RC Electrics
Bob Kopski, 25 West End Dr., Lansdale PA 19446
THIS COLUMN offers a possible breakthrough in motor battery management, and reactions and clarifications to the Sport Speed Control (MA 9/99 and 10/99).
"Breakthrough" is a very strong word, so I offer it here with some uneasiness and some caution. But I suspect this story will be new—and very meaningful, in a dollars-and-cents way—to most readers. This story actually begins years ago.
I fly with Don, who owns and flies my prototype REVOLT! Two or three times a year I bring his REVOLT! home to service or maintain something, and when I have it I usually cycle the transmitter and motor packs. Having done this for two to three years, I began to notice that Don's 10-cell 1.7Ah SCRC pack, which I made for him, always performed "as good as new." This was quite troubling to me, because all my packs of the same vintage deteriorated into the "false peak" behavior syndrome.
Refer to the January 1999 column, where I showed this "false peak syndrome" as actually measured on a motor battery pack. Packs may develop a "funny" characteristic, where immediately upon application of charge, the terminal voltage begins to decrease from a rather-high initial value, instead of increasing from a normal lower value.
This declining terminal behavior can last in excess of five minutes before the pack voltage begins to rise and appear otherwise normal. This behavior confuses some peak-detect chargers because they immediately "think" the pack has reached its peak-charge condition.
When a pack is truly peaked, the terminal voltage begins to decline, and this is the cure for chargers to shut down. For packs with the "false peak" behavior, peak-detect chargers shut down prematurely and often—usually beginning a few minutes after charging starts. The reference graph and column text clearly show this.
The reference also shows that this condition usually goes away after the first agonizing charge/discharge. The second and subsequent charges of the season appear normal, but "false peaking" begins anew after the pack has not been used for a while. A "while" can be hours to days, depending on the pack.
Friend Don flies in an unusual manner; he prefers "shorter" flights and never empties a pack on any flight: he recharges the batteries and flies again. Having observed this and knowing his pack history and behavior, I began to wonder about a possible connection; now it seems that Don may have unwittingly made a significant contribution to E-aeromodeling.
Having made this observation, I chose to explore the idea of a possible cause-effect by modifying the BEC (Battery Eliminator Circuit) portion of my then-evolving Sport Speed Control circuit design to incorporate what is now called the "motor cutoff function." This was nearly two years ago.
Any BEC speed control used with aeromodels should include the feature of automatically shutting the motor down before the battery gets low enough to put the receiver/servos at risk. For most BEC-equipped speed controls this is about 5.6 volts. No matter the number of cells in the motor pack, the typical speed controller will stop motor operation when the pack reaches this lower "receiver-safe" voltage level. If it is a six-cell pack, a cutoff around 5.6 volts does not permit the pack to go deep into discharge. However, for packs of greater cell count, by the time 5.6 volts is reached, at least some cells go into "deeper discharge."
In the more-general case where BEC is not used, deep discharge or even reversal of some cells can occur as one simply tries to stretch the last seconds of flight from any motor pack—a "natural" tendency for most aeromodeling humans, I believe—except Don!
I modified the early SSC BEC function to make it adjustable—to permit motor cutoff to be set for any number of cells, whether I had the BEC in use or not. So far I've used the ratio of 9 volts per cell.
For example, for a 12-cell pack, I set motor cutoff to 10.8 volts using the procedure given in the SSC article. In this example, the SSC begins to retard motor power when the pack drops to 10.8 volts, will not let the motor run the pack lower, and never lets the later pack get into "deep discharge" so long as the pack has reasonably matched cells.
I am pursuing this subject in-depth now because I have just (8/99) passed the nominal one-year mark of outfitting several airplanes with new packs and SSCs adjusted as above. To date, not one case of false-peak syndrome has occurred. Prior experience has "false peaking" happening in just a few months.
To me this is a possible "breakthrough" situation—a costly problem resolved with a "fix" already included in the SSC design! I do not know of any other electronic speed control (ESC) that includes such a feature, so manufacturers may want to look into this for future designs.
For now, if you want to minimize your own battery pack jeopardy, muster some considerable personal piloting fortitude, always resisting all temptation to sustain flight to the bitter end (of your pack), or dig out the September and October 1999 issues and let the SSC become your personal "strength!"
Let's all thank Don for leading us in the right direction! By the way, the SSC's motor cutoff function does not "fix" packs that already display the false-peak behavior. It appears so far that it prevents this behavior from developing in new packs. For how long? I don't know—yet.
As expected, the Sport Speed Control has brought in a variety of reaction.
The SSC article described a motor pulsing at cutoff—a "warning" sound of sorts. (The motor does not stop "cold turkey.") This pulsing will be of different rate and sound depending on many variables, including the particular pack behavior, motor, gears or not, the covering on the airplane, etc. Some pulsing sounds are very noticeable; other installations have barely audible sounds. In any case, you will notice the power falloff—time to ease up on the throttle and bring 'er in.
The "Sport" in Sport Speed Control is really suggesting that this may not be the best choice for competition purposes. The article describes it as a larger and heavier than contemporary product; surely one wants the smallest and lightest for contest work. And the brake is "delayed and soft." I'd guess "instant and hard" is what's needed for brakes where trophies are to be had.
Using preprinted holeboard for the Signal Board has brought positive and negative reaction. Some really like the idea because of ready availability; no messy etching needed, and no drilling. Still others prefer a custom pc board for easier assembly, while another questioned the reliability of holeboard. The latter concern deserves some comment.
Holeboard, otherwise called "protoboard" or "perf-board," has been around for at least four decades, is available in many varieties from many vendors, and is a proven, established way for the electronics industry and hobbyists alike to build circuits in limited quantities. In fact, as I write this, a coworker is preparing to demonstrate a developmental control circuit (built on holeboard) in a customer's full-scale aircraft.
Reliability is far more associated with the quality of workmanship (especially soldering) than the medium of construction—be it pc board, holeboard, hard-wiring, or combinations of all. My coworker is having his circuit assembled by trained, qualified solderers for this reason. No matter what you build on whatever medium, good soldering is a must.
I have eight SSCs in use right now, with parts on hand for six more. I wouldn't put so many (if any) of my airplanes at risk with an "unreliable" assembly!
A few have expressed real interest in the SSC but said that it looked too difficult to them. But look carefully at the published Assembly Sequence. Note that each and every step is very simple. Just take one step at a time—even if you choose to spread the work over several days.
For those who just want to blast ahead, I just finished another Signal Board following the Assembly Sequence in a single three-hour, 17-minute sitting. Most wings take longer.
Another suggestion: Buy the parts and build the Signal Board first, delaying the more-costly Power Plate stuff until you see how it goes. You may find what others have with electronic projects such as the End Of Charge Beeper, Servo Cycler, etc.; it's really not so hard!
To the best of my knowledge, the Sport Speed Control offers more features than any other controller. It does have all the familiar "me too" stuff (BEC, brake, high rate, etc.), but also has the unique motor cutoff circuit discussed above, a nonlinear transfer curve, and a minimally interactive two-screw setup to match your throttle stick signal range.
The nonlinear transfer curve is something that's more difficult to describe than to experience. It makes the throttle stick much more smooth and comfortable—and meaningful. Do you know that the normal "linear" ESC applies less than 20% of full power at so-called "half stick"? Not so with the SSC; see the associated graphic in Part Two.
I have many ordinary ESCs on hand, but I no longer want to fly with linear controllers—they just don't "feel good" anymore!
As for the "minimally interactive adjustment," anyone who's tuned up an ordinary two-screw analog ESC knows the frustration that can be experienced in this process. The idea is simply to fit the controller response to the throttle stick throw, but the usual problem is that the two screws interact—a lot—so the process can take a good deal of patience.
Not so with the SSC. The "lo" and "hi" screws are called just that—no confusion—and they work smoothly and are mostly noninteractive. Just follow the instructions: set one, then the other, and go fly. Only the most fussy may revisit the first screw after setting the second.
So far, those who have built and flown one SSC now have two and/or parts on hand for more! Please note three typos readers have discovered: in the Schematic, IRL3705 should be IRL3705N (like in the Parts List); in the Parts List, under Semiconductors, the column titles "Catalog #" and "Part #" should be switched; and 1N5822-ND should be 1N5822CT-ND in the just-corrected "Catalog #" column.
If you have continuing questions or concerns, please write. I stand firmly behind the design, buildability, and functionality of the SSC. For this or any subject, please include an SASE with any correspondence for which you'd like a reply.
Sincere best wishes for a happy holiday and a great Electrifyin' New Year. MA
Transcribed from original scans by AI. Minor OCR errors may remain.
