I just though the replys were a bit much, but it could have just been me, no harm, no foul. Sorry… whistles casually, take a few steps back…
I just couldn’t resist. It seams like a lot of the “super servo” problems are due to trying to keep everything in the tiny standard servo case. I figured the performance will increase dramatically if a “real” gearhead motor is used, thinking outside the box, lol.
Np really, sorry again. Can’t wait to see that tracked ROV going!
Hmm. Why do my worst posts give the best feedback ?
I guess my comments were not all approporiate to this thread.
Though badly expressed in my post, I do stick to my premise that safe operation and reliability is a design parameter, that we be adjusted in accord with other objectives, and I prefer that to be in my control. As a consumer and engineer, I too was very upset when I started losing servos a year ago. I had not factored healthcare costs into my budget. Maybe myself and fellow robot users are too soft, but frankly it is still great value hobby at 20% per year approx… Sure as an engineer and consumer I still feel affronted by this high failure rate, but it does not worry me like it did. Maybe best to leave further thoughts on this to a less technical thread.
On the stepper Vs. Servo, looks like I was wrong. As soon as I saw the example you posted, I wanted to build a robot with them. I think your math is wrong, and these are still slightly less capable than the servos, but they sure are contenders, and have changed my view on steppers. Will someone build with these and give us some information on their dynamic capability.
I apologise for any marketing bias in my posts. I just feel that Hitec are getting quite a bashing, and I already find them rather conservative. I fear they will become more so. They appear reticent to release the programming cable for the HMI servos, maybe, because this will enable users to make unsafe settings on the servos. Indeed, I also am cautious to release the full HMI programability of the 5990 for this reason.
I do have to agree with other posts that the lack of accurate specification and communication from Hitec has made the situation worse. Thanks for this forum to fill the gap.
On more engineering issues, I did check the 5990 overload protection, and it does indeed power off after 10 sec of overload. I need to do further tests to determine if say a 50% load powers off after 20 sec etc. Also the 5990 does not actually measure current, like the open servo. The HMI read current value is actually the PWM value to the motor, and I guess that is the overload value too.
The heatsink is well thermally connected to the motor, where I still think any problems lie. The processor does not measure actual temperature as far as I can tell.
Kondo Servos have a feature called ICS on their red servo’s. On the earlier bots this feature was visible but not functional.
On the recent releases it is there in all its glory and boy can you reck them if you wanted to.
I shy away from it as I know very little about them but from the settings I’ve seen as default, everything is hiked right up to the limit.
Not a good place to start playing
Ribbotson, I enjoy reading anything you have to say, so please don’t feel like you have to hold anything back.
I think everyone here is capable of having a lively discussion without anyones feelings getting hurt.
Carry on… 
the nagging feeling is quite possibly that I hand waved the gearbox, ignored efficiency, and generally rounded the numbers to make the math more obvious. like I said I banged out a quick reply. 
Anyway, stepper based servos still have to deal with heat, the electronics are more complicated not less, there are mechanical issues not obvious like resonance, and you have similar issues with a relatively small field of suppliers addressing the small form factor and high energy density motors with quality products. I think that as you scale an application up its best efficiency will see-saw between dc motor servos and stepper servos a bit and then stay with dc motor servos up until you start moving towards pneumatic and hydraulic designs as you get really big.
You can pretty much forget the stepper motors for the types of bots generally described here. The steppers are used where precision positioning is the main requirement and not force. I mentioned it in another thread that I think a lot of the expensive servos are sold on hype and not necessarily improved performance. The real key to keep from burning out the servos is to design such that the servos are not under constant strain.
Below is an interesting design which limits the amount of force that the servo has to supply while holding the weight of the bot. It has some other potential issues, but might be worth a look.
yea i was thinking 200oz.in. is probly the med for longitivity versus torque for a standard size servo ,those super servos are amazing but there too hot for most hobbiest i mean it wont take much of error in coding to break something , if u pop the clutch and floor a race car and its in neutral it goes boom while you could put a brick on the gas pedale of my pinto while u tuned it 
but like some body said already if u buy them and moan about all the flaws they,ll build a better one next time,
P.S.
I cant see how a stepper motor could,nt have more potential power(its a bigger motor!! )dont quite know how u would package it but sounds like a winner to me 
The main problem with steppers is to achieve the same amount of power they have to be much larger. The motors in servos gain their power through gearing down the output speed of the motor. Steppers do not achieve the same rotational speed of a dc motor and consequently would require much less gearing to achieve the same 180 degree movement a normal servo provides. Less gearing produces less power.
Steppers do not have a built in method of positioning as a servo does. A stepper requires you to generate a known number of pulses and it will move that amount. You would need additional external rotational tracking to equal a servos accuracy.
Also, like servos, steppers suffer from stalling. You can just apply a high speed series of pulses as the stepper will immediately stall. The pulse rate has to be ramped up and ramped down to achieve the higher speeds.
To complicate this, unlike a servo, steppers can slip. If you apply a sufficient force you can actually hold a stepper stationary. Without adequate rotational tracking you will never know the stepper stalled or slipped. One possible advantage of this may be that it would act as a form of clutch although be it only momentary.
A stepper can achieve what a servo does, even the super-servos but they cannot do it in the same compact form. They become much larger and complicated. There are also very few steppers on that market that are suitable for running off a battery. Most stepper driver circuit require the use of a multitapped toroidal transformer in the power supply.
One extremely good implemetation of steppers I have seen is to fit the stepper with a worm gear which in turn drives a conventional gear. This has an inherently high gear ratio and has the advantage that because the gear is at 90 degrees to the worm rather than in the same plane it is impossible for any amount of force on the gear to drive the worm the wrong way. It would just shear teeth and completely protect the stepper motor itself…This method is used a lot in Lego to achieve a good ratio with high power.
Thanks for all the informative posts guys.
You know what peaves me off the most. I stuck an HS-645 into the elbow so I could continue my testing and I have been using it without a problem at 6 Volts. I have done 45 minutes of continuous movements and numerous 15 minute testing sessions. No problems with the HS-645.
Like I said, the servo wasn’t under much load. And the HSR-5995 was way overkill for the application which is why I chose it. I wanted to have an abundance of additional power on reserve. The HSR-5995 I have on the base is starting to get very hot as well. I might as well pull it out and put something else in before it fries too.
So far my $30 servo is out-performing my $130 one in longevity. Go figure.
I dont remember the exact words but I’m reminded of Bladerunner when the doctor is explaining to Rutger Hauer about short but very bright lives…
LOL
I kind of get the feeling that applies here.
Paul, um I don’t know what kind of stepper you are thinking of here but the kind of stepper drives used in DC applications like a battery system use two h-bridges with independent chopper drives driving two sinusoidal current sequences offset by 90 degrees. when they add torque compensation it distorts the sinewave profile but I forget the equation. Intelligent Motion Systems (as an example imshome.com ) makes many different hybrid microstepping stepper controllers that operate over many different voltages and currents. Lin Engineering themselves (the stepper motors I referenced earlier) make a number of small drive controllers and have partnered with (or own, not sure really) Trinamic Microsystems to make their own microstepping ASICs, both with and without integrated MOSFETs. I guess it seems easy for some folks to just dismiss the potential of stepper based solutions based on old experiences or quick glances but this field like so many others is really benefitting from newer technologies in both semiconductors and materials science. I would tend to agree that in the form factor of a standard hobby servo you are not going to find a stepper based solution yet, but holding a position with a constant torque is really brutal on a DC motor over time. As you move up the size scale of a project there will be several size ranges where for the same money and power you can do better with stepper based servo designs than dc motor based servo designs, particularly if the durability of the servo motors themselves factors heavily into your requirements / specifications.
exxxxxactly,the 5995 is like a 645 with a blower,its not suited for all apps we just needed to know its limitations before we bought it but it is the ferrari of servos ,
and to compare steppers to servos u have to remember u can gear stepper motor down too!! and add pots for positioning again ITS A BIGGER MOTOR a bigger more powerful motor with the same gear ratio as a servo would have to be more powerful,But its gonna be BIGGER
I was about to have the same problem w/the hsr5995tg… About to, because thanx to these posts there’s no way I’m gonna use these servos for the arm I was building - plexiglass and pine, sprocket and .25 chain - I was still making the parts. Now I’ll rethink it all. What I would like to ask of you is if someone here knows of a standard sized high torque servo that’ll stand up to a reasonable amount of abuse without frying. You see my plan is to put this arm on a mobile platform made of pine with a PC brain (Via Epia Mini ITX), 8 in. touchscreen VGA, Speakers, Mic, Camera, etc… Using Lead Acid batts I was hoping to get this monster to have at least an hour autonomous function. It just wouldn’t do to have the arm set fire to the bot after 3 minutes of use. Maybe I’m just a dreamer but I’d like to make a bot capale of doing something practical - at least bring me a beer and sandwich from the kitchen or fetch the remote control…
So I got 5 high torque plastic bricks, a large investment with a very low potential of return but at least I didn’t have to burn anything and who knows maybe I’ll find an acceptable use for them. (paperweights, christmastree ornaments, etc)
Thanks again
Jim
Hi Guys,
sorry to interrupt but be careful with the 9958 MOSFET! I was looking after some info how to fix my Hitec 5645 servo and ended here.
I’m using in my power boats as a rudder servo. It seems Hitec is using the 9958 chip for many servos, like 5625, 5645 and based on the this topic for the 59xx series as well. I have never tried the servo with more than 5 NiMH cells (assuming the receiver will not survive that).
Here comes the issue: there are TWO different kind’a 9958 mosfet. One manufaytured by Vishay the other made by Fairchild. But these two are NOT the same. Please check the link you attatched earlier and will see that one product is a complementary (P and N channel) mosfet the other is P channel only. Since this is my 1st notice I cannot attach here links but enter to google 9958 + mosfet and you will get Fairchild’s homepage immediately for the dual P channel mosfet.
You will see there that it rated for 2,9 Amps, voltage is way much higher than the other electronics would appreciate in the servo circuit.
The complementary refers to the link mentioned earlier. If you would try to fix it at home (part worth less than a buck) first clarify what you’ve got in you servo.
I assume all has the dual P version however unsure. I haven’t checked yet only trying to get some valuable info.
I’m not making robots but 99% positive that you can overload this chips easy. While the robot is squezing something or lifting heavier things it might happen. The chip will not be damaged in case of peak loads but surely will when the load is permanent. I burned my servo when the rudder arm was blocked and the servo is tried to get it out. Even if the 5955 pcb looks different, the result is similar than in my 5645 servos. Only one chip burned, the other still ok. Will figure out how to fix it and let you know the result after.
Take care,
Hici
Thanks for the informative post. If you figure something out, let me know, I’ve got a couple that were “killed in action”, that I would like to revive.
