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Motion Control Concepts

Being able to accurately control motion depends on an understanding of motion control concepts like interpolation. In this article we present the most common methods for accurate motion control.

Point-to-point (PTP) motion is the most basic form of motion control. It is used when precise start and stop position is important, but the path is irrelevant. In a PTP movement from point A to point B, the motors for the Y and X axes will move at the same speed, so as you can see in the diagram below, the motor for Y arrives at its destination before X, and then stops. X still has to move 3 mm, so it arrives later. The result is a path that looks like a broken line. But the path is irrelevant in this example; it is the position that matters.

Linear and Circular Interpolation
If you need more control over the path you can use linear or circular interpolation. Circular interpolation is normally used in 2D applications, while linear interpolation is common in both 2D and 3D applications.
For a 2D linear interpolation movement, you can use a vector with the motion card's intelligent ASIC (Application Specification Integrated Circuit) function instead of setting the speed for each axis like PTP. The result is a straight path from A to B. The motors for all axes will adjust their speed according to commands from the ASIC, so no matter how far a motor has to go, all motors will arrive a position B at the same time.

For circular interpolation two synchronous motors follow a path that is based on parameters for a circle: the circle's radius, start angle and travel angle. The radius is the arc radius in counts for servo axes or steps for stepper axes. The start angle is a number between 0 and 360°, where 0 is along the positive X axis. Increasing the value moves the position counter-clockwise along the radius. The travel angle is the value in degrees to be traversed, so a positive travel angle would mean counter-clockwise movement, while a negative travel angle would mean clockwise movement.

Continuous and Non-Continuous Interpolation
This is best explained by two application examples. In the first application, you need to glue two shells together and need to use motion control to apply an even amount of glue along a 4-point path. Naturally, you do not want the motors to stop when it reaches a point in the path, as the glue would be unevenly applied, so continuous interpolation is used to calculate the natural path of movement and achieve a constant speed along the path.

In the second application, you need to drill four holes along the path. So this time, you want the motors to stop when reaching a point, wait for the drill to finish, and then move to the next point. This is the situation that non-continuous interpolation is used for. When the motors start moving from point A to point B, the controller will speed up to the highest speed, then slow down and stop when it arrives at point B, do the drilling, and then speed up again as it continues to point C.

By using the right kinds of interpolation for your application, you can achieve more precise motion control and also give less stress to your motors.