7、CONTROLLING A MOBILE BASE （控制移動基座）
在本章中，我們将學習如何使用一對差動驅動輪和一個用來平衡的被動腳輪來控制移動基座。 ROS還可以用于控制全向基座以及飛行機器人或水下車輛，但是陸基差動驅動機器人是一個很好的起點。
In this chapter we will learn how to control a mobile base that uses a pair of differential drive wheels and a passive caster wheel for balance. ROS can also be used to control an omni-directional base as well as flying robots or underwater vehicles but a land-based differential drive robot is a good place to start.
7.1 Units and Coordinate Systems （機關和坐标系）
在向機器人發送運動指令之前，我們需要了解ROS中使用的測量機關和約定的坐标系使用。
Before we can send movement commands to our robot, we need to review the measurement units and coordinate system conventions used in ROS.
使用參考坐标系時，請記住，ROS使用右手約定來定向坐标軸，如上圖所示。 食指和中指指向x和y軸的正方向，拇指指向z軸的正方向。 繞坐标軸旋轉的方向由下圖顯示的右手規則定義：如果将拇指指向任何軸的正方向，則手指會沿正旋轉方向卷曲。 對于使用ROS的移動機器人，x軸指向前方，y軸指向左側，z軸指向上方。 在右手規則下，機器人繞z軸的正向旋轉是逆時針方向，而負向旋轉是順時針方向。
When working with reference frames, keep in mind that ROS uses a right-hand convention for orienting the coordinate axes as shown on left. The index and middle fingers point along the positive x and y axes and the thumb points in the direction of the positive z axis. The direction of a rotation about an axis is defined by the right-hand rule shown on the right: if you point your thumb in the positive direction of any axis, your fingers curl in the direction of a positive rotation. For a mobile robot using ROS, the x-axis points forward, the y-axis points to the left and the z-axis points upward. Under the right-hand rule, a positive rotation of the robot about the z-axis is counterclockwise while a negative rotation is clockwise.
還請記住，ROS使用公制，是以始終以米/秒（m / s）為機關指定線速度，以弧度/秒（rad / s）為機關指定角速度。 對于室内機器人，0.5 m / s的線速度實際上已經相當快了（約1.1 mph），而1.0 rad / s的角速度等效于6秒轉一圈或10 RPM的旋轉速度。 如有疑問，請緩慢啟動并逐漸提高速度。 對于室内機器人，我傾向于将最大線速度保持在0.2 m / s或以下。
Remember also that ROS uses the metric system so that linear velocities are always specified in meters per second (m/s) and angular velocities are given in radians per second (rad/s). A linear velocity of 0.5 m/s is actually quite fast for an indoor robot (about 1.1 mph) while an angular speed of 1.0 rad/s is equivalent to about one rotation in 6 seconds or 10 RPM. When in doubt, start slowly and gradually increase speed. For an indoor robot, I tend to keep the maximum linear speed at or below 0.2 m/s.