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Author Topic: Four Servos - External Power Wiring and Hardware  (Read 1251 times)
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Hi,

I am building a vehicle that will have 1 micro roller switch, 3 continuous servos and 1 normal 90 degree servo. How do I wire it to the arduino UNO (do i need a breadboard?) including the external power supply?

Also, will a 4AA battery pack be enough power to operate all the servos at the same time? and do I need a separate power supply for just the microcontroller?

Any input would be greatly appreciated as I have never used an arduino before and am unsure of the hardware and configuration required

Thanks
« Last Edit: April 15, 2013, 06:28:23 pm by rgsp » Logged

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A breadboard's a convenient way of getting it figured out, but for permanence you might like to look at transferring the whole thing to stripboard. For simple configurations, even terminal strip can do the job.

AA batteries might not give you enough current if you hit bad-luck timing and all motors need full current at the same time. A member here, retrolefty, reckons on 1A per motor at stall and it would be a heavy load if all motors wanted an amp each at once. So you could consider having more than one pack.

Then the problem is that 4xAAs giving you only 6v is marginal for the Arduino which wants 7+ at the barrel jack. You could regulate that 6v upwards with something like one of these and still take the 6v to the motors.

Wiring of the motors is simple btw: red and black from the battery, yellow from whatever pin you use for its signal on the Arduino. Arduino power from wherever: eg the regulator as mentioned. Join the Arduino ground to the motor power ground.

If you didn't already do so, read up on the servo library here but disregard the part where it says you can power one servo from the Arduino: always give the motor its own power. Look at the sketches "sweep" and "knob" to see how the coding works.

Lastly, you didn't say what the switch is for, so can't comment on its wiring.

(Btw since you are brand-new to Arduino, you should read Getting Started, work through some of the Tutorial Examples and probably have a look at the foundations page)
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Cheers- the foundations page is helpful.

the roller switch is just a normal input.

would you suggest 7+V or what about 2 AA battery packs (2 servos per pack) and 1 9v battery for the UNO?

what is the best way to get the rwuired voltage and ampage to each component (reliability is important)
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"3 continuous servos and 1 normal 90 degree servo" - if these are standard servos,
meaning ~ 44 oz-in devices, then a 4 AA pack should power them just fine. Best to
use NiMH rechargeable AA batteries, as these have 2200-2500 mAh of energy, and
can be recharged many times, of course. The actual current reqs of these servos will
be more like 300 mA than 1A. OTOH, if you use 200 oz-in servos, then that's a lot
more current.

Hook up your AA pack, and see how long the servos will run for. Some years ago, I bought
a Hitec R/C transmitter/receiver bundle that came with 6 or 8 "standard" servos, and
has a measly little 600 mAh NiCad pack to power them.
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Those 9v batteries have very limited life.... but may be ok for running just the Arduino which might be drawing very little current itself. That would be as much of a cost decision as a technical one!

I've seen AA packs that take 5 cells though. I can't recall where.... one site has a single cell "plug in" for a 4 pack to make it 5.... 5 would give you 7.5 for the Arduino.

Regarding voltage and current per device.... most product sheets will tell you both. Servo sheets seem only to give you voltage, and that's usually 4.8 to 6. You could put your ammeter in series with a servo's power supply and measure the current drawn: make sure you give the servo a realistic mechanical loading, eg by holding the horn to stall it.

For the switch, assuming you want some kind of off / on detection? There's an example of how to read a switch in the tutorials... get the idea of pull-up / pull-down resistors clear... they're explained in the reference under pinmode, I think.
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The current into servos is really rather noisy, and I doubt an ammeter will make a
very good measurement.

1. all kinds of spikes, ripple, pseudo-DC levels, and variations.

2. in addition, the current waveform is actually a nice pulsetrain [each pulse being 2-8
    msec long] when the servo is "not" moving, and which transitions into #1 when the
    servo is moving.
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The continuous servo has the details:
Size:40*20*40.5(servo)
Weight:60g
Wire length:320mm
Speed: 0.14 sec/60 degree at(6V)
Torque: 15kg.cm.at(6v)
Voltage limits : 4.8v-8.4v
Dead Zone setting:4 Microseconds

That torque is equal to about 208 oz-in. I wanted very high torque servos as 2 of these will be driving wheels with a large diameter. The other one is a winch where high torque is required.
The normal servo has a torque rating of about 180 oz-in.

Using rechargeable batteries is not an important factor for me as this vehicle will be used in a competition so it will have a realtively short life.

@oric_dan "if you use 200 oz-in servos, then that's a lot more current." so what amps is that per servo and what would you suggest to power this vehicle, given the new details?

@JimboZA i have attached a picture of a 5 AA battery holder which i can get from a local store. Using alkaline AA batteries, would this be suitable and would I need 2 of these to power the 4 servos and the chip?

If you need more details let me know

Edit: I have not purchased anything yet as I wanted to investigate before purchasing, so i cannot test the current drawn

Thanks - I appreciate the help


* 5AA Battery Pack.jpg (131.49 KB, 591x502 - viewed 24 times.)
« Last Edit: April 14, 2013, 01:50:09 am by rgsp » Logged

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Quote
@JimboZA i have attached a picture of a 5 AA battery holder which i can get from a local store. Using alkaline AA batteries, would this be suitable and would I need 2 of these to power the 4 servos and the chip?

Well a previous post said AAs should be ok, but remember that 5 is 7.5V which is too high for the servos. So maybe a 4 pack for the servos and 5 for the Arduino?
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ok i think ill go with the 9V battery for the UNO chip and then a 4 AA battery pack for the 4 servos. If I find that that all four cannot run from one battery pack then I can get another one (add it to the circuit) and run 2 servos off each battery pack.

Does this sound feasible?

Also do i need to have anything else apart from; the micro switch, bread board, wires, battery holders, power cable to 9V battery and servos? in particular, resistors and transistors etc?

In regard to the roller switch, it will be engaged in the vehicles driving configuration, it only is needed to sense the edge of a table (the switch will disengage when it rolls of the edge).
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As indicated before, 208 oz-in servos are gonna draw a lot of current, well over 1A each,
I imagine. The currents will vary as the torque varies over a range of 5:1 or more.

However, I imagine the NiMH AA-cells will still provide enough power for short periods of
time. I believe NiMH will push more current than alkaline, but you should check this. I'd also
go for the 5-cell pack over the 4-cell pack in this case, as it will last a lot longer.
 
Just try it and see how it goes. That's how you learn how these things work.
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sorry for all the changes but upon doing more research, i came across high voltage and capacity 18650 li-ion and li-po batteries. The are rated at 3.7V and 3000mAh.

I plan on getting 4 of these cells in two different battery holders (2 cells per 2 servos) to power the 4 servos. This will mean a voltage of 7.4V to each servo.

Can anyone point out any flaws in this design? (the arduino chip will be powere by one standard 9V battery)
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Running multiple lithium batteries in series or parallel introduces some balancing issues that can be quite dangerous if you don't deal with them. If you're using this type of battery I suggest you ought to be using a known reliable solution to manage the batteries, and for cells in series that's not trivial; you can't just treat those like NiCd, NiMH etc batteries.
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ok i plan on using 4 3.7v cells but a maximum of two per servo.

How would you suggest i manage them? Is this the same as voltage regulation?

A link to some info or a wiring diagram would be extremely helpful

Cheers
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