# 多轴飞行器 PID 调参指南

## 简介

PX4 使用 P 比例、 I 积分、 D 微分 (PID) 控制器, 是使用最广泛的控制技术。

The controllers are layered, which means a higher-level controller passes its results to a lower-level controller. The lowest-level controller is the the rate controller, then there is the attitude contoller, and then the velocity & position controller. The PID tuning needs to be done in the same order, starting with the rate controller, as it will affect all other controllers.

## 前置条件

• 您已为您的飞行器选择了最接近的 默认机型配置 。 这应该可以让你的飞行器飞起来。
• 您应该已经执行过 电调（ESC）校准
• PWM_MIN 正确设置。 它需要设置一个小值, 但当飞行器解锁时, 需要保证 电机不停转

可以在 特技 Acro 模式 手动/自稳模式 中进行测试：

• 卸下螺旋桨
• 解锁，并将油门杆拉到最低
• 把飞行器倾斜到各个方向, 大约60度
• 确保没有电机停转
• 可以通过 SDLOG_PROFILE 参数，启用高速率日志记录配置文件, 以便使用日志来查看角速率和姿态跟踪性能 (之后可以禁用该选项) 。

## 调参步骤

For safety reasons, the default gains are set to low values. You must increase the gains before you can expect good control responses.

• 调整增益时，所有的增益值都应该慢慢增加, 因为增益过大可能会导致危险的振荡! 一般情况下，每次增益值的调整幅度大约在20%到30%，获得最优增益值后，基于最优值再下调5%到10%。
• 在修改参数之前务必先着陆。 慢慢增加油门，观察振荡的现象。
• Tune the vehicle around the hovering thrust point, and use the thrust curve parameter to account for thrust non-linearities or high-thrust oscillations.

### 速率控制器

• 我们更推荐特技模式， 但这种模式比较难飞。 如果你选择特技模式，记得把把特技模式指数因子都禁用了：
• MC_ACRO_EXPO = 0, MC_ACRO_EXPO_Y = 0, MC_ACRO_SUPEXPO = 0, MC_ACRO_SUPEXPOY = 0
• MC_ACRO_P_MAX = 200, MC_ACRO_R_MAX = 200
• MC_ACRO_Y_MAX = 100
• 手动/自稳模式更好飞，但这种模式也比较难观察姿态和角速度控制器到底调好了没。

• If you notice strong oscillations when first trying to takeoff (to the point where it does not fly), decrease all P and D gains until it takes off.
• 但如果你的飞行器对遥控器的所有指令都没什么反应的话，可以增加P增益试试。

#### P增益

P增益(比例增益) 用来减小误差。 它可以加快响应速度，因此应该在不引入震荡的前提下设的尽量的高。

• 如果P增益太高了，会有高频率的振荡。
• 如果 P 增益太低了:
• 飞行器会对遥控器的输入很迟钝。
• 如果是在特技模式下，飞行器会漂移，你会一直要矫正它来让它水平。

#### D 增益

D增益(微分增益) 用来增加阻尼，可以防止超调。 同样地，这个值应该尽量设大一些来避免超调。

• D增益太大，电机可能会抽搐、发热(也有可能不会) 。这是因为D项同时也会放大震动等带来的噪声。
• D增益太低会导致超调，即相比设定的值「冲过头了」。

#### I增益

I（积分）增益可以「记住误差」。 如果你发现过了一段时间了，角速度还是达不到设定值，那就该增加 I 了。 它很重要(尤其在特技模式下) ，但我们不应该把它设得太高。

• 如果积分增益太高：你会看到缓慢的振荡。
• If the I gain is too low: this is best tested in Acro mode, by tilting the vehicle to one side about 45 degrees, and keeping it like that. 他应该始终保持相同的角度。 如果它往回漂移，增加 I 。 通过观察日志我们也可以发现 I 增益太小的问题，可以看到实际的角速度过很久也达不到期望的角速度。

I 增益一般在0.3~0.5之间，俯仰角的一般要大一点。

#### 测试步骤

You can create a step-input for example for roll, by quickly pushing the roll stick to one side, and then let it go back quickly (be aware that the stick will oscillate too if you just let go of it, because it is spring-loaded — a well-tuned vehicle will follow these oscillations).

### Thrust Curve

To counteract that adjust the thrust curve with the THR_MDL_FAC parameter.

The rate controller might need to be re-tuned if you change this parameter.

The mapping from motor control signals (e.g. PWM) to expected thrust is linear by default — setting THR_MDL_FAC to 1 makes it quadratic. Values in between use a linear interpolation of the two. Typical values are between 0.3 and 0.5.

If you have a thrust stand (or can otherwise measure thrust), you can determine the relationship between the PWM control signal and the motor's actual thrust, and fit a function to the data. This Notebook shows how the thrust model factor THR_MDL_FAC may be calculated from previously measured thrust data.

The mapping between PWM and static thrust depends highly on the battery voltage.

If you don't have access to a thrust stand, you can also tune the modeling factor empirically. Start off with 0.3 and increase it by 0.1 at a time. If it is too high, you will start to notice oscillations at lower throttle values. If it is too low you'll notice oscillations at higher throttle values.

### Airmode & 混控器饱和

The rate controller outputs torque commands for all three axis (roll, pitch and yaw) and a scalar thrust value, which need to be converted into individual motor thrust commands. This step is called mixing.

It can happen that one of the motor commands becomes negative, for example for a low thrust and large roll command (and similarly it can go above 100%). This is a mixer saturation. It is physically impossible for the vehicle to execute these commands (except for reversible motors). PX4 has two modes to resolve this:

• Either by reducing the commanded torque for roll such that none of the motor commands is below zero (Airmode disabled). In the extreme case where the commanded thrust is zero, it means that no attitude correction is possible anymore, which is why a minimum thrust is always required for this mode.
• Or by increasing (boosting) the commanded thrust, such that none of the motor commands is negative (Airmode enabled). This has the big advantage that the attitude/rates can be tracked correctly even at low or zero throttle. It generally improves the flight performance.

However it increases the total thrust which can lead to situations where the vehicle continues to ascend even though the throttle is reduced to zero. For a well-tuned, correctly functioning vehicle it is not the case, but for example it can happen when the vehicle strongly oscillates due to too high P tuning gains.

Both modes are shown below with a 2D illustration for two motors and a torque command for roll r. On the left motor r is added to the commanded thrust, while on the right motor it is subtracted from it. The motor thrusts are in green. With Airmode enabled, the commanded thrust is increased by b. When it is disabled, r is reduced.

![Airmode](../../images/mc_pid_tuning/MC_PID_tuning-Airmode.svg)


If mixing becomes saturated towards the upper bound the commanded thrust is reduced to ensure that no motor is commanded to deliver more than 100% thrust. This behaviour is similar to the Airmode logic, and is applied whether Airmode is enabled or disabled.

Once your vehicle flies well you can enable Airmode via the MC_AIRMODE parameter.