HELP, Sometimes I Lose Control? Dual 55T, 14.4V, Novak SR

[4RnrRick]

PLEASE HELP ME FIGURE THIS OUT....



So I wired everything up like I shown in the above schematic and I've also made a battery plug that is just a jumper so I can unplug one battery so I can run 7.2 V or 14.4 Volts. Well I've ran into a problem.

On 7.2 Volts everything seems to work good but sometimes when I run 14.4 Volts if I get the RC bound up and flexed hard I loose all control of the sevros and ESC. It doesn't take off or anything like that. What I mean by that is the steering servos for no reason turn all the way one direction and the led on the ESC goes out like it lost all power. Basically like you turned the RC on with the the transmitter off.

If I flip the switch on the ESC off and back on about 75% of the time it fixes the problem, the other 25% of the time is does the same thing again. Eventually it will return to normal. The ESC is slightly warm to the touch but not hot by any means. Everything else is just room temperature.

I've tried plugging in a 4.8V battery pack into the Batt terminal on the RX and that doesn't fix the problem. It still does it.

I do have some minor glitching also but I've installed the recommended .1 uF capacitors on the motors which seems to have helped the problem but not cured it. BTW the transmitter uses AM frequency.

Any ideas on whats going on?

[Grizzly4x4]

Well Rick, I have not resonded to this because I have been thinking about this alot and I made a discovery last weekend.

As many people know I had alot of problems this summer with my Super Rooster just completely shutting down, and it would be useless for a couple days. Then for some reason it would werk again.
I always attributed this to using dual digital servos, a 5945MG rear and a 5645MG front. But the thing I overlooked was the motors. When I was having the shut down problems I was running Magnetic Mayhem's but I never tried running the servos through the BEC with the lathe motors.
This last weekend at Jason's I tried running my SR (same one I had shut down issues with), 55T motors, and the dual digital servos with the servo's pulling through the BEC. (I disconnected my voltage regulator.)
Well, it werked flawlessly. The servo's seemed a little weak but my SR never shut down.

How is this related, you may ask. Well I think that it has to do with current draw. Seems to me the Mag Mayhem motors draw more current than the Integy's in the same situation.
For example, if the tires are bound up and I give it throttle and both servos are trying to hold position. The Mag Mayhem's will draw more current to get the tires to spin because the Integy's make more torque that is why my ESC shut down with the Mag's.
So, in your case, I think the ESC freezes up because you are running 14.4V to the ESC. Voltage (V), Current (I), and resistance (R) are linearly related, V = I x R.
If you double the voltage and the resistance stays the same (same load on the motors) the current doubles. So maybe the ESC is shutting down due to current draw just like mine did with the Mag's.

First off, people are gonna say "but the SR is rated for 320 amps!". Yes, I know this. But for some reason my Mag's were shutting it down even though I doubt they were pulling more than 320 amps. Also, that rating is when the mosfets are at room temperature, not after they get warm from crawling.
I can not explain the reason for the shut down but it sounds to me that the problems you have are similar to mine because yours werks fine on 7.2 volts but shuts down on 14.4V.

My advise, try to get hold of an EVX to see if the problem goes away, or get a Novak Super Duty XR.
Why does Dave not have problems and you do? How can bublebees fly...I can't tell you but that's the way it is.

That's my thoughts, hope it helps.

[4RnrRick]

Griz - Thanks for the reply! I was wondering if anyone would after 40 views..... Sounds like a good theory......

I played with it ALOT last night and the thing that kept confusing me was no matter what I did with 14.4Volts it would shutdown eventually. not immedately but it could be anywhere from 10 - 90 seconds of playing with it if the ESC started at room temperature. It would also almost always shut down when the crawler was totally flexed out. my only idea there was there was additional load on the servos which was cauding them to draw current. There was also times That I could possition it so it was okay but if I hit the throttle it would shut down OR if I tried to turn it would shut down. then with it in that position I plugged in a 4.8 V battery pack and it would do the exact same thing! but if I put it at 7.2 volts I didn't have ANY problems. I crawled through almost 1 battery pack at 7.2 volts last night with no shut down problems but then I tried it back at 14.4 Volts and couldn't even get up the first minor obsticle without it shuttign down.

So then I tore into it and rewired it to this diagram.



That BultacoJim recommended in this post:
http://www.rccrawler.com/postt4894.html

He also posted some info here about doing the same thing:
http://www.rccrawler.com/postlite4156-servo.html


Well at first it didn't work because I wired it to the positive side of the ESC but once I got it to the negative side of the ESC battery wires it worked FLAWLESSLY at 14.4 Volts! I drained (2) battery packs probably 1/2 way down at 14.4 Volts and never once did it shut down!! So doing this seemed to fix it.

But now I have another problem. See This is wht first itme I was actually able to play with it at 14.4 Volts for a long period of time on difficult obsticles. Wiring these lathe motors to 14.4 Volts gives you incredible wheel speed and really good low end torque but it makes it difficult to crawl SLOWLY with it at 14.4 Volts. If I leave it at 7.2 Volts there is time that I'm wishing for more power because it seems I can stall it out if I get bound up hard. But I can modulate the low end speed pretty good for climbing a vertical face. Any ideas? (calling SR5Dave.... and others)

[SR5Dave]

My EVX was a hair weird at times, but it was fine 99% of the time. Maybe its time to head over to my RC guru and ask him whats happening. (He's in his late 60s, been in the sport forever)

I know Ace said not to worry about breaking in your motors, but with the 14V its going to make a LOT more punch than with 7.2, and will want a better connection between the brushes and com. I have found, in my experiences; to be essential to use rx packs, or else my junk will run like crap, but I've never run a SR.

Also, if you haven't already, cut off your power switch, solder the wires together, and tape it up. my power switch has caused more problems than anything

And Griz, thats some good advice there Can't deny math. I'm surprised the SR is unhappy with it though.

Rick, my suggestion is to cut off that power switch, and run an RX pack, but with a twist that the other guys will know more about than me.

Griz, you should know more about this since I just hack shit together.. Isn't there a way you can run a battery pack in line with the servos? Like you would w/ a 1/4 scale servo.

Ooh, another good idea.... Could it be that you're powering TWO big servos off ONE plug? Rick, run the rear locked out and unplug that servo, then see what happens. (Cheap lockout: Tighten the knuckles down real hard, and just run a tie rod, I did it for a year or so with minimal problems)

EDIT: looks like I was a second late and you did just what I was trying to suggest in that last paragraph!

[4RnrRick]

Dave, I know you typed your post before you read mine but I just wanted to re-iterate this.....

I have found, in my experiences; to be essential to use rx packs, or else my junk will run like crap, but I've never run a SR.

Running a 4.8V RX pack made NO difference, it still shut down!

Also, if you haven't already, cut off your power switch, solder the wires together........

I don't see what benefit I get from that. With a RX pack the switch doesn't do anything and remember it still shuts everything down even with a RX pack if I'm running 14.4 Volts.

Rick, my suggestion is to cut off that power switch, and run an RX pack,

Again like I said in my very first post, running a 4.8 V RX pack does fix the problem.

Ooh, another good idea.... Could it be that you're powering TWO big servos off ONE plug?

Probably, Rewiring the servos to draw power directly from one battery seems to have fixed the problem....

[4RnrRick]

But now now onto the next subject........


See this is wht first time I was actually able to play with it at 14.4 Volts for a long period of time on difficult obsticles.

Wiring these lathe motors to 14.4 Volts gives you incredible wheel speed and really good low end torque but it makes it difficult to crawl SLOWLY with it at 14.4 Volts. It just had too much RPM to a slow creep even on the first iteration of the speed control. and thats with 9 tooth pinoins.

If I leave it at 7.2 Volts there is times that I'm wishing for more power because it seems I can stall it out if I get bound up hard. But I can modulate the low end speed pretty good for climbing a vertical face.

Any ideas how to get that low end modulation back but still have that high torque? has anyone tried something in between 7.2V and 14.4 Volts - Say 9 cells at 10.8 volts? Or could you tweak the trigger on the transmitter somehow? (calling SR5Dave.... and others)

[Grizzly4x4]

Your solution sounds similar to mine as well Rick. With my Mag Mayhem motors a 6V RX pack fixed my problem. Basicly you have done the same thing, you made the servo's pull juice from a source other than the BEC.

I wonder if a 6V RX pack would have fixed your problem. I wouldn't think a 4.8V pack would help if you are still running 6.0V through the BEC as well. However I think your solution is a good one.

I was going to try my Integy's on a 8 cell pack (9.6V). How do you feel about building battery packs? Maybe Integy's on a 8 cell pack would be a good compromise. Maybe run dual 4-cell packs in series.

[4RnrRick]

Well I've never built a battery pack before. But I was thinking that I could take the outer shell off one of my packs and just solder on some conections mid way thru the pack to see what happens. but I'm kinda leary about trying that with my NEW battery packs. also with how I've designed my chassis, I'm mounting the batteries on the axles themselfs, so it really designed for Dual 7.2 packs....

Is there some other type option? or has any already tried this?

[SR5Dave]

As for the power switch; the power switch wires have the least insulation and are the smallest of any. Very often power switches will try to hit your chassis, causing twitches, get hit and try to fall off, causing your rig to act like crap, and if you are stuck on an obstacle and the motors are floored, you will build up internal resistance which will just nuke your switch and wires, and melts all the insulation off, meaning you have to open it up and solder on new wires and switches.

BTDT! Never again will I touch a power switch!

And I know you ran a RX pack, I was just saying to anyone else reading that it may help them.

[SR5Dave]

Originally Posted by CLODrock

i know exactly why, the super rooster is only used for 6-10 cells, you are using 12, to much voltage

You'd think, but the type of amp the SR uses is rated to like 20+ volts. The only reason they say that, is because if you take a motor that isn't made to run on too many volts, you'll start causing HUGE problems, and it will basically short circuit the motor, which you can imagine is bad for the ESC.

So its a limit they put in to try to ensure the user does not pull too many amps from the ESC. The only limit that matters in a speed controller is current, and about 24V ;)

[BultacoJim]

4RnrRick
I’ve been standing in front of my old Bridgeport designing and building a new crawler chassis and just now saw your post. Even if I did, I would have referred you to the same post that you found on your own. Looks like everybody and you got it figured out.

SR5Dave has a point. Those crappy little BEC switches will get oxidization in their contacts and create high resistance and arcing. Some people never have problems, but I like to run in the snow and moisture is always causing problems for me. Do like Dave & replace it with a straight wire or use one of these small connectors. http://www2.towerhobbies.com/cgi-bin...?&I=LXKX41&P=M Just use the outside two of the three pin and that is your On/Off switch.


CLODrock is right, the SR is only rated for 12 volts. This ESC regulates your motor speed much like a switching power supply does and too much voltage will cause it to “crow bar”. http://www.kbt-dc-supplies.com/crowbar.php . A good circuit designer can keep it from destroying things (which Novak has done), but the voltage has to be removed before things will return to normal.

SR5Dave: Yes, the current that an ESC has to dissipate is the most important factor, but the MOSFETs
http://www.mtmi.vu.lt/pfk/funkc_dari...tor/mosfet.htm used in ESCs are P-channel and N-channel devices that react to voltage differences applied to them. The drain current increases linearly with the applied drain-to-source voltage. Like Grizz said, V=I x R
Change one value and it effects something else. That is why you are on your own when you go beyond the specs published by the designers. Not that it won’t work, but just keep your fingers crossed that it works like you think it will.

4RnrRick
A SR uses Pulse Width Modulation (PWM) to control the current sent to the motor(s). Drive PWM Frequency is the frequency at which the duty cycle information is being sent from the speed control to the motor during forward drive (How many times-per-second the motor is being cycled ON and OFF to control its speed). At maximum speed, the duty cycle or pulse duration of the pulses is longer and therefore provides maximum current to the motors and maximum efficiency.

At partial throttle, an ESC becomes less efficient and ends up turning battery power into heat. As a result, the overall speed and run time of the vehicle is reduced. The result is a drastic reduction in performance at partial throttle. That is why you want your gearing setup to reduce (increase torque) your “working” speed to at least 2/3 throttle. This gives you a little “turbo” in reserve for the vertical stuff.

One benefit of the SR is that it uses Polar Drive Technology, the drive transistors are driven in such a way that losses during partial throttle are reduced to a minimum without sacrificing radio range. Using this same technique on the brakes increases the recharge to the battery pack, allowing longer run times.

But, it is still pushing the design envelope of the SR at low speeds using 14.4 volts. Dissipating the excess heat from 14.4v at low speeds is causing the poor performance at low speeds. The higher voltage is causing the MOSFETs to become super saturated and produce thermal noise or thermal run-away. It can also cause what the RC world calls "glitching", which you discovered.
For our engineering students: http://kabuki.eecs.berkeley.edu/~rsn...EE231/proj.pdf

Remember, the Specs say 10 cells or 12 volts. You get good top end with voltages greater than 12v, but you pay for it at low speeds. Almost like that “Hot” cam everybody was putting in their cars when I was growing up a long time ago.

Try Grizzly’s suggestion. The lathe motors were really designed for a max of 12 volts. 9.6 volts might be a good compromise. My new chassis is being designed for two custom 4-cell packs to keep the weight and physical length to a minimum, yet have more voltage than 7.2V for my lathe motors. I’m still experimenting though. And like SR5Dave said, running the lathe motors on 14.4 volts mean you have to break them in properly to get the brushes to seat properly. Otherwise you’ll get lots of electrical noise from the arcing and that can hose up your Rx.

So my 2 cents worth of advice is to either reduce the voltage to 12 volts or less or get a Super Duty XR $$$$$

Keep us posted, if your crazy little chassis works better than mine, I’m going to kick myself for spending so much time on mine.

[4RnrRick]

BultacoJim - Thanks for the detailed reply!

So has anybody else tried something in between 7.2 & 14.4 V for a Lathe motor...........