4.2.4. Acceleration mode

The acceleration function is active by default. The acceleration is used for smooth start or finish of the rotation without shocks that are inevitable when the predefined speed is reached instantly. Moreover, inertia of rotor and the other components of stage usually doesn’t allow instant gathering of high speed which results in the loss of steps as well as failure in rotation during the operation without feedback. While operating the motor with feedback via encoder, the speed will increase as quickly as motor limiters allow. High acceleration makes the rotation unstable as well as it makes more noise and vibration. That’s why the acceleration mode is recommended. The acceleration function provides reaching of both maximum speed and sustainable motion even for motors with intermediate torque value.

The acceleration/deceleration mode works in the following way: during the speeding-up, while the required speed value is higher than the actual one, a gradual acceleration is performed at the Acceleration value which is measured in steps per squared second. After the required speed is reached, the controller switches to predefined speed rotation mode. Approaching the destination position, the controller begins to decrease the speed as the speeding-down would equal to Deceleration value and the motor would stop exactly at the destination point. Thus, this mode provides a trapezoidal speed profile. If the displacement distance is small, then the acceleration may change directly to deceleration; this will result to triangular speed profile. Turning the acceleration mode on and off, as well as setting of acceleration and deceleration value, is possible using XiLab software (see the Settings of kinematics (stepper motor) section) or by set_move_settings() command described in Programming guide.

The Acceleration value is adjusted independently from the Deceleration value – and there is a reason for it. Usually, due to friction effect resisting acceleration but contributing to deceleration, the maximum acceleration value is less than deceleration one. Therefore, for the fastest response of the stage either preset profiles should be used, or the acceleration/deceleration values should be established experimentally, according to what your stage may provide. For stepper motors working without feedback these are the values that do not lead to the steps loss. For motors with feedback the trapezoidal speed profile should be controlled using XiLab charts. The acceleration/deceleration values should be taken 1.5–2 times less than those resulting in speed profile distortion or step loss.

Note

Turning the acceleration/deceleration mode off is sometimes useful for multiaxis systems control where, during the motion along multi-dimensional paths, a continuous speed projection on each of the axes is required.

Note

The acceleration value is not displayed in XILab main window.

Note

Acceleration/deceleration values should be set as to allow the motor to reach target speed or decelerate from top speed to zero in less than 5 minutes. If the acceleration/deceleration on kinematic settings page is set outside of this range, then the controller will return an “incorrect value” error and accelerarion/deceleration will be changed in controller to applicable value.