feat: 更新第九章

chapt9/example_ultrasound/lib/PidController/PidController.h

@@ -0,0 +1,30 @@
+#ifndef __PID_CONTROLLER_H__
+#define __PID_CONTROLLER_H__
+
+
+class PidController
+{
+    public: 
+        PidController() = default;
+        PidController(float kp, float ki, float kd, float dt);
+    
+    private:
+        float target_;
+        float kp_;
+        float ki_;
+        float kd_; 
+        // pid
+        float error_;
+        float error_sum_;
+        float derror_;
+        float prev_error_;
+        float intergral_up_ = 2500; // 积分上限
+    
+    public:
+        float update(float current); // 提供当前值，返回下次输出值，也就是PID的结果
+
+
+}
+
+
+#endif // __PID_CONTROLLER_H__

chapt9/example_ultrasound/lib/README

@@ -0,0 +1,46 @@
+
+This directory is intended for project specific (private) libraries.
+PlatformIO will compile them to static libraries and link into executable file.
+
+The source code of each library should be placed in an own separate directory
+("lib/your_library_name/[here are source files]").
+
+For example, see a structure of the following two libraries `Foo` and `Bar`:
+
+|--lib
+|  |
+|  |--Bar
+|  |  |--docs
+|  |  |--examples
+|  |  |--src
+|  |     |- Bar.c
+|  |     |- Bar.h
+|  |  |- library.json (optional, custom build options, etc) https://docs.platformio.org/page/librarymanager/config.html
+|  |
+|  |--Foo
+|  |  |- Foo.c
+|  |  |- Foo.h
+|  |
+|  |- README --> THIS FILE
+|
+|- platformio.ini
+|--src
+   |- main.c
+
+and a contents of `src/main.c`:
+```
+#include <Foo.h>
+#include <Bar.h>
+
+int main (void)
+{
+  ...
+}
+
+```
+
+PlatformIO Library Dependency Finder will find automatically dependent
+libraries scanning project source files.
+
+More information about PlatformIO Library Dependency Finder
+- https://docs.platformio.org/page/librarymanager/ldf.html