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servos - analog or digital — Parallax Forums

servos - analog or digital

David BDavid B Posts: 592
edited 2011-05-31 15:03 in Robotics
I have a project that needs a dozen or more individual motions, and I'm trying to keep it inexpensive, so I started looking around for any and all cheap motion devices - steppers, servos, muscle wire, relays, pneumatics, solenoids, whatever will do the job.

I've got some stepper experience, but only have a few steppers in the junkbox, and even with salvaged steppers, buying lots of controller chips quickly gets expensive. So I'm looking more at servos.

I'm a total servo beginner. I have an old Vex robotics servo, have played around with a couple of OBEX servo programs and have gotten it to work, but that's about the extent of my experience.

I went to the website for Hobbytown (a local hobby store) and see that they offer both analog and digital servos. They show a small analog servo at $9, which is about as cheap as I've seen around.

Is anyone here familiar with the analog/digital distinction in servomotors? Even the Parallax store doesn't say anything about analog/digital in its's servo offerings.

Which sort do the OBEX servo programs operate? Or can they operate either?

Does anyone know about this stuff?

thanks.

Comments

  • prof_brainoprof_braino Posts: 4,313
    edited 2011-05-31 11:43
    A guy at the Chicago robotics club got 20 of these at $2.77, they are not the most powerful but they might be the cheapest
    http://www.hobbypartz.com/topromisesg9.html

    Analog servos are not so accurate and don't do "extra" functions as digital servos might (position reporting, software reversable. etc).

    The servo thing is all about the control protocol. Every 20ms , you send the servo a pulse of 1.5ms +/- 1ms (aprox). 1.5 ms pulse commands the device to the "center" position, increasing the pulse duration commands it to positions in one direction, less than 1.5 ms commnds it to positions in the other direction. Each servo is different, but the range is usually less than 180 degrees, and at some point "hits the stops" until you modify it or "bash through the stops"
  • David BDavid B Posts: 592
    edited 2011-05-31 13:04
    They look good - for that price, for my needs, those cheap little servos look perfect. Thanks.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2011-05-31 13:16
    As I understand it, analog draw more current the farther the way they are from the goal position. Digital servos draw full current even if they are very close to the goal. Digital servos use more power and are faster (so I've been told).

    If also read it's not a good idea to use a digital servo as continuous rotation servo (I read this somewhere in a Parallax tutorial, website or book).

    I know at least some of the digital servos can receive refresh pulses more frequently than 50Hz.
  • ercoerco Posts: 20,256
    edited 2011-05-31 13:24
    If you're starting from scratch and on a budget, it would be hard to go wrong with a bunch of cheap analog servos and a dedicated servo controller. Parallax' own 16-servo PSC is nice, see http://www.parallax.com/StoreSearchResults/tabid/768/List/0/SortField/4/ProductID/595/Default.aspx?txtSearch=servo+controller and Pololu's Maestro series is quite nice; I have the 6-servo Maestro Micro and it is works very well. Nifty software, full control. Here's their 12-servo unit: http://www.pololu.com/catalog/product/1352
  • David BDavid B Posts: 592
    edited 2011-05-31 14:06
    I just ordered 10 of the cheap servos from HobbyPartz.com.

    Those servo controllers do look pretty handy, especially for the sockets, since making so many physical connections can get so awkward for those of us that are somewhat mechanically challenged.

    It sounds like digital servos will behave like analog servos when receiving a standard servo drive signal, but give you the option of increasing the pulse rep rate for a bit faster control.
  • ercoerco Posts: 20,256
    edited 2011-05-31 14:22
    You can do slightly better on price on Ebay if you want to wait for shipping from China. Here's 10 9g servos shipped for $23:

    http://cgi.ebay.com/10X-Micro-9g-Servo-RC-Futaba-helicopter-Trex-450-SG90-/320669312759?pt=Radio_Control_Parts_Accessories&hash=item4aa96166f7

    I have bought several, and they work fine for non-critical apps.
  • schillschill Posts: 741
    edited 2011-05-31 14:34
    David B wrote: »
    It sounds like digital servos will behave like analog servos when receiving a standard servo drive signal, but give you the option of increasing the pulse rep rate for a bit faster control.

    Depending on the digital servo, you also pick up some additional functionality (if you buy a programmer). I'm talking about servos that are controlled the same way analog servos are (and not things like the Dynamixels).

    For instance, you can often set the max and min allowed angle, max speed, etc. These are typically done before using the servo in your application and not changed from within the device/robot/etc at run time.

    Digital servos often have higher maximum torques and more precision (but they don't have to).

    I have used digital servos in applications where I wanted to set the angle range to prevent damage and not rely on the code running on the device controlling the servos. I have also used them when I needed more torque. They can be a lot more expensive so I usually just use analog.

    The digital servos I've used are all from Hitec and the programmer is relatively cheap ($20-$30). The last time I looked the programmers for other servos could get pretty expensive.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2011-05-31 14:57
    shill, I've often wondered about how the digital servos are programmed.

    I'm pretty sure each brand of servo has it's own programing protocol and you can't interchange programmers, right?

    Does your Hitec programmer connect to the computer?

    I suppose it should be possible to listen in on the signal line as a servo is programed and figure out the protocol. That way your robot could change the servos programing on the fly. I think this would be more work than it is worth though.

    Thanks for the extra information.
  • David BDavid B Posts: 592
    edited 2011-05-31 15:01
    I like the "Similar Threads" at the bottom of this forum page; thanks to them, I can see that I'm not the first to ask this! Duh! Gotta start using "advanced search" a little more.

    A couple of people mentioned this site, with lots of information -

    http://www.societyofrobots.com/actuators_servos.shtml#digitalanalogservos

    I don't mind $30 that much as compared to $23; they're both so much better than buying 10 of a $14 motor!
  • schillschill Posts: 741
    edited 2011-05-31 15:03
    Duane Degn wrote: »
    shill, I've often wondered about how the digital servos are programmed.

    I'm pretty sure each brand of servo has it's own programing protocol and you can't interchange programmers, right?

    Does your Hitec programmer connect to the computer?

    I suppose it should be possible to listen in on the signal line as a servo is programed and figure out the protocol. That way your robot could change the servos programing on the fly. I think this would be more work than it is worth though.

    Thanks for the extra information.

    I believe this is the programmer I've got:

    http://www.hitecrcd.com/products/digital/digital-servo-programmers/hpp-21.html

    It connects to a PC by USB. A simple program on the PC is used to setup the servo.
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