Racing mini-quads ask a lot from their power systems. Not necessarily in the sense of raw continuous power, but in the demands they make on an ever changing throttle value. During some acro maneuvers, it is not uncommon for a motor to fluctuate from near-idle to full throttle then back to idle in a split second. It should come as no surprise then that you hear lots of stories about miniquads falling out of the air because of motor issues.
This article deals with one particular issue that many miniquad pilots may experience when their motors reach the upper-echelons of their power level: lost synchronization or “desync”.
In benign situations, desync simply manifests as a momentary stutter from the motor, which can be audibly heard. Ideally, it will happen to all motors relatively concurrently, causing your quad to start falling out of the air. In truly bad situations, only one or two motors will fail, causing your quad to unexpectedly somersault.
When desync occurs in flight, the proper response is to lower the throttle. A desynced motor will continue to spin, perhaps even producing enough thrust to keep the quad airborne. Flying a motor that is desynced will almost certainly destroy either the motor or the ESC or both. Lowering the throttle generally allows the ESC to “catch up” to the motor. This should cause the issue to go away, momentarily at least.
There are a few potential causes for lost motor synchronization:
Older ESCs, such as those running SimonK firmware, are known to have stuttering issues when paired with certain motors. If at all possible, try to either upgrade your ESC or install BlHeli on it.
This article assumes you are using BlHeli but most of the suggestions are equally applicable to SimonK ESCs.
Lost synchronization can occur when the ESC is not able to send the required power to the motor on one of the phases. If this is the case, it is because one of your motor to ESC leads is partially shorted or has a shoddy connection. If you have just a single motor giving you trouble, this is the most likely cause.
You can check for this by:
- Carefully examining the wire shielding on the wires between the problematic motor and ESC.
- Checking the solder connections on the ESC and any bullet connectors. Do not assume that since these connections were soldered at the factory that they are fine. I have purchased several ESCs that had faulty solder connections which partially failed after a short time.
Checking the wire integrity at the motor. This can be done by gently tugging on the wire where it comes out of the motor. If any of the wires have significantly more give then the others (you can reference your other motors as well), it is likely that it is starting to come apart inside of the motor. You will probably need to buy a new motor in this case.
The motor timing, which is generally adjustable on the ESC, is the go-to adjustment to fix stuttering issues. Put simply, the timing adjustment changes how much time the ESC waits between power cycles in a motor phase. The lower you can get your timing, the more efficiently your motor will run. However, if you are having desync issues, try slowly bumping up your timing one setting at a time.
Going from “Low” to “High” is not necessarily a good idea. Some motor/ESC/prop combos will work poorly at both “Low” and “High” timing, and might prefer to operate at “Medium”. Step through the motor timing steps one a time from low to high, testing each setting on a bench to see how it affects your stutter. Once you’ve eliminated the stutter, stop adjusting.
Check out the BlHeli manual for more documentation on how motor timing works and how to adjust it.
Some builders use mounting screws which are too long for the quadcopter. These screws can protrude into the motor windings, causing them to be damaged or short. Whenever you are done with a build – you should look very closely at the bottom of the motor where the screws will be protruding through. If these screws are contacting any of the windings, remove them and replace them with shorter ones.
Spin your motors through with your hands. Most brushless motors will feel “notchy”, but they should not make any scratching or rubbing noises through their rotation. Nor should they get caught up at any point. If you have a motor that has either of these two problems, check the following:
Motor not centered properly
Check to make sure your motor is centered correctly on your quadcopter’s arms. The bearing should be visible and concentric with the hole in the center of the arm tip. Most quadcopter arms have adjustable motor holes so that you can move the motor around if it is not lining up.
Sometimes the glue that holds the magnets to the bell of the motor will degrade and allow one or more of the magnets to come loose. This may not be evident because the magnets can actually hold themselves in place due to the magnetic fields existing inside of the motor. Remove the bell and gently prod each magnet with the tip of a pick or small screwdriver. If it moves easily, the glue has let loose. You should either buy a replacement bell or attempt to glue the magnet back in. Superglue works for this, but it can be difficult to get the placement right and you stand a good chance of unbalancing your motor.
Small bends in the motor shaft can cause parts to scrape against one another. The best way to check for this (without a runout gauge) is to spin up the motor without the prop on. If the shaft (or bell) gets slightly blurry, it is bent. You will need to replace it.
Shaft / Bell Retainer Lost
The shaft and bell are held in place by either a C-clip or a single screw in most motors. Both of these attachments are prone to failure, which leaves your motor vulnerable to disassembling itself in flight. You can check if you lost your shaft rentention mechanism by firmly holding the quadcopter and pulling up on the motors from the prop. If the bell comes up more than a few millimeters, it is likely you have a problem. Look on the bottom side of the affected motor to see if something is missing. Most motor manufacturers sell extra C-clips and screws for just this occassion. If you are using screws, make sure you loc-tite them back in place.
ESC Signal Wire Connection / Ground Loops
This is actually an issue I ran into recently when re-doing my Krieger build. Up until then, I had always omitted the ground (black) wire when plugging the ESC into the flight controller. While you can get away with this in many builds, you can’t always.
After re-assembling, I noticed that the motors had synchronization issues at high throttle values. After doing some research, I found that others have had similar issues when removing the ground wires from their ESC->flight controller connection. Apparently the master ground connection from the PDB to the ESC is so noisy on some ESCs that the fluctuations on the line can mask PPM commands from the flight controller. As the motor unwinds due to the lost command, it receives the command again and powers back up. As it draws more power, the electrical noise increases again and causes it to unwind. This mimics a desync staggeringly well.
After grounding all of my Krieger’s ESC signal wires, the problem instantly went away. I won’t be omitting these in the future on any of my builds.
If you are having a stuttering issue and you are only using signal wires from your ESC, I recommend you run the ground wires too, as this is very likely your problem.
An ESC that has had a component like a resistor or capacitor torn off or burned out will obviously not work right. How it fails is dependent on what exactly was compromised. Lost capacitors, however, are notorious for acting up in flight since a lot of what they accomplish is power buffering and smoothing – which only really plays a role when they are under power.
If you are running into a continuous stuttering issue and have exhausted all other options, try out a fresh ESC or closely inspect your problematic ESC for missing components. It may be hard to spot the problem so using a working ESC from another arm for comparison might help.
If you notice the issue with the “Damped Light” feature from BlHeli enabled, try disabling it on all of your ESCs. If the issue goes away, your quadcopter is giving you a hint as to what the problem is. The big difference between damped light mode and non-damped light mode is that the ESCs generate backcurrent when damped light is enabled. Think of the motors like tiny electrical generators that send out bursts of electricity every time they are braked by damped light mode. The only way that your quadcopter can handle these bursts of electricity is by it’s electrical capacitance – which is essentially the sum total of capacitors you have in your system. Most ESCs have a single power capacitor, so your quadcopter will have at least 4. It is likely other electronics like your FC have some too. If these capacitors begin failing or degrading for any reason, you can start to see issues with damped light mode as the voltage in your power system fluctuates way out of spec.
The solution is adding a capacitor. A nice big fat electrolytic capacitor, rated at 22V (or higher if you are running 6S – you want an overratted capacitor) and with at least 200uF; more is better but not necessarily required. These would work fine – or you can get smaller ones if space is a concern. You can install the capacitor anywhere in the power system. It should bridge the main battery negative and positive terminals. On a recent quadcopter that had this happen, I installed the capacitor right underneath the power connector and heat shrinked it between the wires. Just make sure the two leads don’t contact – and when installing the capacitor remember that there is a negative terminal and a positive terminal. If you use a capacitor with too low of a voltage rating or reversed, it will explode – very quickly. It’s happened to me – it wont catch fire or do any damage but it does send little aluminum shards everywhere that would really suck to get in your eye. I would suggest wearing eyewear the first time you plug in your modified quad, for that reason.
Adding a software filter
Adding a filter may help vibration and allow for smoother flight. Check out our article on adding a notch filter using Betaflight.