jdtl

So I"m building a system that involves a Hitec HS-645 Ultra Torque Servo moving a load near the limit of its ability. My system also runs 2 dc motors (for driving) through a Sabertooth 2x5 RC motor driver (http://www.dimensionengineering.com/Sabertooth2X5.htm). I'm operating using a Spektrum DX5e transmitter and Spektrum AR500 receiver, which is powered through the Sabertooth motor driver. I can control everything just fine if I don't plug in the servo, or if I use it unloaded, but when I load it up, my drive motors won't move. Does anyone know why this is? I'm thinking somehow the servo is hogging the current, but I talked to Spektrum and they said their receiver can control 5 high torque servos pretty well, so I'm not sure why I'm having this problem. I was initially running this off 8 1.5V AA batteries (from CVS), but was told by a Spektrum representative to try a higher quality voltage source (NiMH probably). I got my hands on a 7.2V LiPo (charged up to 8.2 I think), and this really made no difference.



Might also be useful to note that I also have a 3rd DC motor doing other stuff onboard, that's controlled through a FingerTech tinyESC (http://www.robotmarketplace.com/prod...T-TINYESC.html) small single channel speed controller. I can control this motor just fine under any circumstances.

If anyone has suggestions for what I should do, I'd be very grateful.

firehopper

to me that sounds like you need to drive the servo via its own power source. or try a different servo.

the motor BEC may not be able to give enough power and is being close to overloaded.

lejongleur

My Feedback: (10)

Hard to imagine that some AA batteries could possibly support the needs of 2 DC motors and a 133oz/in. servo. It's not so much the unloaded voltage of the batteries, but their ability to supply current under load, and how much (the 'C' rating relates to this.) I agree that a Spektrum receiver can support several decent servos - that's what their meant to do - but this would usually be driven by a BEC that can supply 5-10A continuous at 5-6V - way too much for your flashlight batteries.

I think your power needs are causing a 'brown out', and the solution (or at least an easy test) would be to plug in a more serious battery. At 7.2V (2S lipo), your Fingertech device takes 2+A, you may need another 2+A for your servo(s), plus whatever your motors suck up - let's say a total of 8-10A at least for testing purposes - so a 2S 1000mah 10C lipo might just cover it. 2S 1000mah 20C would be better, or 1800mah 20C - depends how much weight you can carry.

Alternatively, have you tested the servo using a servo tester?

Hope this helps...Chris

jdtl

I haven't used a servo tester nor do I have one/access to one. Your analysis of the current requirement seems to make sense, although I don't have access to a higher C-rating lipo. I'm thinking maybe I can use firehopper's suggestion of using 2 power sources might help solve the current problem. As it stands, the servo gets its power from the receiver, which is powered by the 2 servo pigtails coming off my Sabertooth speed controller, whereas the FingerTech device gets and gives no power to the receiver and just powers its motor directly from the battery. It might make sense then that the speed controller considers the receiver higher priority than its motors, so if more current is required there, it's allowed.

Someone told me I could pop out the power pins from the servo pigtails on the Sabertooth speed controller to prevent it from giving the servo any power. I'm thinking then I could drive my DC motors off of my 12V, and then plug the lipo into into the battery port of the receiver to power the servo. My lipo doesn't have a servo pigtail or anything, but I'm assuming I could just connect it to power and ground and forget about the signal pin. Does this seem sound?

lejongleur

My Feedback: (10)

Because I know almost nothing about the robotic configuration you are using, I can only usefully tell you that you normally do not directly power a receiver (and hence its servos) from a lipo - usually a small UBEC is used, or a BEC that is integral to a speed controller. I see that the spec for an AR500 receiver actually says 3.5-9V is OK, but the actual supply voltage is (I believe) the voltage that is fed through to the servos - in your case, the HS-645 servo requires 4.8-6V (which is normal.)
So I think you need a UBEC and a good receiver battery to run your servo. Oh, and if you want one, you can get a servo tester for $7 plus shipping:
http://www.hobbycity.com/hobbycity/s...idproduct=8296

jdtl

So is the main reason not to use a lipo because its voltage isn't stepped down below 6V for the servo? I'll try to get my hands on a UBEC from a hobby shop, but for a quick fix now, would there be something wrong with draining the battery down to 6V before using (I read some not so good things about using a normal voltage regulator http://www.societyofrobots.com/actuators_servos.shtml). I'm sort of under a time constraint with this project so I don't know if I have time to order something online.

I'm perhaps not doing a great job of describing my configuration, but at present the 5V BEC from my Sabertooth speed controller is what's powering the receiver, and I'm pretty sure now that this is why the drive motors (that get controlled by the Sabertooth) do nothing when the servo's under full load. This makes sense to me because the 3rd speed controller with no BEC and no contribution to the receiver power has no trouble operating when the servo's under full load. This is why I'm thinking I should power the receiver separately.

Thanks again for all the input

jdtl

Or is there anything wrong with connecting a 4-pack of 1.5V AA's to a 2pin plug and plugging that into the receiver as power?

dknovick

Yeah, you can fry a servo if you go above 6V. A 2S lipo, fully charged, would be 8.4V (4.2V per cell). Lipos are considered "dead" at 3V per cell (6V total for a 2S). Going below that could kill the battery. So draining the lipo is out of the question. It's not bad to use a linear voltage regulator, but you'll be wasting power (if you care).

How much current can the Sabertooth provide?


This should be fine. It's how use used to power the receiver before BEC. There used to be a BATT input in the receiver. Double check with yours before you do anything, though.

-Dave

jdtl

The sabertooth provides up to 5A continuous, 10A peak per channel

I got my hands on a 6V NiMH RC battery (~7 fully charged). It worked fine with the problem I had, but apparently my small speed controller (Fingertech TinyESC without BEC), does nothing and I think it's because 7V is too much for it (specs say Receiver Voltage: 2.7-5.5V) especially since it works ok when I switch back to powering the receiver with my 5V BEC off the Sabertooth.

Would it be ok to drain my NiMH battery down to 5.5 or is this approaching killing the battery?

The BATT input is what I started using with this NiMH battery, so it seems like using 3 AAs would be my best bet to make this work, at least for a quick fix.

jdtl

Or what about cutting the leads on the NiMH battery and splicing in a 5V regulator? I'm hesitant about regulators because I don't really want to risk overheating.

dknovick

I wouldn't drain the batteries. NiMH runs at about 1.25V (per cell), and then drop to 0.8-1.0V at 80% (where they are considered dead). If you take your 6 cell pack and take it down to 5.5V, there would be only about 0.9V per cell, and the pack would be just about dead.

How long does this have to run?

Man, you are having to solve a lot of problems. First, I'd try the 4 AA (or C or D?) and make sure everything works.

-Dave