Submitted by shutterdrone on Fri, 07/30/2010 - 01:03.
EDIT: Whoops! You changed the topic on me there, guess I should wait a bit before responding =) -- I'm in the airport ATM, and just happened to notice the topic come up.
Everyone has been using a wide variety of steppers and DC motors, but the ones I use are primarily the Anaheim Automation steppers (I also have some vectric motors, but I do not have a part number for them). The stepper we used in the early dolly prototype was the Anaheim automation 17YPG302S-LW4 with a 19:1 planetary gearbox. I have also used the much smaller 15Y001S-LW4 on my pan and tilt units, but would suggest using the 15Y102S-LW4, as it is less difficulty with the easydriver (better current limit on the 102S).
On the DC motor dollies, we are currently (and intend to for the shipping version) using the Dayton 8.75 RPM Gearmotor from Grainger. We were previously using the 0.45 RPM motor, but found it too slow for most uses.
-end edit
Hi there! To be honest, I don't have anything against servos, but the ones we're usually talking about (hobby servos) are not really designed for use in systems like this. They have two primary weaknesses:
1: They usually don't rotate 360 degrees or more.
2: Their control signals usually limit them to minimum movements of 1 degree
(1) is a problem because we can't effectively use them for linear axes (like a dolly move), as they will rotate less than one revolution. While many hobby servos can be modified to do continuous rotation, at that point they really cease to be servos and are just geared DC motors. In continuous-rotation modified (hobby) servos, if you leave the control interface in-place, you lose lots of speed control. Many I've seen just allows you to say turn right or left, and offer little control over speed (hobby servos are usually designed to go to the requested position at maximum speed).
Regarding (2), hobby-level servos have interfaces designed to move to desired position, at maximum speed (as mentioned above), and usually the input is specified in degrees. So, with a hobby servo with 260' rotation capabilities, there are only 260 positions we can put it in. This is very limited for pan and tilt axes, and generally not usable for timelapse or gigapixel photography unless one is using a very wide-angle lens.
Now, there are industrial servos out there, that can continuously rotate and offer accurate positioning. Effectively, they are geared DC or AC motors with encoders attached. They are often quite expensive, and quite accurate. However, as we note they are basically just DC motors with encoders attached, we can achieve the same end results (and cost, if you like =) by attaching an encoder to the output shaft of the DC motor.
The dollyshield is designed with "servo" support, by providing a set of inputs for encoders. While the firmware doesn't currently support it, it will be added soon, as I intend to create a tutorial for replacing the electronics in the motorized telescope heads a lot of people have been using with the dollyshield. These heads have geared DC motors with encoders for positioning (making them servos, effectively), and it is possible to support them with the dollyshield hardware.
That being said, it doesn't support a traditional "hobby servo interface" where a PWM signal indicates a position in the servo's rotation, but it could be easily modified to do so if one needs.
Submitted by SimplicityGuy on Fri, 07/30/2010 - 06:31.
Apologies for the edit. :)
I somewhat answered my own question, that servos have a 180 degree max limitation in rotation (if left unmodified). And when modified, they are not as precise from what I've read. However, 180 degree rotation shouldn't really be a problem for tilt under typical circumstances (well, a standard tilt head is limited in a similar fashion). And for panning, the 180 degree is somewhat a limitation, but perhaps not a big one.
ServoCity has some nice high-torque servos, such as this one, that I've been considering using for pan and tilt. For linear motion I was thinking of using a stepper.
Pardon my ignorance, but I'm not familiar what the planetary gearbox is used for. Can you elaborate?
Submitted by shutterdrone on Fri, 07/30/2010 - 19:37.
Robert,
Don't forget that the servos only move in 1' increments! (1' between each output video frame.) That's unusable for most pan and tilt operation w/ timelapse, unless you want really fast movements (like high-speed video motion). I've experimented with gearing them down, and even at 1/3' increments, it's still too fast for most TL pan/tilt operations (even at 17mm focal length).
A planetary gearbox is just a gear box with a planetary arrangement of gears (as opposed to worm, or spur gear arrangements). It allows for a more compact gearbox than a spur gear arrangement, but not as compact as a worm gear arrangement.
The gearboxes are used to "gear down" the motors - it has two effects, to slow down the motion, and to increase the torque. For example, that servo you listed has a spur gear chain inside of the case attached to a high-speed DC motor. They use it to achieve the 222 oz/in of torque.
I wouldn't suggest using that high-torque servo, unless you only plan to do real-time video (i.e. not timelapse), for the reasons above in the first paragraph.
Submitted by SimplicityGuy on Sun, 08/01/2010 - 03:18.
Thanks for the answers! This has been very helpful.
I've spent some time reading more about steppers, and definitely agree that they are the way to go. The servos seemed like a good choice at the time since there are many pre-made accessories (like the ServoCity Gearboxes), but there are many drawbacks with the servos.
Luckily I'm still in the planning phase of my project. :)
Submitted by shutterdrone on Sat, 12/11/2010 - 23:44.
Expect to drop the torque down quite a bit unless you want to use 2.8 Amps of power *grin* Otherwise, look great - look very similar to the anaheim motors. Pretty low price though!
Submitted by joaobarros on Sun, 12/12/2010 - 00:29.
You're so right, I somehow missed that...too many stepper specs in front of me right now =)
The Anaheim you're using on the v1 prototype uses 0.8A which is just a notch over the Easydriver 0.75A limit
Would you recommend it for timelapse *and* video?
The problem is buying from Anaheim here in Europe, UPS Customs handling fees and Customs taxes almost double the original price :(
I found *one* Anaheim distributor in the UK and I'm waiting for a quote.
For the record: http://www.totalmotionsystems.co.uk/products/anaheim/index.html
Submitted by shutterdrone on Sun, 12/12/2010 - 16:43.
Joao,
Yes, you can use it for video and timelapse, as you can run it at 1000 PPS (and maybe even higher), presuming you've given yourself enough torque headroom. 0.8 is close enough to 0.75, and running it at 12V lets you adjust down the current requirements a bit.
I totally understand customs! It's a pain in the butt, we're lucky here in the U.S., we don't get charged customs on most incoming shipments! (The Gov. gets its pound of flesh out of just about everything else, however...)
I'd also check and see if they'll sell you just the gearbox, and let you pair it with another motor... That may open up your options a bit!
I don't know why, but the gearbox only option from Anaheim ends up costing 4 times as much as the motor with gearbox option...
(better quality gear box I guess??)
I tried several of the Anaheim steppers with gearbox (I can walk to their office from my house) and finally ended up settling on the same model used in the Dolly prototype v1 here. The others I tried just didn't have the right torque to move my dolly. I also designed my dolly to have quick change motors so I could swap out the Stepper with a DC motor/gearbox from Anaheim and use the MX2 Dolly Engine. :) The downside to Anaheim is the long wait times on some models...
EDIT: Whoops! You changed
EDIT: Whoops! You changed the topic on me there, guess I should wait a bit before responding =) -- I'm in the airport ATM, and just happened to notice the topic come up.
Everyone has been using a wide variety of steppers and DC motors, but the ones I use are primarily the Anaheim Automation steppers (I also have some vectric motors, but I do not have a part number for them). The stepper we used in the early dolly prototype was the Anaheim automation 17YPG302S-LW4 with a 19:1 planetary gearbox. I have also used the much smaller 15Y001S-LW4 on my pan and tilt units, but would suggest using the 15Y102S-LW4, as it is less difficulty with the easydriver (better current limit on the 102S).
On the DC motor dollies, we are currently (and intend to for the shipping version) using the Dayton 8.75 RPM Gearmotor from Grainger. We were previously using the 0.45 RPM motor, but found it too slow for most uses.
-end edit
Hi there! To be honest, I don't have anything against servos, but the ones we're usually talking about (hobby servos) are not really designed for use in systems like this. They have two primary weaknesses:
1: They usually don't rotate 360 degrees or more.
2: Their control signals usually limit them to minimum movements of 1 degree
(1) is a problem because we can't effectively use them for linear axes (like a dolly move), as they will rotate less than one revolution. While many hobby servos can be modified to do continuous rotation, at that point they really cease to be servos and are just geared DC motors. In continuous-rotation modified (hobby) servos, if you leave the control interface in-place, you lose lots of speed control. Many I've seen just allows you to say turn right or left, and offer little control over speed (hobby servos are usually designed to go to the requested position at maximum speed).
Regarding (2), hobby-level servos have interfaces designed to move to desired position, at maximum speed (as mentioned above), and usually the input is specified in degrees. So, with a hobby servo with 260' rotation capabilities, there are only 260 positions we can put it in. This is very limited for pan and tilt axes, and generally not usable for timelapse or gigapixel photography unless one is using a very wide-angle lens.
Now, there are industrial servos out there, that can continuously rotate and offer accurate positioning. Effectively, they are geared DC or AC motors with encoders attached. They are often quite expensive, and quite accurate. However, as we note they are basically just DC motors with encoders attached, we can achieve the same end results (and cost, if you like =) by attaching an encoder to the output shaft of the DC motor.
The dollyshield is designed with "servo" support, by providing a set of inputs for encoders. While the firmware doesn't currently support it, it will be added soon, as I intend to create a tutorial for replacing the electronics in the motorized telescope heads a lot of people have been using with the dollyshield. These heads have geared DC motors with encoders for positioning (making them servos, effectively), and it is possible to support them with the dollyshield hardware.
That being said, it doesn't support a traditional "hobby servo interface" where a PWM signal indicates a position in the servo's rotation, but it could be easily modified to do so if one needs.
!c
Apologies for the edit. :) I
Apologies for the edit. :)
I somewhat answered my own question, that servos have a 180 degree max limitation in rotation (if left unmodified). And when modified, they are not as precise from what I've read. However, 180 degree rotation shouldn't really be a problem for tilt under typical circumstances (well, a standard tilt head is limited in a similar fashion). And for panning, the 180 degree is somewhat a limitation, but perhaps not a big one.
ServoCity has some nice high-torque servos, such as this one, that I've been considering using for pan and tilt. For linear motion I was thinking of using a stepper.
Pardon my ignorance, but I'm not familiar what the planetary gearbox is used for. Can you elaborate?
Thanks for you help!
Robert.
Robert, Don't forget that the
Robert,
Don't forget that the servos only move in 1' increments! (1' between each output video frame.) That's unusable for most pan and tilt operation w/ timelapse, unless you want really fast movements (like high-speed video motion). I've experimented with gearing them down, and even at 1/3' increments, it's still too fast for most TL pan/tilt operations (even at 17mm focal length).
A planetary gearbox is just a gear box with a planetary arrangement of gears (as opposed to worm, or spur gear arrangements). It allows for a more compact gearbox than a spur gear arrangement, but not as compact as a worm gear arrangement.
The gearboxes are used to "gear down" the motors - it has two effects, to slow down the motion, and to increase the torque. For example, that servo you listed has a spur gear chain inside of the case attached to a high-speed DC motor. They use it to achieve the 222 oz/in of torque.
I wouldn't suggest using that high-torque servo, unless you only plan to do real-time video (i.e. not timelapse), for the reasons above in the first paragraph.
!c
Thanks for the answers! This
Thanks for the answers! This has been very helpful.
I've spent some time reading more about steppers, and definitely agree that they are the way to go. The servos seemed like a good choice at the time since there are many pre-made accessories (like the ServoCity Gearboxes), but there are many drawbacks with the servos.
Luckily I'm still in the planning phase of my project. :)
I've found some interesting
I've found some interesting Steppers with gearboxes and they're "cheap"
http://www.active-robots.com/products/motorsandwheels/bipolar-stepper-motors.shtml
Anyone had any experience with these?
I'm thinking of getting one of these:
Europe: http://www.active-robots.com/products/motorsandwheels/NEMA-17-Bipolar-26-8-1-Planetary-Gearbox-Stepper.shtml
US: http://www.phidgets.com/products.php?category=23&product_id=3311
shutterdrone, any comments? :)
Expect to drop the torque
Expect to drop the torque down quite a bit unless you want to use 2.8 Amps of power *grin* Otherwise, look great - look very similar to the anaheim motors. Pretty low price though!
I'd say go for it =)
!c
You're so right, I somehow
You're so right, I somehow missed that...too many stepper specs in front of me right now =)
The Anaheim you're using on the v1 prototype uses 0.8A which is just a notch over the Easydriver 0.75A limit
Would you recommend it for timelapse *and* video?
The problem is buying from Anaheim here in Europe, UPS Customs handling fees and Customs taxes almost double the original price :(
I found *one* Anaheim distributor in the UK and I'm waiting for a quote.
For the record: http://www.totalmotionsystems.co.uk/products/anaheim/index.html
Joao, Yes, you can use it for
Joao,
Yes, you can use it for video and timelapse, as you can run it at 1000 PPS (and maybe even higher), presuming you've given yourself enough torque headroom. 0.8 is close enough to 0.75, and running it at 12V lets you adjust down the current requirements a bit.
I totally understand customs! It's a pain in the butt, we're lucky here in the U.S., we don't get charged customs on most incoming shipments! (The Gov. gets its pound of flesh out of just about everything else, however...)
I'd also check and see if they'll sell you just the gearbox, and let you pair it with another motor... That may open up your options a bit!
!c
I don't know why, but the
I don't know why, but the gearbox only option from Anaheim ends up costing 4 times as much as the motor with gearbox option...
(better quality gear box I guess??)
I tried several of the Anaheim steppers with gearbox (I can walk to their office from my house) and finally ended up settling on the same model used in the Dolly prototype v1 here. The others I tried just didn't have the right torque to move my dolly. I also designed my dolly to have quick change motors so I could swap out the Stepper with a DC motor/gearbox from Anaheim and use the MX2 Dolly Engine. :) The downside to Anaheim is the long wait times on some models...