Robot Movement Principle & Motion Types

    I would like mention about movement principle of robots and some important motion types. So let's start with the Movement Principle of industrial robots.

Movement Principle

    Industrial robots aim to go from one point to the specific position which can be named as "pose to pose movement". It means clearly "move from the current position to the new position". All story starts with this motion concept. Besides that, robots make some special tasks using industrial equipments(we can call them as tool in some states) such as material handling, palletizing/depalletizing, painting, arc welding/spot welding on the specified path. Robot programming includes some special robot movement commands and when moving on the path robot run special commands to control the environment such as PLC, sensors, actuators and valves. Path can be explained as "The continuous locus of positions(or points in three dimensional space) traversed by the tool center point and described in a specified coordinate system(R15.05-2)." Motion types determine how robot follow the path, too.

Friendly Reminder: If you have a basic knowledge of any "Low Level Programming" like C Language, your adaptation to the industrial robot programming will be easy than the others who don't know coding. You should probably learn motion types and some other special commands&functions for robotics.

    If we categorized how to handle the robot start and end points

• It could be directly teaching the robot by the programmer
• It could be the result of some computations(offset programming)
• It could be directly came from external devices such as PLC or Camera.
• With Artificial Intelligence(AI) and Machine Learnings.

    Robots movements strictly depend on the Tool Center Point(TCP) and Coordinate Systems that are selected. These two topics are very important for industrial robotics and programming. So I will take a look at this topic later deeply.

Motion Types

    We are going to continue with the Motion Types of the robots. I will take a look at three important(most used) types of motion. These are;

• Linear Motion: In the Linear Motion(Linear Interpolation) the TCP moves along linear(straightly as much as possible) path. During linear interpolation, the TCP travels travels along the straight line between start and end point. It may cause more cycle time so it must use only if it is required. The requirement depends the tasks on the path when accuracy of movement is at the forefront such as camera, palletizing and welding applications. Imagine this motion type like that: "Put you finger onto the a side of the table and move your finger along a side of this table with following the straight line. This makes you slow and gains more accuracy."

Fig: An Example Of Linear Interpolation

• Joint Motion: Joint interpolation is basically independent movements of the joints from start point to the destination point. The main purpose in this motion type, robots try to go the destination point at high speed with own calculation of each axes. So it reduces the accuracy in the name of specified movement. On the other hand this easiness gains more cycle time. This motion type is mostly used(or preferred) when quick motion is desired than path accuracy through start point to the destination point. Again "Put you finger onto the a side of the table and move your finger along a side of this table without following a line. You just go from start point to the next(final) position. You are free to choose the way that you follow. This makes you more faster and reduces accuracy."

Fig: An Example Of Joint Interpolation

• Circular Motion: The TCP moves along the circular path. To define circular movement robot need three point which are start point, mid point and the destination point. This motion type is mostly used in welding robots.
  
• Spline Motion: When performing operations such as welding, cutting and priming, using the spline interpolation makes the TEACH-IN for workpieces with irregular shapes easier. The path of motion is a parabola passing through at least three points. It makes the movement more smooth. 

Fig: An Example Of Spline Motion

Important Note When Teaching Path To The Robot: Teach points in the way that the distances between the three points are roughly equal. If there are any significant differences, an error will occur during playback and the robot may perform an unexpected and dangerous movement. Ensure that the ratio of distances between steps m:n is within the range of 0.25 to 0.75. If you don't apply this you may face some kind of problems. 

Fig: The Ratio Between The Points

    I have prepared a YouTube video to show some examples and differences about the Linear and Joint Movement. I have a future plan to create a video too for circular movement. If I prepare a video I will update this post. I dropped the video below which is related to the Linear and Joint motion types.

Note: Yaskawa Motoman Robot and MotoSim Simulation environment were used for this video.

Video: Linear and Joint Motions

Thanks For Reading...

Okan Okumuş

23.06.2020