doc : optical_flow.md

i'm affraid i did not translate correctly the part at line 92:
the speed will be controlled such as Optical Flow "values"? do not exceed 50% of the "given parameter"?

docs/en/optical_flow.md

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+# Use of Optical Flow
+
+Running the technology "Optical Flow" offers the possibility of POSCTL flight mode, and autonomous flight operating on a camera pointed downwards that detects changes of ground texture.
+
+## Enabling
+
+> **Hint** It is recommanded to use [special PX4 firmware for Clever](firmware.md#прошивка-для-клевера).
+
+The use of a rangefinder is essential. [Connect and setup laser-ranging sensor VL53L1X](laser.md), according to the manual.
+
+Enable Optical Flow in the file `~/catkin_ws/src/clever/clever/launch/clever.launch`:
+
+```xml
+<arg name="optical_flow" default="true"/>
+```
+
+Optical Flow publishes data in `mavros/px4flow/raw/send` topic. In the topic `optical_flow/debug` is also published a vizualization, that can be viewed with [web_video_server](web_video_server.md).
+
+> **Info** Correct connexion and [setup](camera.md) of the camera module is needed for proper functioning.
+
+## Setup of the flight controler
+
+When using **EKF2** (parameter `SYS_MC_EST_GROUP` = `ekf2`):
+
+* `EKF2_AID_MASK` – flag 'use optical flow' is on.
+* `EKF2_OF_DELAY` – 0.
+* `EKF2_OF_QMIN` – 10.
+* `EKF2_OF_N_MIN` – 0.05.
+* `EKF2_OF_N_MAX` - 0.2.
+* `SENS_FLOW_ROT` – No rotation.
+* `SENS_FLOW_MAXHGT` – 4.0 (for the rangefinder VL53L1X)
+* `SENS_FLOW_MINHGT` – 0.01 (for the rangefinder VL53L1X)
+* Optional: `EKF2_HGT_MODE` – range sensor (cf. [rangefinder setup](laser.md)).
+
+When using **LPE** (parameter `SYS_MC_EST_GROUP` = `local_position_estimator, attitude_estimator_q`):
+
+* `LPE_FUSION` – flags 'fuse optical flow' and 'flow gyro compensation' are on.
+* `LPE_FLW_QMIN` – 10.
+* `LPE_FLW_SCALE` – 1.0.
+* `LPE_FLW_R` – 0.1.
+* `LPE_FLW_RR` – 0.0.
+* `SENS_FLOW_ROT` – No rotation.
+* `SENS_FLOW_MAXHGT` – 4.0 (for the rangefinder VL53L1X)
+* `SENS_FLOW_MINHGT` – 0.01 (for the rangefinder VL53L1X)
+* Optional: `LPE_FUSION` – falg 'pub agl as lpos down' is on (сf. [rangefinder setup](laser.md).
+
+[The `selfcheck.py` utility](selfcheck.md) will help you verify that all settings are corectly set.
+
+## POSCTL flight
+
+Setup POSCTL to be one of PX4 flight modes and then select POSCTL.
+
+## Autonomous flight
+
+The module [simple_offboard](simple_offboard.mb) enables autonomous flight.
+
+Example of take off and leveling at 1.5m above the ground:
+
+```python
+navigate(z=1.5, frame_id='body', auto_arm=True)
+```
+
+Flying forward for 1m:
+
+```python
+navigate(x=1.5, frame_id='body')
+```
+
+[Navigation using ArUco-markers](aruco_marker) and [using VPE] are available when using Optical Flow.
+
+## Additional settings
+
+<!-- TODO: статья по пидам -->
+
+If the copter has an unstable position using VPE, try to increase the *P* coefficient of speed PID controler - parameters are `MPC_XY_VEL_P` and `MPC_Z_VEL_P`.
+
+If the copter has an unstable height, try increasing `MPC_Z_VEL_P` coefficient or getting better hover throttle - `MPC_THR_HOVER`.
+
+If the copter is consistently yawing, try:
+
+* recalibrate gyroscopes;
+* recalibrate magnetometer;
+* different values for `EKF2_MAG_TYPE` parameter, that indicates how data from the magnetometer is used in EKF2;
+* changing values of `EKF2_MAG_NOISE`, `EKF2_GYR_NOISE`, `EKF2_GYR_B_NOISE` parameters.
+
+If the copter's height is deviating, try:
+
+* increasing the value of `MPC_Z_VEL_P` coefficient;
+* change the value of `MPC_THR_HOVER` parameter;
+* add `MPC_ALT_MODE` = 2 (Terrain following).
+
+When using Optical Flow, the maximal horizontal speed is farther limited. This is an indirect influence of the parameter `SENS_FLOW_MAXR` (maximal reliable "angular speed" of the optical flow). In normal flight mode, the speed will be controlled such as Optical Flow values do not exceed 50% of the given parameter.
+
+## Errors
+
+If errors of `EKF INTERNAL CHECKS` occur, try to restart EKF2. To do so, enter in the MAVLink-console : в MAVLink-консоли:
+
+```nsh
+ekf2 stop
+ekf2 start
+```