Electric R/C Flight
By Heather Mardon
Part 5: Electronic Speed Controllers
It’s June already and I was scratching for a subject for this months topic when speed controllers where suggested to me. All of a sudden there was plenty of inspiration for my column. For the purposes of simplicity I have only described the operation of ‘Brushed motor’ controllers.
The term ESC is often used as an abbreviation for ‘electronic speed controller’. An esc’s basic function is to vary the amount of power to the motor from the flight battery based upon the position of the throttle stick.
In early speed controllers used in R/C cars & boats they used a variable resistor with a wiper that was moved by a servo.
This method works ok at full throttle as the battery is connected directly to the motor, however at part throttle settings the current flows through the resister causing power to be wasted in the form of heat. As a model aircraft will spend most of its time at part throttle this is not a very practical means of controlling the power.
Modern speed controllers vary the power to the motor by rapidly switching the power off and on. The switch is a ‘Mosfet Transistor’ instead of a mechanical device, and the rate at which it is switched is about two thousand times a second. Therefore the power to the motor is varied by changing the amount of On time, versus off time in a given cycle.
Here is a very simplified circuit and waveform diagram that may help with the explanation.
When the MOSFET is turned on, current ramps up as the magnetic field in the motor windings increases. When the MOSFET is turned off, magnetic energy stored in the windings has to be absorbed by the ESC. By wiring a diode across the motor, we return the energy back into the motor as current, which ramps down as the magnetic field collapses (without a diode the current would drop rapidly, but a large voltage spike would occur which could damage the MOSFET)
Bells & Whistles
There are several other features that speed controllers can have and they are described as follows:
This is a common feature that as the name implies eliminates the need for a separate battery to run the radio receiver and servos. The principle is that the receiver is feed a regulated 5volts through the throttle channel cable from the main flight battery. The following picture shows the normal wiring.
BEC equipped controllers usually have a rating of how many servos they can driven on a given number of cells in the flight battery. For example a certain speed controller may be able to handle 4 servos on 7 cells but only 2 when run on ten cells. This is due to the regulator circuit having to ‘drop’ a larger amount of power when run on higher cell counts, and the more power lost across it the hotter it will get. Make sure you read the specifications of your controller very carefully as these ratings vary considerably across makes and models.
Note: If your speed controller does not have a BEC or you wish to run more servos than it is rated for, you will have to connect up a separate receiver pack. If you wish to disable the onboard BEC disconnect the 5v supply wire on the throttle channel connector (Usually the red wire). You cannot use the onboard BEC and a receiver battery at the same time.
So what happens when you are using a BEC equipped controller and your battery is running out, do you lose power to the radio too?
No, what occurs is when the voltage gets below a certain threshold the speed controller cuts the power to the motor leaving the last few precious electrons to operate the radio an servos. The point of cut off will vary according to make and model. One thing to be aware of is the higher your cell count, the flatter the battery will be when the LVC kicks in. This because most ESC’s have a fixed voltage cutoff point that does not vary with the number of cells you are running, so if the cutoff is 6v and you are running on 10 cells the individual cells are down to 0.6v per cell. However if you were running on 7 cells it would be 0.86v per cell.
A lot of the modern microprocessor controlled ESC’s have a feature that enables you to get one or two more bursts out of your motor before you are dead stick. This is usually activated by reducing the throttle to zero after the motor cuts, and then advancing it again. This may enable you to just get over that fence in time….
A brake equipped ESC will switch a Mosfet on wired across the motor when the throttle is reduced to zero. This has the effect of shorting out the motor windings causing a brake effect. The main purpose of this is to stop the prop from ‘windmilling’ and allowing the blades of a folding prop to fold backwards.
Most brake circuits only activate for a few seconds at a time, just enough to get those blades folded.
Some ESC’s have the ability to enable or disable this feature. It may be advantages in some cases to have the brake turned off if you wish to use the windmilling prop as a sort of airbrake on a landing approach. Or if you are running a large prop through a gearbox the brake may be too harsh causing gears to be stripped.
This term describes the way the signal from the receiver to the ESC is processed. In the case of an Opto Coupled ESC the receiver is completely electrically isolated from the motor and flight battery by using light to transmit the signal (Inside a sealed chip). Because of this isolation a BEC circuit cannot be used as it would defeat the purpose. In theory there is no way for interference to travel back down the signal or supply wires to the receiver.
What do all those number mean? Here is a commonly used speed controller and its specifications.
40A continuous/ 50A short Duration
6-12 cells (6-16V)
2A BEC, selectable EMF Brake, undervoltage Cutoff.
These ratings are fairly self-explanatory. The only thing that it does not tell you is how many servos it will run. However it does rate the BEC at 2A which should be able to cope with up to four servos as long as there not real current hogs.
Another variable with speed controllers is there throttle response. Some better ones have smooth linear response while others have a lot of exponential causing them to be at full throttle before you get to half stick. Others require you to calibrate there end points every time you power them up. Read your instructions carefully for how to carry out this procedure.
TIP: If your speed controller is not very linear, try setting some positive exponential on your throttle channel, I have one controller like this and by setting 100% expo on channel three it now uses the whole travel of the stick.
What’s on the building board
The same thing that was there last month! I’m making progress on my new slow flyer a "Lethargik". Ian has already finished his and here is a picture of it before it’s maiden flight. PS: it uses a cute little Kontronik ‘Rondo 300’ speed controller that solders directly onto the back of the motor and can handle 8A continuos 6-8 cells, BEC, and only weighs a few grams.