project pattern
C
Redux
ontinuing with the programming of the electronic speed control (ESC) for Yuri, the remaining pa- rameters to be set are for the be- havior of the brake. How the braking func- tion works is one of the most misunderstood operations of the ESC. The brake function is not a governor (at any rpm or throttle posi- tion); it is not active when the motor is run- ning (at any rpm); it is only active when the throttle input pulse to the ESC is below the “run” threshold. When set for a low idle, or freewheel (no power to the motor and no ac- tive brake), the prop rpm on a typical Pat- tern airplane can “spool up” substantially in a downline.
Additionally, after exiting a long down- line, the lower end of the throttle stick will have a variable amount of “dead zone” until the rpm settles. Dead zone meaning adding throttle (from idle) will not result in addi- tional airspeed until the increased throttle catches up with elevated rpm (from spool up in the downline), or the rpm decays to match the throttle position. With a properly set up brake, the downline spool up can be mini- mized, allowing more constant downline speeds, and a reduced amount of dead zone. The amount of spool up (and dead zone) is much greater on direct drive motors in com- parison to geared motors, owing to the me- chanical drag of the gearbox.
For Castle Creations ESCs, the motor will run when the input pulse is >1.2 milli - seconds, and the motor will either freewheel or the brake will engage (if programmed) when the input pulse is <1.2 milliseconds. The run threshhold for Jeti ESCs is >1.1 milliseconds. With YGE and OS/Futaba ESCs, the run threshhold is set using a throttle range calibration procedure. When the brake is active, the ESC is es- sentially blind to the armature position due to “noise” of the magnetic fields generated
by dave lockhart with algirdas ungulaitis You can reach Dave Lockhart via e-mail at
davel322@comcast.net
PHOTOGRAPHY: DAVE LOCKHART
When properly set up, an ESC brake can be very effective at limiting prop “spool up” in downlines, and make it much easier for the pilot to present a constant speed pattern.
during braking. The brake function works by effectively shorting the motor wires to- gether for a variable amount of time (contin- uously at 100% strength). This effect can easily be observed. Take a motor with bare leads, and spin the motor. Then, pinch (short) a pair of the motor leads together, and spin the motor; the resistance to spin- ning will be substantially increased. When in brake mode, the motor is effec- tively acting as a generator, and current flows through the ESC into the Li-Pos. The amount of current depends on the brake strength, but with typical 10S Pattern set - ups, I have seen peaks as high as 10 amps with sustained current flow of about 3–5 amps (as recorded by an Eagle Tree data logger).
Most ESCs can generate a substantial amount of braking force, and in some cases, the force can be enough to impart substan- tial torque (as the prop is rapidly slowed) to the spinner, prop, prop nut(s), motor mount, and fuselage. Each brand of ESC addresses the behavior of the brake a bit differently, but the ultimate goal is to have the brake engage smoothly and reliably without caus- ing any damage to components. While beyond the scope of this column, it should be noted that the ESC’s job of restarting the motor is a much more chal- lenging task after the brake is engaged in comparison to restarting the motor after a period of freewheeling; increasing the im- portance of having the braking parameters set correctly. If the braking parameters are not set correctly, “hard” re-starts (effectively a timing error between the ESC and motor) can occur, and in some instances can be vio- lent enough to kick loose the spinner and/or prop.
Programming the brake parameters for the Castle Creations Edge 80HV is accom- plished on the third tab, “brake”, of the Cas- tle Link. The programmable parameters are brake strength, brake delay, and brake ramp. If the value for brake strength is left at 0% (the default value), the braking func- tion is disabled, and when the throttle stick is at the lowest position, the ESC supplies no power to the motor, and the motor simply freewheels. When a value for the brake strength is entered, the value is relative to 100%, which is the maximum braking force that can be generated (which is dependent on the motor).
Castle Creations ICE and Edge series ESCs allow a wide range of parameters to be logged at variable frequency and can be downloaded for storage and/or detailed analysis.
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Brake delay is the amount of time the ESC will allow the motor to freewheel prior to ap- plying the brake; I set this value to zero. Brake ramp is the duration of time in which the ESC gradually increases the brake strength from zero to the programmed value.
SEPTEMBER 2013
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