An overview of controlling a linear motion mechanism (operation and velocity characteristics) with a ballscrew and a motor will be explained here. Since linear motion mechanisms with ballscrews and electrical motors have better controllability as compared with the ones with cams and cranks, they can achieve both high speed and high accuracy performances which have contradicted functions under using cams and cranks mechanism.

(1) Operating characteristics

The characteristic of a linear motion operation can be expressed in how the system performs stops at both ends, and at random positions in between the ends.

(a) Stops at both ends

In general, the end stop positions are regulated by placing sensors at the ends (prevention of overruns). Pneumatic devices such as air cylinders have end stop limitations due to their construction.

(b) Intermediate position stop

Stop position accuracies are required for most intermediate position stop applications. This affects the mechanism design, overall rigidity, and drive/control methods used. Applications requiring 0.01mm or so accuracy can be accomplished with a ballscrew and a servo motor with position sensors utilizing proper ramping profiles (for an acceleration/deceleration control) and backlash compensation.

(2) Velocity characteristics (see Fig.1)

In order to shorten the positioning time and achieve high positioning accuracy, component deformation produced by inertial force (inertial torque) must be reduced. To do this, acceleration (α) needs to be made as small as possible. However, the rise/fall times will become or make longer if acceleration (α) is simply reduced. A countermeasure for this is a multi-step velocity control method where acceleration is reduced to near zero at the start and the end, and largely increased during acceleration/deceleration.

(a) Multi-step velocity control method

This is one of the methods to achieve higher positioning accuracy at an intermediate position stop. By reducing the velocity in multiple steps as the target position approaches, the effects of decelerating load inertia is reduced. The multi-step velocity settings can be pre-programmed as high/med/low speeds, or motor pole counts can be sequentially switched to do this.

(b) Acceleration/Deceleration control method

This is a method where acceleration is reduced to near zero at the start and the end, and largely increased during acceleration/deceleration.