Low Cost High Speed Brushless DC Motor Drive

Leg Current Sensing Triple-Half Bridge

The core of the motion solution is a digital triple-half bridge which switches the HV voltage into the various coils of the Brushless DC motor under control of the Juno IC. Dropping resistors measure the current flow through each of the three coils. Leg current sensing allows the use of less expensive ground-referenced operational amplifiers and delivers much less noisy current measurements. Current measurements are taken at specific portions of the PWM excitation cycle but these detailed timing considerations are handled automatically by the MC73113 IC.

Accurate Commutation Initialization

Since there are no Hall sensors in this application (which saves cost) we will use the quadrature encoder to commutate. A key question then becomes how the phase angle is initialized. The answer is a procedure applied before normal motor operation which energizes the motor coils and observes the resulting position of the motor. The MC73113 handles all of this using a method called Pulse Phase Initialization. The only requirement is that the motor be able to rotate freely.

Getting Brushless DC Motors to spin efficiently at high speed is a challenge familiar to many applications, even some on the water! Manta5 of New Zealand needed a compact, cost-effective solution to control the main propulsion drive on one of the world’s first hydrofoil bikes, the Hydrofoiler SL3. Their solution harnesses PMD’s Juno IC, featured in this application note, to control the power of the BLDC motor.

Learn more about Manta5’s interesting and fun new product at manta5.com.

Field Oriented Control (FOC)

In this application a drive technique called Field Oriented Control is used to give the best performance. You should make sure the motor is wound for sinusoidal commutation for best results. If not, you can use an alternate technique called third leg floating (sometimes also called trapezoidal commutation) to attain high speeds. But generally speaking, field oriented control gives the smoothest and highest speed output for a given motor. The diagram below shows how FOC works, although to some extent this can be considered an exercise left to the reader. The MC73113 IC allows the selection of either FOC or third leg floating control depending on the motor type used.

FOC Decoupling

A further technique used within the FOC controller is worth a mention, namely FOC decoupling. FOC decoupling improves control performance by addressing situations where the independent PI loops, one for D and one for Q, saturate due to a lack of available voltage (one or more of the A, B, C coil voltage commands drive at 100%). The result can be a reduction in drive efficiency. FOC decoupling addresses this by limiting the output of the Q loop and cross-coupling the D and the Q loop outputs rather than having them be entirely independent. From the user’s perspective the only additional control parameter needed to activate FOC decoupling is the motor’s electrical time constant.

Juno Family of ICs

The Juno family of ICs are perfect for building your own low-cost, high performance Brushless DC motor controller. Juno's excel at velocity and torque control, with features such as FOC, profile generation, high/low switching amplifier control signals, leg current sensing, and more. Available in packages as small as 7mm x 7mm and costing $12 in quantity, these ICs are an ideal solution for your next your next high speed motor controller design.

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Stepping and Servo Motor Commutation

written by Kevin McCarthy, CTO, Dover Motion

Multi-phase motors need to switch among phases as they move, which is referred to as commutation. Most systems with incremental encoders require an initial commutation phase finding step upon power-up. Depending on the specifics of the application, this can range from quick and easy to problematic.

Click here to download this white paper to get a solid grounding on the subject of commutation and to learn about a robust algorithm that ensures accurate commutation even under challenging conditions.