Servos controlled by microcontroller

Hi,
im completely new to electronics so please bear with me.

i am trying to select some high torque servos (4) to control with an mbed microcontoller for use in a pair of robotic legs.

i was initially going to use dynamixel rx-28 servos, but they would be difficult to control with an mbed wouldnt they?

i am now thinking of using Hitec 5990TG servos. would they be a more simple option? could they be easily controlled with this servo library
mbed.org/cookbook/Servo

also, what sort of power supply would be needed for 4 of those.

any comments / suggestions would be appreciated

thanks in advance.

What’s an mbed? Maybe you should ask your question on their (?) forum?

Alan KM6VV

It’s an oversized stamp that is C programmable. Yes the 5990 can be controlled with the servo library you linked to. The power required for for of those servos is dependent on how much work they are doing. I would recommend 3-4amps. The mbed forum may not have servo savvy people. You are welcome to post here all you want. :slight_smile:

thanks, i appreciate that :slight_smile:.

do you mean 3-4 amps for controlling all 4 at once, or for each one. what is their approximate current drain for average normal use.

could you recommend a battery pack that should handle all 4 at once?

Again it depends on how much work they are doing, but I would think that would handle all 4 of them.

It will be much easier to find a suitable battery. How long do you need it to operate?

not particularly long. maybe an hour or so of non continuous use. operating time is not really an issue

maybe something like this?
a123rc.com/goods-139-RC+Batt … +trex.html

i really have no idea what to look for

It would be far too much for what you need !

It’s a 40 C, which means it can deliver 6*40 = 240 Amps !!!
And with a 5 amps draw (if you include all electronics and considering the badest situation) you will get around 72 minutes of operation.

You will pay less if you take something in the like of 10 C (generally the kind of battery used in receiver, but I don’t know if there are 5000 mAh in this range).

You could try to go for something like that :

hobbyking.com/hobbyking/store/__14077__ZIPPY_Flightmax_2500mAh_6_6V_5C_LiFePo4_Receiver_Pack.html

It’s a LiFePO4 6,6V 5C 2500 mAh battery pack, and check the price, you could go for 2 of them.
With a lower voltage, less risks of destroying the servos.
LiFePO4 are much safer than their big brothers Li-Po.
5C will mean able to handle 12,5 Amps discharge current.

EDIT :

To reply to your other post, there are mainly 4 types of battery used nowadays.
In robotics, the most commonly used was Ni-Mh because they are last long proved technology and easy to use, there is no risks to use them.
But, they are weighty and take a HUGE amount of time to be recharged (last generation is a little better with charging time of around 2 hours, whereas old ones took at least 8 hours). Also, they can’t handle fast discharge.

In comparison, you have the Li-Po (lithium-polymer), the most powerful batteries on the market, capable of handling hundreds of Amps. They are the lightest, with a power density up to 200 Wh/kg. And they can be charged at 2C, which means they are fully charged in 30 minutes. But, their biggest drawback is that they are quite “tempered” ! :mrgreen:
You can’t do what you want with Li-Po, they must not be overcharged or overdischarged, each cell should be balanced within a specific tolerance. And so you will need a special charger and a special controller between battery and electronics. (don’t be alarmed, the last generations are a lot more stable than the previous ones)

The third ones are LiFePO4, another technology based upon lithium. They are not quite so powerful as the Li-Po, but their cell voltage is lower, so less dangerous for the servos.

There is also the Li-Ion used mainly in small electronics appliances (like notebook, cell phone, cameras, …) but I don’t have any experience with them.

thank you for your reply, and i apologize for my delayed response.

so if i were to go for 2 of the zippy flightmaxes you posted, you think that would do the trick?

a few questions:

-does having 2 just mean you double the operating time as compared to just one?

-roughly what sort of operating time would 2 of those give me?

-how do you calculate the operating time from the battery specs?

thanks a bunch!

hobbyking.com/hobbyking/stor … _Pack.html
or would 2 of those be suitable?
that size would be much more appropriate for my needs

Excuse me for being late at the reply but I’m really busy working on R&D for the space industry ! :mrgreen:

So :

-having two mean doubling the capacity and hencefore doubling the operating time
-the two of them would give you : 2*2500 = 5000 mAh and if you consider drawing 5 amps continuously, it gives you :

5 Ah/5 A = 1 hour !

(as simple as that ! :smiley: )

The battery you mentionned is the highest quality grade available. So it can handle fast charge ( 15 minutes for full charge).
The only drawback is the price : 32 $ against 9 $ ! :blush:

Thank you very much :slight_smile:

thats a big help.

Hey Jimnoy,

Just wanted make a point of clarification. When these guys are telling you to use two batteries there are two ways you can do that. There’s a parallel configuration and a series configuration. A parallel config is additive for the mAh content, meaning 2500mAh+2500mAh at 6v. A series config is additive for voltage meaning 2500mAh at 6V+6V=12V. The series would be very harmful to your servos. Here’s a little pic to help out.
batts.JPG
Not the greatest drawing, I’m on my netbook and have only a tiny trackpad, but it should illustrate my point.

Hope this helps!

EDIT:
I checked the specs of that servo. It says that it’s able to handle 7.4V and that higher voltage gives you more torque. So, I might recommend this hobbyking.com/hobbyking/store/__9437__ZIPPY_Flightmax_5000mAh_2S1P_20C_hardcase_pack.html. It’s a decent battery at a decent price. Easily capable of handling the 22A spike that could happen if all the servos stall (not likely, but possible). And it rids you of worry about two batteries instead of one. Also, do you have pics with dimensions of the legs you intend to use these servos with? And a speculated weight of what they’re supporting? These are quite expensive and extremely heavy duty. It’s possible that these could be overkill depending on your application.

thanks for that marcham.

i have already bought the batteries - i had a pretty small size restriction so went with the one i linked to as it’s more compact.

i dont think the servos are likely to stall and the higher voltage is probably not necessary as i had a pretty large safety factor with my torque requirements.

cheers again for everyones help