Driver ICs Elevate Design of Stepper-Motor Control

Aug 1, 2007 12:00 PM
By Guido Remmerie, Director of Industrial ASSPs, and Peter Cox, Product Manager for Industrial ASSPs

Parameterization Mode

A major concern of every system designer using a stepper motor is how to drive this actuator without losing steps. Many parameters influence the correct motion, and it is not always easy to find the right combination.[1] Randomly changing settings to find the optimum set of parameters can lead to a long development time.

Using highly integrated stepper-motor-driver positioners eases the programming of the key parameters without compiling and requalifying the software. A step-by-step guide is available to help designers.[2]

In the first programming step, the required torque is calculated. This is a system-level requirement in most cases. Knowing the velocity and the required corresponding torque, one can determine the required current, also called the run current, which can be selected from a table containing 16 values varying from 59 mA up to 800 mA by sending a 4 bit in a single I2C command. The same principle is applied when setting the holding torque, which is generated in the motor by supplying a small dc current in the coils. In a separate table, a holding current is selected using a similar 4-bit word.

In the second step, the motor dynamics are considered. Every stepper motor has its own resonance frequency, also called the forbidden or Eigen-frequency (Fig. 6). During acceleration and deceleration, this forbidden frequency zone should be crossed as quickly as possible. A solution offered by the AMIS-30624 is the selection of two velocities and two corresponding acceleration and deceleration profiles. The first velocity, the minimum velocity (VMIN), is chosen above the forbidden zone. The second velocity, the maximum velocity (VMAX), is the nominal speed of the motor. Both velocities can be selected from two velocity tables.

The motion starts at VMIN, which means that the acceleration to reach VMIN, is theoretically infinite. A second acceleration determines the time to reach the VMAX value, and can be set. Because the deceleration is symmetrical, these settings are also used to set the time to slow the rotor to a complete stop. These timings are part of the system requirements in most cases. Based on these timings, the minimum acceleration can be calculated. The upper limit is, as explained in Application Note AN_AMIS-3062x_04,[2] a function of the Eigen frequency of the motor. Again, the acceleration/deceleration can be set by simply sending a 4-bit word using a simple I2C command.

Once all parameters have been calculated, they are sent on the I2C bus to the motor driver/positioner. When they are proven to be stable, the system designer has the option to burn this set of values into a nonvolatile memory embedded in the IC.

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© 2007 Penton Media, Inc.