Servo tension - shake

fuichris · 2014-10-05 09:41

Hi all!
Rookie here with a simple pully project with a Basic Stamp Homework Board USB and Parallax Standard servo. The servo shakes when it is over-extended (see video Vimeo link: https://vimeo.com/108051024)
Sorry for the dumb question, but do I just need a stronger servo?
I do need to pull harder for my pully mechanism to work, but wondered what my options are.

Thanks!

Publison · 2014-10-05 09:46

Welcome to the forum!

Neither of your videos are view able.

When you say over extended, do you mean you are hitting the stop in the servo, (usually at -90 degrees ans +90 degrees)?

You may be looking for a Continuous Rotation Servo since you say it on a pulley arrangement, but I can't tell while there is no valid video.

Also your servo may be drawing too much power. It may need a separate battery.

Publison · 2014-10-05 10:33

Finally got the vimeo video to display. Look like you are using the Parallax Standard Servo. What is not in the video is the the item you are trying to drive. Also what board are you driving this with? A Basic Stamp?

fuichris · 2014-10-05 10:54

Publison wrote: »

Finally got the vimeo video to display. Look like you are using the Parallax Standard Servo. What is not in the video is the the item you are trying to drive. Also what board are you driving this with? A Basic Stamp?

The project involves moving petals... See video (https://vimeo.com/99092288). Essestially trying to get smooth controlled movement. And yup, I'm using a Basic Stamp homework board. Tks!

Publison · 2014-10-05 11:03

Nice project!

Without seeing the linkage involved, the first thing I would do is reduce the highest PULSEOUT value. You might be hitting the mechanical lock in the servo.

You might need a bigger servo. ServoCity has many. and I use them for all my needs.

Publison · 2014-10-05 11:21

Have you looked into Muscle Wire? Pretty neat stuff. May be the perfect application.

One supplier:

https://www.sparkfun.com/products/11900

Duane Degn · 2014-10-05 12:13

fuichris wrote: »

wondered what my options are.

How are you powering the servo? If you're using a a battery what kind of battery is it?

Many servo issues are caused by a power supply not being able to supply the current needed by the servo.

If you're using a good power source, then you may want to use a more powerful servo.

I like HobbyKing's HTX12K metal gear servos but there are probably lots of other servos which are stronger then the Parallax standard servo.

BTW, I thought the petals looked great!

fuichris · 2014-10-05 19:27

Thanks Publison! I tried to play with the PULSEOUT value but it didn't make a difference.
But everytime I loosening the strings at the strained point, it doesn't shake so I'm certain it is a tension power issue. Is the shaking a way for the servo to tell me "you've hit the limit of my servo strength!"?
Thanks for the Servocity link!

fuichris · 2014-10-05 19:30

Publison wrote: »

Have you looked into Muscle Wire? Pretty neat stuff. May be the perfect application.

One supplier:

https://www.sparkfun.com/products/11900

Thanks for the Muscle Wire link ... but I think the wire mechanism to make the petals curl up needs to be very strong and the Muscle Wire might not do it.
But it's a nifty product nonetheless.
I was actually looking for a super thin flexible spin for the petals too, but I'll post that separately. Tks!

fuichris · 2014-10-05 19:51

Duane Degn wrote: »

How are you powering the servo? If you're using a a battery what kind of battery is it?

Many servo issues are caused by a power supply not being able to supply the current needed by the servo.

If you're using a good power source, then you may want to use a more powerful servo.

I like HobbyKing's HTX12K metal gear servos but there are probably lots of other servos which are stronger then the Parallax standard servo.

BTW, I thought the petals looked great!

Hi Duane,
I'm powering the Parallax standard servo using an ac/dc adapater (outputs into my homework board with 9v. Plugged, no batteries). I needed the project to run indefinitely if possible...like mths to yrs.

More powerful servo: ok, noted. I'm new with servos, so how do I know it's a strong enough servo to power the petals? torque figure? The standard servo says 36oz-in torque (??).
I'll go take your advice on the HobbyKing HTX12K metal gear servos. 10kg sounds strong ;p

DEPENDABILITY: Thanks for the compliment on the petals! My problem is trying to move the petals dependably without it wearing down over time. Which is why this shaking / strength of servo worries me.

Any advice is greatly appreciated!!!

fuichris · 2014-10-05 20:03

Here's the project basically ... (not battery powered).

erco · 2014-10-05 21:24

fuichris: Neat project and welcome to the forums. You are pulling at the maximum outer radius of that servo horn, which requires too much force and stalls your servo. Fortunately you want slow motion, so you can make a simple mechanical change and hopefully use your existing servo. Move the round brass roller from the outermost hole on the servo horn to the innermost hole and you should see some immediate improvement. For any given servo torque, operating at half the radius gives you twice the tangential force. You can easily reprogram for faster servo movement to make up for the slower speed at a smaller radius.

fuichris · 2014-10-06 06:38

erco wrote: »

fuichris: Neat project and welcome to the forums. You are pulling at the maximum outer radius of that servo horn, which requires too much force and stalls your servo. Fortunately you want slow motion, so you can make a simple mechanical change and hopefully use your existing servo. Move the round brass roller from the outermost hole on the servo horn to the innermost hole and you should see some immediate improvement. For any given servo torque, operating at half the radius gives you twice the tangential force. You can easily reprogram for faster servo movement to make up for the slower speed at a smaller radius.

Erco: Worked like a charm!! Thanks! Sometimes on the way back while it's super taut, it jumps a bit. Is that preventable? Secondly, in the future, what happens when this new 'inner-horn' position doesn't work and it starts to shake again. Does it mean that I will definitely need a stronger 10kg servo? Or do you have another trick up your sleeve ;p I'm trying to gauge if I actually have the right equipment at the moment. I'm doing over 20 different types of flowers so some may require more pull.

KNOT: Is using a bunch of knots on the fishing line a good way to pull with the easy connector? (or make another post?)

(ps: Additional string per petal: FYI - I may also add an additional string to pull a single petal and see if that gives me more strength)

Many Tks!

erco · 2014-10-06 08:45

A better idea is to toss the servo horn and make a small diameter pulley to pull your string. You said knots in fishing line, hopefully you are using nylon leader, not monofilament?

fuichris · 2014-10-06 19:02

erco wrote: »

A better idea is to toss the servo horn and make a small diameter pulley to pull your string. You said knots in fishing line, hopefully you are using nylon leader, not monofilament?

ROUND HORN: I'm thinking of the round horn (see pic)
FISHING LINE: I'm using a fireline (see pic).. i think that's florocarbon which i think is ok (see pic).
Tks!!

erco · 2014-10-07 15:44

Still looks like monofilament to me, but if it works for you, great.

When I use strings in mechanisms, I use braided nylon fishing leader since it's very strong, flexible, durable and is easy to knot. Once it's in place and working I hit the knot with a drop of thin CA glue and it will never come out.

Gorilla (glue) also makes a very strong braided nylon string. We had a spool of it at work which I loved, but I can't find it online right now. Will advise if I find a link.

fuichris · 2014-10-07 23:28

erco wrote: »

Still looks like monofilament to me, but if it works for you, great.

When I use strings in mechanisms, I use braided nylon fishing leader since it's very strong, flexible, durable and is easy to knot. Once it's in place and working I hit the knot with a drop of thin CA glue and it will never come out.

Gorilla (glue) also makes a very strong braided nylon string. We had a spool of it at work which I loved, but I can't find it online right now. Will advise if I find a link.

Tks Erco! The fishing line is a flourocarbon braid so shld be ok...I testing as we speak. Great tip on a spot of glue on the knot. Will do. Tks!!

Here is an update: https://vimeo.com/108051024

Publison · 2014-10-08 08:14

It looks a lot smoother!

I suspect you are loosing some power due to friction at the 90 degree entry to the tube. Can you change the orientation of the tube to get a straighter entry of the cord?

I would also think that over time, the cord would wear a grove in the tube, causing more friction in the future.

fuichris · 2014-10-08 08:32

Publison wrote: »

It looks a lot smoother!

I suspect you are loosing some power due to friction at the 90 degree entry to the tube. Can you change the orientation of the tube to get a straighter entry of the cord?

I would also think that over time, the cord would wear a grove in the tube, causing more friction in the future.

Friction and angle of the string to the servo will be key.... need to figure a way to avoid the friction.
Tks!

erco · 2014-10-08 09:53

+1 to Jim's comments. Either move the servo to make the string pull inline with the tube, or put a pulley there at the 90 degree turn.

Domanik · 2014-10-08 10:46

fuichris,

I really like what you're doing. While watching the video it occurred to me that the rate of movement of the string changes as it passes through the arc of the servo horn. That is the servo could have a constant RPM at the pivot on the horn but the angle of the string constantly changes and therefore so does the speed. A servo_setramp might be used at each end of the cycle. It might also be possible to use a small drum to replace the horn so that the speed is constant and therefore the flowering speed could directly be controlled by a servo_setramp statement.

Thanks for sharing what you're working on, it's really nice,

Dom..

Courtney Jacobs · 2014-10-09 08:20

fuichris,

If your original question has been answered satisfactorily, please mark this thread as solved.

fuichris · 2014-10-09 18:06

Courtney Jacobs wrote: »

fuichris,

If your original question has been answered satisfactorily, please mark this thread as solved.

Hi Courtney... I was just about to do this yesterday, but I couldn't find where to changed UNSOLVED to SOLVED. Can you advise? Is there an edit button somewhere? Tks! Chris

fuichris · 2014-10-09 18:13

Domanik wrote: »

fuichris,

I really like what you're doing. While watching the video it occurred to me that the rate of movement of the string changes as it passes through the arc of the servo horn. That is the servo could have a constant RPM at the pivot on the horn but the angle of the string constantly changes and therefore so does the speed. A servo_setramp might be used at each end of the cycle. It might also be possible to use a small drum to replace the horn so that the speed is constant and therefore the flowering speed could directly be controlled by a servo_setramp statement.

Thanks for sharing what you're working on, it's really nice,

Dom..

Hi Dom,
Great tip!
STRING SPEED / ANGLE: The angle of the string is a BIG issue for me. Actually, the space between the BOX and the FLOWER will be a table. The string will go through the table and into the box mechanism so the angle will be less dramatic. A pully in the box will help to alleviate the fraying of the string at any angle, but not sure where to place it and how. That's another thread ;p I've never tried the servo_setramp before but I'll give it a try...interesting you say that I could control the speed directly by this statement...hmmm. See below the current super simple code I'm using.
SMALL DRUM: do you mean a circular piece (see att pic)
Tks!
Chris

a VAR WORD
Servostart VAR WORD
ServoMid VAR WORD
ServoEnd VAR WORD

Servostart = 800
ServoMid = 1500
ServoEnd = 550

Main:

PAUSE 500

FOR a = servostart TO servoend STEP 1 'goes from 200->1200 one step at a time'
PULSOUT 10, a 'send signal to pin 10, and refers to 'a' variable'
PAUSE 10 'for every step, pauses for 0.015 seconds. 1000 = 1 sec'
NEXT 'must have NEXT after FOR'

PAUSE 10 'pauses for 1 sec (1000) after it completes servostart -> servoend'

FOR a = servoend TO servostart STEP 2 'returning back to the start'
PULSOUT 10, a
PAUSE 10

NEXT

GOTO Main 'repeats again'

Domanik · 2014-10-09 18:54

Chris,

Here's a schematic with the pulley close to the mouth of the tube and a small drum to replace the servo horn (like the wood bobbin from sewing thread).

As the drum turns at a constant speed the string winds at a constant speed on the drum. The pulley can accommodate any angle of the string to keep it centered into the tube.

Dom...

fuichris · 2014-10-09 18:59

Domanik wrote: »

Chris,

Here's a schematic with the pulley close to the mouth of the tube and a small drum to replace the servo horn (like the wood bobbin from sewing thread).

As the drum turns at a constant speed the string winds at a constant speed on the drum. The pulley can accommodate any angle of the string to keep it centered into the tube.

Dom...

Wood bobbin! Good idea!
Attaching it to the 2mm tube will be tricky but it has to be done somehow otherwise the fishing line will fray after 3-4 months. Thanks!

c.

Courtney Jacobs · 2014-10-10 08:29

To change the prefix of your post title:
- Go to your original post in this thread,
- click "Edit Post" in the bottom right-hand corner
- Click "Go Advanced" in the bottom right-hand corner
- Now you should be able to choose a new Prefix in the dropdown menu.

Servo tension - shake

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