Improving Performance

ROSflight supplies several methods to improve the performance of your aircraft. Tuning gains, adding feed-forward torques and tuning the estimator are three ways to get your aircraft flying great!

Gain Tuning

Because there are a wide variety of multirotors out there, no one set of PID controller gains will work for all vehicles. The default set of gains is relatively conservative for most platforms, and should be somewhat airworthy in most cases, however, depending on the inertia-to-torque ratio on your MAV, you may have to change these gains considerably. There are some great tutorials online on multirotor gain tuning, this is another tried-and-true method used to great effect at BYU.

If you are unfamiliar with PIDs, then you should probably go read about them before trying to tune a multirotor. Getting an understanding for what is going on will definitely guide your decision making process as you try to figure out better gains.

While tuning controller gains, there is a very likely scenario that the multirotor will oscillate out of control. To handle that, I generally add what I call "training wheels" to the multirotors I'm tuning, which amounts to thin carbon rods in the shape of an X zip-tied to the landing gear. This widens out the base of the quadcopter so if you come down on a hard oscillation, chances are you'll end up upright, hopefully without a prop strike. Here is a video of the maiden flight of ROSflight with "training wheels" attached.

Now, for the procedure on tuning.

Tuning Roll and Pitch

Here is a flowchart describing my PID tuning process for roll and pitch:

tuning_flowchart

You may want to do another D-tuning iteration, and sometimes it is helpful to do a little tweaking to eek out a little performance from the differences in roll and pitch dynamics.

Notice, I didn't include any I tuning. As a general rule, try to keep the I gain as low as possible. It will always slow your reponse rate to input, and it can induce low frequency oscillations.

You should only have I gain on roll and pitch if one of the following is true:

  1. You expect your CG to change or
  2. you expect your rotor geometry to change.

Both of these are pretty rare. Instead, use your RC transmitter to trim the aircraft so it hovers on no stick input. In the RC trim calculation section, we will use the RC trim to calculate a feed-forward torque on roll, pitch and yawrate.

Tuning Yawrate

Dynamically and algorithmically speaking, there is really not much advantage to using a D gain in yawrate. Controlling with derivative requires differentiating gyro input, which tends to be pretty noisy. In our experience, putting D in rate controllers on multirotors has always decreased performance.

Tuning Yawrate is generally pretty easy, basically, keep cranking it up until you feel like it's "locked in". Sometimes a little bit of I (on the order of 0.1P) can help with this as well.

The problem with too much P on yawrate generally manifests itself in motor saturation. Some, especially larger, multirotors have problems getting enough control authority in yaw with the propellers being aligned flat. After you're done tuning, you might want to look at a plot of motor outputs during a fairly agressive flight. Underactuated yaw will be pretty obvious in these plots, because you'll see the motor outputs railing. To fix this, you can put shims underneath the motors to tilt the motors just a little bit in the direction of yaw for that motor.

RC trim (Feed-Forward Torque Calculation)

In the vast majority of cases, your multirotor will not be built perfectly. The CG could be slightly off, or your motors, speed controllers and propellers could be slightly different. One way to fix this is by adding an integrator. Integrators get rid of static offsets like what we are talking about. However, as mentioned above, integrators also always slow your response. In our case, since this offset is going to be constant, we can instead find some "feed-forward" or equilibrium offset torque that you need to apply to hover exactly.

Use the RC transmitter to find the "equilibrium torques" about the x, y, and z axes to keep the multirotor level. This is done by trimming the aircraft with the RC trims. These are usually the little switches next to sticks on your transmitter. Adjust these until you can hover the multirotor without touching the sticks.

Next, land the multirotor, disarm, center the sticks and perform a trim calibration with rosservice call /calibrate_rc_trim. ROSflight then uses the trim settings on your transmitter to find these equilibrium or feed-forward torques that need to be applied post-controller to keep the multirotor level. These torques will be applied to all future commands (both from the onboard computer and RC), so you will need to zero out your transmitter trims after calibration.

Estimator Tuning

ROSflight uses a non-linear complementary filter, based on the quaternion implementation of "Non-linear complementary filters on the special orthogonal group " by Robert Mahony1 to estimate attitude and angular rates. The implementation has been improved with suggestions from "Attitude Representation and Kinematic Propagation for Low-Cost UAVs" by Robert Casey2. A write-up of the derivation and implementation details can be found in the LaTeX report in reports/estimator.tex (You'll need to be able to compile LaTeX sources to view the PDF).

In addition to the complementary filter, accelerometer and gyro measurements are filtered using a simple low-pass filter (LPF) to cut out noise from vibrations. A block diagram of the estimator is shown below for reference. y_{gyro} and y_{acc} are gyro and accelerometer measurements, respectively and \beta_{gyro} is the estimated gyro biases.

CF_diagram

Tuning the Low-Pass Filter Gains

The ACC_LPF_ALPHA and GYRO_LPF_ALPHA parameters are used in the following low-pass-filter implementation. (see lines 98-106 of estimator.c).

x_t = (1-\alpha)y_t + \alpha x_{t-1}

where y_t is the measurement and x_t is the filtered value. Lowering \alpha will reduce lag in response, so if you feel like your MAV is sluggish despite all attempts at controller gain tuning, consider reducing \alpha. Reducing \alpha too far, however will result in a lot of noise from the sensors making its way into the motors. This can cause motors to get really hot, so make sure you check that if you are changing the low-pass filter constants.

Tuning the Complementary Filter

The complementary filter has two gains, k_p and k_i. For a complete understanding of how these work, I would recommend reading the Mahony Paper, or the technical report in the reports folder. In short, k_p can be thought of the strength of accelerometer measurements in the filter, and the k_i gain is the integral constant on the gyro bias. These values should probably not be changed. Before you go changing these values, make sure you completely understand how they work in the filter.

If you do decide to change these values, you should stick to the following rule of thumb.

k_i \approx \tfrac{k_p}{10}.

  1. Mahony, R., Hamel, T. and Pflimlin, J. (2008). Nonlinear Complementary Filters on the Special Orthogonal Group. IEEE Transactions on Automatic Control, 53(5), pp.1203-1218. 

  2. Casey, R., Karpenko, M., Curry, R. and Elkaim, G. (2013). Attitude Representation and Kinematic Propagation for Low-Cost UAVs. AIAA Guidance, Navigation, and Control (GNC) Conference.