This article was translated to English, and was originally published for deviceplus.jp.
Hello, everyone. This is Mocchan. This is Part two of our series on tuning a compact micro drone that you can even fly indoors to create an airframe with original flight characteristics. Part one. Now we already introduced the process to change the hardware aspects such as the battery, propellers, and motor to craft a more powerful airframe. However, in this article I would like to cover how to tune the software to craft a drone with even better flight handling.
・About the Rateprofile “RC Rate” setting
・About the Rateprofile “RC Expo” setting
・Adjusting the calibration to craft a responsive airframe
・About the proportional control system calibration
・About the gyro calibration
・Crafting an airframe with mild characteristics
・Resolving difficulties flying in the vertical direction
In part one, we covered the tuning of the hardware aspects including the battery, propellers, and motor. This time, I would like to discuss how to use the “Betaflight” software to change the settings of the flight controller installed in the drone to change the airframe characteristics. Furthermore, “Betaflight” can be used free of charge. Install the software to your PC from the following URL.
The following is an introduction to the settings which are available under PID Tuning in “Betaflight.” Because this is software-based tuning, these settings are designed to be changed. However, these adjustments can achieve a wide range of results, and creating the airframe characteristics that are close to your desired image will make the drone easy to handle.
The name of the tab is “PID Tuning.” PID is an abbreviation of “Proportional-Integral-Differential Controller” which is one type of feedback control within control engineering.
Putting aside the discussion of difficult topics, there are many settings other than PID which can be adjusted in this tab. One such adjustment item is the “Rateprofile setting.” This setting adjusts the characteristics for operating the drone. The three items are “RC Rate,” “Super Rate,” and “RC Expo,” and each of the values can adjust the roll, pitch, and yawing axis. First, let’s understand with respect to these three rates what these items are and how they should be adjusted.
Before doing so, allow me to explain the range of effectiveness of “Rateprofile.” Rateprofile shows no changes except for yaw in Angle mode and Horizon mode. This setting is limited to “AIR mode” and “ACRO mode” which are capable of acrobatic flight. Many people think that AIR mode and ACRO mode are the same, so I will provide a supplementary explanation.
・AIR mode: This mode activates when the mode switch is assigned to AIR mode.
・ACRO mode: This mode activates when the mode switch is not set to anything.
In this manner, you can switch between AIR mode and ACRO mode. The difference is that AIR mode correction is turned on during AIR mode. A representative example of this difference is the “FPV Angle Mix.” For this setting, ACRO mode operates in a three-axis gyro linear mode with absolutely no correction. You have to handle all of the steering yourself, and you should think of it as a mode in which the drone flies completely under your control. Select this mode according to your piloting skills.
The “RC Rate” item configures the basic operating quantity. To make this easy to understand, let’s disable all of the other rates. To do so, set the numbers for “Super Rate” and “RC Expo” to 0.00. Did the graph in the lower rate preview change to a completely straight line? Next, change the “RC Rate” value to 1.00. I believe that the “maximum velocity [deg/s]” on the right side should be 200. The meaning of this number indicates the speed at which the airframe rotates. Explaining this in terms of the ROLL axis, when the aileron stick is fully pulled down, it means that it will rotate horizontally by 200° in one second. Since the characteristic curve is a straight line, when the stick is pulled down halfway, the airframe will rotate horizontally by 100° in one second. Therefore, when this number is increased, the airframe movement becomes more agile. However, if this number is increased too much, the center vicinity becomes too sensitive, and even hovering becomes uncontrollable. The key point with this value is to increase it until just before it starts to feel difficult to handle. This numerical value determines the basic operations of the drone, so be sure to properly adjust it.
Once the basic operations are decided, next we will make the drone slightly easier to handle. The “RC Expo” setting makes the response in the center vicinity of an overly sensitive stick more moderate, and the stick operating quantity can be adjusted by a range of 50% from the center.
If the area around the center of the stick feels a little bit difficult to handle during hovering, changing this numerical value will make the sensitivity more sluggish. If the rudder is too sensitive and the motion around the center vicinity of the stick feels a little bit difficult to handle, adjust this numerical value until the handling does not feel too sensitive to you.
If this number is 0.00 and it handles normally, then the value of the previous “RC Rate” is too low. The value of “RC Expo” should be set between 0.10 and 0.20. Conversely, the proper value of “RC Rate” is just right at between the 0.10 and 0.20 of the “RC Expo” value.
The final setting is the “Super Rate.” This item changes the stick response across a range of 50% to 100%. This setting is particularly related to the maximum velocity when the stick is pushed all the way down.
As an experiment, let’s set this numerical value to 0.90. The maximum velocity changes to 1,998 deg/s. This means that the airframe rotates 1,998 degrees or 5.5 times in one second. It actually cannot rotate at such a speed unless you use a rather extreme motor and propellers. Realistically speaking, a “Super Rate” of about 0.80 is the limit of what the airframe can withstand. Even at this setting, the maximum velocity is 1,000 deg/s, a calculation which means that the airframe will rotate 2.8 times in one second. In FPV, the drone will rotate at a speed which is completely unrecognizable, so it will be almost impossible to stop the drone at the desired angle. Typically, this value should be set between 0.70 and 0.75.
Because the name of the tab is PID Tuning as shown above, people tend to think that these are “PID” adjustment items, but the entry of these three item values is extremely important. Carry out multiple test flights to find the values that work the best for you.
Many people who become initiated into racing drones have previous experience flying toy drones. Compact toy drones often have airframes with extremely sluggish stick operations. However, if they fly a racing drone with the same feeling, the airframe will fly out of control due to the good maneuverability, and the drone will be unable to even properly hover.
This is a phenomenon which frequently occurs when a racing drone is flown with the same flying feel as a toy drone which ignores sloppy operations. That being said, it requires a considerable amount of time to master a delicate proportional control system stick. Until you are able to do that, you will likely drop and bump the airframe or damage the airframe and abandon the drone.
Until you get used to the airframe, reducing the maneuverability to that of a toy drone will not be good practice. However, reducing the maneuverability by a certain degree will reduce the number of times that the airframe flies out of control and enable you to tune the settings to make it easy to fly. In this part, I would like to introduce you to tuning methods that will create a responsive and easy to fly drone.
This content is not only limited to beginners but will also be a reference for those who can fly drones to a certain degree. In particular, when the calibration is properly adjusted, the airframe characteristics become responsive, and the drone becomes extremely easy to fly. Sometimes you see people who are able to fly drones to a certain degree forcefully flying an airframe that is unable to provide this level of responsiveness. In contrast, others will understand the importance to the level of an expert and perform the calibration in a thorough manner.
Now, generally speaking, there are two types of calibration. The two types are telling the proper horizontal position to the airframe gyro sensor and properly setting the signal sent from the transmitter.
First, let’s perform a neutral calibration of the transmitter. Open the “Receiver Tab” in the “Betaflight” settings. What is important in these adjustments are the median values of “Roll,” “Pitch,” and “Yaw.” It is important to set these median values to 1,500 ms. If these numerical values are off, the drone model displayed in the “Preview” of the same tab should slowly rotate. You need to make adjustments so that the drone model comes to a dead halt.
In particular, when using the proportional control system manufactured by Futaba Corporation (Futaba) that many Japanese pilots use, the center value is exactly 1,520 ms. To make minor adjustments, change the Sub-Trim item of the proportional control system.
Open the setting screen of the proportional control system and adjust the numbers displayed on the sliders within the Receiver tab so that they get as close as possible to 1,500. I believe that the numbers will change slightly during operation. Try to get the numbers as close as possible to 1,500.
Next, I expect that there will be some flicker and deviation with respect to 1,500, but we will configure it to disable that range. This is set in the item called “RC Deadband” which is a feature that absorbs flicker and other forms of jitter to disable operations in that range. Set this number to 10 or less. Any additional deviation or flicker is due to the performance of the transmitter. Due to problems in the sensing potentiometer and the accuracy of the gimbal that the stick is attached to, this number tends to increase in proportional control systems that are used for a long time and inexpensive models. With the Futaba proportional control system mentioned above, each individual number should be about four. When these settings are completely finished, I expect that the drone display in the “Preview” should stop moving.
Furthermore, also keep an eye on the endpoints when changing the Sub-Trim. Move the stick to the full extent and check again that the numbers on the slider display 1000 or less to 2000 or more. This is because the numbers are changing between the maximum value and the minimum value when the sub-trim is moved.
Now that the proportional control system calibration is finished, next we will calibrate the gyro. This is the “Horizontal Calibration” item in the “Setup” tab. When configuring this setting, it is important to maintain the position of the airframe in a horizontal location. If the calibration is performed in a tilted position, the airframe will try to stop in a tilted position during actual flight.
When the proportional control system calibration and the airframe horizontal calibration are complete, the airframe will hover in a stable manner. If the airframe hovers with almost no maneuvering, that proves that the calibration has been perfectly configured.
If the airframe appears to float away, stop making adjustments with the trim feature of the proportional control system. Making further adjustments is harmful without doing any good at all. In fact, when looking at the transmitter screen of people flying drones showing erratic behavior, I have noticed many times that the trim is not at the center, and they are adjusting the flight with the trim. Instead of adjusting the trim as an emergency measure, perform the calibration properly to create the conditions in which the airframe stays in place even when you are not touching the stick.
Once the basic adjustments are complete, we will change the airframe characteristics for beginners. The items for adjusting the airframe flight characteristics are under “PID Tuning.” However, there are not many parameter items to set for these adjustments when making the operation more moderate. The original objective is to fly fast, so making moderate adjustments is not something that this software is very good at. That point may be a factor in why beginners feel that it is difficult to control a racing drone.
Now let’s make the adjustments. For beginners, it is best to first practice by using the “Angle mode” which corresponds with the six-axis Gyro mode, so here we will proceed by mainly focusing on the adjustment items in Angle mode.
As stated previously, while we are calling this “PID tuning,” there are also many non-PID items. First, we will adjust the three items consisting of “RC Rate,” “Super Rate,” and “RC Expo” which were used to accelerate the airframe movement. As stated before in the section on making the airframe faster, these settings affect only the yaw. Therefore, if it moves too quickly in the yaw direction, it will become difficult for a beginner to handle. In particular, they will be unable to point the airframe in the direction that they wish to proceed during FPV flight and move unsteadily toward the gate, so it is recommended that the movement be gradually accelerated according to the piloting skill.
First, we will adjust it so that it is easy to orient the drone even with rough stick operations when a beginner is unable to perform minute rudder stick operations. Open “Rateprofile” in the PID Tuning tab. For the time being, set the value of “Super Rate” to 0.00 in the “YAW” setting item. I think that a value of 0.10 for “RC Expo” is good for now. Under these conditions, let’s change only the “RC Rate” value and try flying the drone. Initially, we will start from about 0.50. When the calibration is complete, hovering should not be that difficult. Do your best for now to deal with the vertically unsteady movement. We will adjust this later.
While in a hovering state, move the rudder stick (left stick) all the way in either the left or right direction. In contrast to previous movements, the airframe should rotate in a very slow manner. It should slowly turn even with rough stick operations, so it can be easily stopped when facing the desired direction. Gradually increase the “RC Rate” value so that it turns at a speed that you can handle according to your level of skill.
Fly the drone over and over, make an adjustment, and then repeat the process to find just the right value. Naturally, the secret to doing this is to adjust the value so that it turns just right when the stick is moved halfway instead of pushing it all the way down. Do not flip the stick back and forth like a switch.
After the “RC Rate” adjustment is finished, configure the “Super Rate.” This “Super Rate” is like a control input booster for the proportional control, so the effect can be seen when the stick is pulled halfway or more. I think it is a good idea to also start this setting at about 0.50.
When the adjustments in the yaw direction are finished, next we will adjust the sensitivity in the roll and pitch directions. Typically it is standard to change the PID values here to change the characteristics. However, if a beginner changes these values slightly, it is difficult to understand the changes, which makes it difficult to feel the differences before and after the changes. With the exception of models that have undergone special tuning, many models adopt the default PID values. Therefore, it is better to think that the balance between the PID values uses numbers that have been proven to work. We will use these numerical values as they are and reduce the effective range to moderate the characteristics.
The parameter to change is the “Strength parameter” under the “Angle Horizon” item. I believe that the value is set to 50 by default but decreasing this number will moderate the response with respect to stick operations. In addition, lowering the number to about 30 will significantly reduce the maneuverability. The airframe will not follow even rough stick operations, so the movement should feel mild.
It is important to note that the response will be slower when operating the stick. To put it another way, when the stick is returned to the center, the airframe will return to the horizontal in a similarly slow manner. This behavior differs from that of a toy drone, so be careful not to set the angle strength to 30 or less. It will take time for the drone to tilt significantly, but the same can be said for the return to the horizontal, so it might be a good idea to lower the number for the angle upper limit so that it does not tilt to a significant degree.
If you have made it this far, the drone movement should be rather moderate. However, perhaps the airframe is moving up and down going back and forth between the ceiling and the floor? Such movements should be particularly noticeable in those who have flown a toy drone with an altitude maintenance feature. Regarding this issue, the only thing I can say is “practice a lot,” but I will introduce some items to adjust.
When it comes to throttle operation, the handling of the proportional control system is more important than habituation. The reason why is because when aileron operations in a Mode 1 proportional control system and rudder operations in a Mode 2 proportional control system are performed, the throttle has changed.
Even if the operator intends to move the stick directly to the side, if the actual movement pulls the aileron and the rudder stick to the inside, the throttle decreases. Conversely, when the stick is pulled to the outside, the throttle increases as when the stick is moved in a diagonal manner.
This is quite noticeable with those who have small hands, but it is a problem with the range of thumb motion and the indirect position. Those of you who have a proportional control system handy can try picking it up. From that position, try switching the stick completely to the side. Unless you really think about it, I suspect that you will be unable to pull the stick directly to the side. This is because people move the throttle up and down in a completely unconscious manner. In order to reduce this effect, let’s make the throttle operating sensation in the hovering state more sluggish.
Now let’s try to make the drone hover. If the previous calibration is complete, hovering should not be that difficult. When you are able to get it to hover, remember the throttle stick position at that time. This time, the stick position was slightly above the center. Let’s make the throttle operation at this position loose.
We will change the “Throttle MID” number in the “Rateprofile” setting of the PID Tuning tab. The default is in the center at 0.50, so we set it to 0.60. The default for the “Throttle EXPO” next to it is a linear characteristic set to 0.00, but this time we tried setting it to a slightly larger 0.50.
As a result, the throttle operation at about 60% becomes a sloped curve in the horizontal direction. Let’s try flying it again right away. If these settings work beautifully, you should have crafted a drone with comparatively moderate characteristics. If it is still shaky in the vertical direction and feels difficult to control, make adjustments under “PID Tuning.” The reason why is because there are no throttle related PID values. Therefore, you change it by changing the “D value” which is not needed for the yaw control. This was originally a parameter for suppressing the yaw maneuver, but it has the effect of applying a filter to the throttle. However, this number should be set to a maximum of 20 or less. If possible, I think it is a good idea to lower it to about 10. This is because increasing it will cause a significant deterioration of the response and cause the drone to significantly shake when the power is turned on.
So what did you think of Part one and Part two of our series about tuning the “TinyWhoop” micro drone? Were you able to craft a high-speed, high-power machine or build an airframe with mild characteristics according to your own piloting skill?
There is no right answer when it comes to drone configuration. I hope that you use a drone that you crafted on your own to enjoy FPV flight while enjoying the process to configure it according to your own flight style!