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16 Servo's — Parallax Forums

16 Servo's

GreenGiant83GreenGiant83 Posts: 43
edited 2006-09-07 21:11 in BASIC Stamp
I am thinking about using my Basic Stamp 2 to run between 12 and 16 standard servo's.· I dont know the ratings right now, but they are roughly equivelant to the one that comes with the kit.· Is there any problem for me hook up them up straight to Vdd, Vss and one pin each?· I would be sending their PWM pulses consecutively :

PULSOUT 0, val0
PULSOUT 1, val1
PULSOUT 2, val2
...
PULSOUT 15, val15

I dont want to fry my chip. I am also wanting to use a wall converter to supply the 9volt to the chip, since I will be wanting to run these servo's more or less constantly for a few hours at a time.· I think I read somewhere they using those wall converter type things is sometimes a bad thing.

Comments

  • allanlane5allanlane5 Posts: 3,815
    edited 2006-09-07 15:01
    A couple of issues.

    1. The Servo signal is a 1 to 2 mSec pulse, repeated every 20 to 50 mSec. You CAN drive 16 servo's, give a 8 mSec pause, and drive them again. This can work.

    2. A servo takes about 100 mA or more when it's 'moving'. 16 x 100 mA == 1.6 AMPS. 1.6 AMPS is a lot of current -- most wall-warts max out a 1 Amp (1000 mA). Most regulators max out at 1 amp, too, so trying to draw 1.6 amps through it will definitely cause it to over-heat and shutdown.

    #2 above means you'll have to design some way of powering all these servo's -- two wall-warts, driving 8 servo's each would work. Make SURE you tie all power supply grounds together, as the Servo signal won't work otherwise. A stock BOE board only has one linear regulator, so can't supply all that current by itself.
  • GreenGiant83GreenGiant83 Posts: 43
    edited 2006-09-07 18:09
    So there is no problem sending the pulses straight from the stamp's pins.· The problem your saying is that there is not amps from a normal wall power supply to run· up to 16 servos at once under load.· I need atleast 1.6 amps at whatever voltage the servo's are rated for.· I think 5-15 is typical?· I know 5 volts comes from the stamp and I have no problem running two servo's on that.· I drew a mock up schematic(i use the term loosely).· Would you take a look at it, and tell me if I am understanding you correctly?
    Schematic.gif
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  • Mike GreenMike Green Posts: 23,101
    edited 2006-09-07 18:57
    I would not connect the wall-warts in parallel since they may differ slightly in output voltage. There is no problem for the Stamp to produce the power for the servo pulses, but the Stamp can drive only one servo at a time. The maximum pulse width is about 2ms and the servos have to get pulses every 20ms. That leaves time for only 10 servos and no processing time for anything else. I would suggest using a Parallax Servo Controller which can drive all 16 servos simultaneously with control of speed and speed changes and leave most of the Stamp's processing time available. I would also suggest getting a power supply rated at maybe 2 amps. Jameco has several switching "wall-wart" regulators in their catalog for $10-$14. There should be no problem running them for hours since you'd be well within their rating.
  • allanlane5allanlane5 Posts: 3,815
    edited 2006-09-07 20:06
    Alright -- see the attached.

    If you do as you've show, you MUST have +5 or +6 volt "REGULATED" wall-wart power supplies. "UNREGULATED" is much more typical, but you'll have to add 2 7805 Regulators, with input and output capacitors for both, if you use UNREGULATED supplies.

    And yes, the Parallax Servo Controller (PSC) is much better suited to this sort of thing.

    Post Edited (allanlane5) : 9/7/2006 8:14:39 PM GMT
    BS2_Multi_Servo_Power.gif
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  • GreenGiant83GreenGiant83 Posts: 43
    edited 2006-09-07 20:52
    I am hoping to not have to spend another $40 on the controller. I am actually only planning on using 8-10 servo's right now, I just figured if I was gonna ask, I would go all the way.· I dont know how much digital potentiometers cost, but i was thinking of using a potentiometer in conjuction with a capictor and a 556 IC.· I could modify the value of the potentiometer whenever i needed to and the 556 would just keep sending the pulses even while stamp was busy with other things.· Any thoughts on that method?
  • Mike GreenMike Green Posts: 23,101
    edited 2006-09-07 21:11
    That sounds like a good way to do it. Physically it's much bigger and it's a lot of wiring, but it's fairly cheap to do. The DS1844 is a quad 64 tap digital pot in a DIP package for about $2-$3. You can get dual pots with 256 taps for about the same price. I'd still go with the servo controller.
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