Hi all. You may remember that my daughter has been working on a project in college - some of you had some input early on. She is building a "mirror" that will eventually be composed of about 900 servos. Each servo, the team decided, will turn a polarized disk in response to a video camera signal going into a computer.
The degree of turning of this disk will create shades of gray - and thus a low res version of the video image will be reproduced by these 900 disks. (The entire mirror will have a polarized piece of plastic on its face.)
This is what the test mirror looks like today.
The question at hand is this. How to provide wall outlet power to the mirror?
Currently she hopes to figure out how to power 2 Polulu microserial servo controllers, each on attached to 8 servos (Hitek microservos). Thus 16 servos in all.
Eventually, as I mentioned she needs to move up to powering about 900 servos (30 pixels by 30 pixels is the final resolution of the mirror).
Currently she is using a 9 volt battery for each of the controllers and a 4 cell (4.8 Volt) nicad battery to power the servos connected to the controllers.
Anyway, any help you can offer regarding what approach she might take to move away from the batteries and to the world of wall outlet electricity would be most appreciated.
That's a pretty neat project. It must be art, since I don't see a purpose, but it's still pretty neat art. :)
Just get a 5V power supply from Radio Shack (or your preferred electronics dealer), I would think. Actually, when you're powering 900 servos, you might need something able to handle a higher current than the typical wall warts can do -- something more like a lab bench supply, which is quite a bit more expensive. A good electrician can often adapt a power supply from an old computer, and get the same effect for considerably less money.
On the other hand, when you're prepared to drop a grand in servo controllers and close to ten grand on servos, a few hundred bucks for a lab bench power supply is chump change.
I love Pololu products, including the micro serial servo controller -- I have one myself, which I'm using on my 4-servo walker:
...but, in all honesty, when you're dealing with this many servos you should by using the Lynxmotion SSC-32 controller. It's by far the cheapest controller, at a little over $1/servo. (She'll still need 29 of them, so she might contact them directly and try to negotiate a quantity discount.)
Lynxmotion engineers also hang out on their forums (at lynxmotion.com) and I bet would be happy to provide advise on this project, especially on the best way to power 900 servos and 29 SSC-32s.
Basically, you'll need a bunch of power supplies. To keep things efficient, you'll want the output of the power supply to be around 6v.
The servos are going to consume most of the current. According to the spec, the HS-81 is the least expensive and draws 220 mA (no load operating).
So you're looking at 1/4 amp per servo. That's about 225A assuming all of the servos can move at one time. If you can stage the movement, then you can reduce the current consumption.
The cheapest power supplies are going to be something like 400w PC power supplies, which are essentially commodity items.
You can pick up surplus stuff sometimes, like these guys have a 5v 100A supply
Tigerdirect.com has a 400 watt supply that's rated to give 5v @ 40A for $35.
You can probably run 3-4 of these from a single AC circuit.
I'd recommend getting something like the LynxMotion SSC-32. It's only $39 and can control 32 servos. This will be cheaper per servo than the Pololu servo controllers.
Then you have to figure out how to get 30 of these connected together. Instead of using full blown RS-232, you could connect all 30 RxD lines together to a single TxD line from the PC, and broadcast to all of the nodes. You could modify the protocol (the SSC-32 has full source code available) and precede each command by a node id.
If you use the TTL version of the serial port, you could even connect all of the Tx lines together providing you disable the TX portion of the UART when not transmittings and configure the pin as a GPIO input.
Anyways - that should give you some points to ponder....
I guess that you've been through all this, but I do wonder about all those servos - its going to be noisy for sure. Since what you want is 90 degree movement and the load is negligable, I'd go for the cheapest motor I could buy, mount the disc directly on the shaft and place a return spring and a couple of end-stops. Then the deflection is proportional to (average) current. The current only flows in one direction, so a simple transistor switch driven directly from a micro port would be about as minimalist as you could get.
Power supplies and distribution for a project like this are things not to be taken lightly - large currents can easily create common mode 'noise' that will cause chaos.
Its a fun project, but don't underestimate some of the engineering challenges ...
Power is probably the easiest part. I'd use a large 12v lead acid battery with a ~10 amp auto battery charger connected to it all the time as the base supply. To power the servos (which should actually have little load on them), I'd use a 7805 voltage regulator chip (rated for 1 amp) with a diode on the ground lead to supply ~5.7v output for the servo. As the servos will be using little actual current, you probably could have several servos powered via each regulator chip. The below page has a simple schematic showing how I do this with my servo setup. As the student design is some what of a clone of the bottom circle mirror, my achedemic expectation would be for the students to be more inovative and significantly reduce the number of servos and servo controllers used to have a significant reduction in the cost of the project (cost reduction will be the name of the game when they graduate).
Yes, that's correct. But, if you use a single supply at 162 amps (from calculation below) you will need very large wires running from the supply to your distribution panel. Doing some research on the net, it looks like you something like 4 gauge wire to do 150 amps. I don't even know where you buy that size wire.
Good question. I just looked up the spec of a standard servo (Hitec HS-311) and it says it draws 180 ma at 6V with no load (your disks are probably the same as no load).
900 servos times 180 ma is 162 amps. That's very high and that doesn't include whatever your control boards will add to that. You should try to measure the current draw on the servos you are actually using to see what it is. You should consider using the lowest power servos you can if you aren't already using low power servos. The lowest current draw servo I could find was 100 ma. That would be much better at 90 amps total but those servos cost twice as much as the standard servos which draw 180 ma.
A high wattage PC power supply (420 watts) can deliver 42 Amps at 5 volts. So you would need 4 or 5 of those if you could run your system at 5 volts. PC power supplies are at least cheap.
162 amps at 6V is 972 Watts. A single 15 Amp outlet can go up to near 1500 watts so it's possible. But it depends on the efficiency of your power supply (or supplies). I have no idea how efficient they are so I don't know if it will work or not (my guess is they are fairly efficient). I think it will probably work but it's close. If you servos use more power, or the control boards us a lot of power, you might not be able to run it on a single outlet.
Ah, I just found a spec on a PC power supply that said it was 70% efficient. Which means if you need 972 watts for your load, the power supply will be sucking 1388 watts from the wall - which is near the point that you will trip a 15 A home circuit. So it could be close.
If you take care to limit servo power, you should be able to make it work (you might need to write the software to limit the speed of the servos or limit how fast they are all turning at once to limit the max power.
If you intend to run it in commercial buildings (like a school) it will probably have 20 amp circuits which will give you a bit more room to work with.
You should hook up an amp meter to your prototype and see how much current it's drawing and then do the math to see what happens for 900 servos.
Basically, power supplies in that range are special beasts that will cost you a fortune (I couldn't find any searching the net - the highest I could find were 100 Amp and those were very expensive). You are probably going to have to split up the load and use multiple power supplies because I doubt you will find anything big enough to run your entire system at a reasonable price.
A previous post suggested you use a 12V battery and a "high current" 10 amp charger. If you need 162 amps, an "high current" 10 amp charger won't even be close. A car battery can deliver many 100's of amps but not for very long.
Sometimes, but I don't recommend it. Servos are electrically noisy, and in my experience often cause the controller to reset. Life is less painful when you power the controller from a separate power source.