Finished the latest InsideHeli yesterday. Comments!
=== Made in the U.S.A. ===
This is just classic. I’m a Californian, so you can guess that I rarely ever see the “Made in USA” tag (except maybe on some clothing and those classic wooden toys). Frankly, I just don’t give it much weight anymore. Trying to buy only American is like trying to buy all your groceries at the Farmer’s Market (there’s a lot of stuff that America just doesn’t bother to make in-house in this global economy). Just buy from the companies who actually care about quality and customer support.
=== Digital Servos ===
Once again, Lucien was thorough and accurate! Admittedly, I’ve only paid the money for a few digital servos for myself, but I’ve worked with the industrial actuators quite a bit at work. It’s fascinating to put the motor commands on the scope while testing movements.
To my knowledge, the better digital servos do not necessarily go “full power” with any movement, but the voltage command is definitely extremely sharp compared to an analog servo. There still should be a dead-band, or you’d risk killing the motor (with zero deadband, the motor would continuously twitch back and forth…you know, like those cheap Spektrum servos).
As a controls engineer, I could bore you guys to tears talking of the pros and cons of analog versus digital actuators (er…servos). Like any new technology, the benefits have limits. You’ll be a lot happier with a high-quality analog servo than a poor-quality digital servo. A digital servo with a poor-quality potentiometer will be twitchy and buzz a lot. If the controller has no dead-band or does not use a decent command curve, it may burn out the controller pre-maturely. Also, some servos handle stall conditions better than others: a bad one will just strain at full power until it smokes.
Analog servos tend to vary the motor power depending on how far they are from the “goal” position. If the servo only needs to move a little bit, it powers the motor only a little. The problem is that if your control surfaces have any friction, then you get slow responses and memory (where the servo never quite returns back to center). Analog servos are still a bit cheaper, don’t torture your BEC, and usually have a longer lifespan since the motor and controller aren’t slamming at full power all of the time.
If you’re flying an airplane with smooth-working control surfaces, you’ll be hard-pressed to feel the difference between an analog and a digital servo of the same quality. Sometimes I think the claims of improvement are mostly the placebo affect. If your control surfaces have a bit of friction yet demand precision (biplane 4-wing ailerons comes to mind), then you’ll get some benefit from how a digital servo aggressively applies whatever power it takes to follow your movement.
Helicopters are an entirely different matter. EVERYTHING fights those servos. That spinning swashplate and frantic tail-rotor are constantly pulling and yanking in every direction as that beast of mechanical chaos twists in the air. Digital servos (GOOD digital servos, that is) are a real benefit that you can feel.
So, I robbed my planes of the digital servos to upgrade my other helis. They all have separate BECs anyways.
=== Linear BEC trouble ===
I’m not sure if Lucien specifically mentioned this or not, but one problem with the built-in linear BECs in ESCs is that they do not put out constant voltage. The voltage drops under load. So, it’s not so much that the BEC quits or fails when the servo load gets high, it’s that the voltage drops to a point where the newer receivers suffer a “brown out.” Those of you in California with our failing electric grid know what I’m talking about.
Switch BECs will but out constant voltage no matter the load. That’s what makes them so great. However, there is a slight frequency wave from the switching that can’t be entirely eliminated with the capacitor. That wave can interfere with the receiver and some gyros. Castle Creation had to update its BEC to be compatible with the new Spektrum receivers. So, if you get some no-name BEC for your heli and start having receiver or gyro problems, you may want to consider slapping on a small 4-cell NiMH receiver pack for a flight and see if the problem goes away. If it does, then you need to go shopping for another BEC.
That’s it from me this week. Time to fly!
- Jim