Electronic Design

Add Ramp To Stepping Motor

Due to their low cost, stepping-motor controllers are often chosen over full-blown, software-driven controllers for a number of applications. However, stepping motors provide rather low torque at high speeds, thus they usually require a starting ramp to prevent them from stalling at startup.

This circuit is a one-chip solution to that problem (see the figure). Lying at the heart of the circuit is the 555 timer U1, which is configured as an astable multivibrator. In this case, a 50-m on time was selected to be compatible with the stepping-motor driver. The off time is approximately equal to 1.1R2C2. A 10-ms off time was selected for the final speed of the stepping motor.

The ramp time is approximately R3C3. A ramp time of one second was desired, so the values were selected as indicated.

The ramp is exponential and asymptotic to the final speed. Therefore, no discontinuities exist in the startup response.

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