Developing more efficient motor control designs
Motor control-specific DSPs are designed primarily to address the requirements of large off-line motors. Off-line motors are typically AC induction or brushless DC motors operating from 110 to 480 VAC and ranging from 1/4 to 100 HP. Motor control-specific DSPs are often too costly for small motors control systems.
This article will provide software examples of motor control designs that use C8051F3xx MCUs to illustrate their use in controlling various types of motors. While the examples are relatively simple, they demonstrate effective solutions for the various motor types. While a typical motor control system often requires additional features and higher functionality, these software examples may be used as a starting point for the development of more complex motor drive systems.
DC motor control
DC motors are the most common and least expensive of all small motors. In this article the term "DC motor" refers specifically to a brush-commutated, permanent-magnet motor. DC motors are used in a wide range of applications in the automotive, consumer and industrial market segments. Brushless DC (BLDC) motors promise improved reliability, reduced noise, and potentially lower cost. However, BLDC motors have only supplanted conventional DC motors in a few specialised high volume applications—disc drives and computer fans.
The characteristics of a DC motor make it the easiest motor to use in a variable-speed system. The torque speed characteristics are shown in figure 1. The no-load speed of a DC motor is proportional to the voltage applied across the motor. The voltage-speed characteristics of a DC motor driving a constant-torque load, linear-load, or exponential-load are also continuous, positive-slope, and predictable. Thus, in most cases it is feasible to use open-loop control.
By simply varying the voltage across the motor, one can control the speed of the motor. Pulse width modulation (PWM) can be used to vary the voltage applied to the motor. The average voltage applied to the motor is proportional to the PWM duty cycle (ignoring the second order effects of the motor inductance and discontinuous operation).
Figure 1: DC motor characteristics.
A basic example provides simple speed control of a DC motor using F3xx MCUs. This example reads the position of a potentiometer using the ADC and outputs a corresponding PWM signal using the PCA 8bit PWM mode. The hardware configuration is illustrated in figure 2.
A single N-channel Power MOSFET Q1 is used to drive the DC motor. The Power MOSFET should be chosen for the particular motor voltage and current requirements. A free-wheeling diode D1 is connected across the DC motor. When the MOSFET is turned off, the current through the motor inductance will continue to flow. The MOSFET drain voltage will rise to one diode-drop above the motor supply voltage. The current will then flow through the free-wheeling diode.
Most low-voltage motor drive circuits employ Schottky power rectifiers for the free-wheel diode. Schottky rectifiers have a low forward voltage and a very fast reverse recovery time. Both are important factors in a motor drive application.
Figure 2: DC motor characteristics.
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