn, yes that is the way I plan to do it.
So the main question, even if a motor is held in a stall, (assume a dozen college kids got on the hammock and the frame did not collapse) as long as the currrent is monitored and the voltage/current is ramped up to a point within the limits of the motors specs there should be no problem. I will also use logic found in garage door openers as I just fixed mine (did not grease the plastic gears for 20 years) I accidently moved the little device on the end of the motor shaft that counts the rotation. Once I found the error code listing for the blinking light I was able to fix it. So I will create an error code blinking led for the device as well. Also I will put a tempreture sensor on the motor.
I will take some pictures and show the results when I am done. Finally we have some rain this weekend and I can clean up my bench in both my house and garage otherwise I would just be in the hammock.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!
I suggest using some kind strain guage between the motor and string. This way, you can measure the "tug" on the motor, and constantly correct the timing using this measurement. Ideally, when the motor's frequency is matched to that of the hammock, there would be virtually NO tug at all... But in reality, there is always some kind of dampening effect such as air resistnace, and internal resistive forces in the hammock. All of those forces will slow the hammock down, and eventually stop it, so in th mean time there will be some tug on the strain gauge to measure.
Ok here is my suggestion. The bar that sits between the hammock and frame has a vertical post hanging down with a circular segment on the bottom that makes contact with a wheel mounted on a motor.
The motor is turned on for a brief time to drive the hammock in one direction (motor goes clockwise).
When the hammock starts to swing in the other direction it will turn the wheel, motor, and optical encoder and the motor will be driven in the other direction for a brief time.
The direction of the motor is determined by the encoder direction and the length of the motor on time will determine how far the hammock swings.
If you used a permanent magnet DC motor you could use the reversal of the EMF to switch direction and do away with the optical encoder.
How's that?
I had not considered a gimballed cup holder since drinking from a cup would require sitting up to avoid spilling. I was thinking of a long flexible straw.