The trend in aircraft design is toward more efficient brushless DC motors. A integrated motor drive module for brushless DC motors from International Rectifier (El Segundo, CA) reduces the effort of programming the motor control and simplifies certification. Because the device is configured for sensorless field oriented control, the elimination of Hall sensors improves reliability while reducing component count. It enables the aircraft to achieve greater fuel economy and lower maintenance costs.
The module’s main feature is its robust construction that allows it to operate in a very low temperature or a very high temperature with high humidity. The device can transition these temperature extremes many times due to the designed-in materials that are closely matched for TCE (temperature coefficient of expansion). The device is also designed to withstand the vibration profile of an aircraft and to operate at high altitude.
To improve fuel economy and environmental and hydraulic control in aerospace and military applications, mainframe manufacturers are adopting an alternative approach to motor drive control. Instead of fixed speed induction motors with mechanical gearboxes, new motor control architectures are moving toward variable speed permanent magnet (PM) motors due to their smaller size and lighter weight. These motors, which exhibit a high torque-to-current ratio, efficiency and power factor, are designed to drive fuel and liquid cooling pumps, environmental control fans, compressors, and actuation of flight surfaces more efficiently.
Facilitating the transition away from geared motors, the integrated motor module for sensorless vector control of variable speed PM motors utilizes a new class of dedicated control engine with an embedded pre-configured sinusoidal field oriented control (FOC) algorithm. One significant advantage of this control engine is the very short computation time to complete closed loop control algorithm with deterministic timing. Fast computation directly influences the dynamic performance of torque and speed of a servo system. The faster the update rate of closed loop current control, the higher the band-width of torque control.
The digital controller integrates benchmark gate drivers for the analog interface, power MOSFETs and feedback electronics, to provide complete drive control in a robust, compact lightweight plastic package.
Sensorless DC Motor Control
The dedicated control engine embedded in the digital controller, called a Motion Control Engine (TM), comprises configurable control blocks (i.e. Proportional plus Integral, Vector rotator, Clark Transformation, etc.) needed to perform closed loop controls and motion hardware peripherals (Space Vector PWM, motor current feedback interface, encoder or resolver feedback etc.). Unique flow control logic is used to structure the control blocks into a configurable control loop that enables parallel execution and multi-loop control. Therefore, no multi-tasking operating system is required. A synchronous execution mechanism of closed-loop velocity control and closed-loop current control can be included in the control structure. Flexibility is retained by implementing configurable parameters in the control blocks and peripherals. The parameters are accessed through a real-time host register interface that can be read or written by a host controller or a companion MCU.
The digital IC allows the designer to control motors up to 500 VA without custom software or Hall sensors to determine the rotor position. Without Hall sensors for position sensing, the rotor must start in a known location. In order to gain control of the motor, the rotor windings must be spun and parked to the known position. The goal is to maintain an orthogonal relationship of the stator and rotor windings. The stator winding back EMF is a function of the motor speed, and the stator currents are a function of the back EMF and applied voltage. The rotor position can be calculated, and the signal fed to the space vector PWM modulator and the appropriate gate signals applied to switch the MOSFET. In addition, inverter leg shunt current sensors are used to feed back phase current to the digital controller in order to provide the maximum torque and maintain speed. The rotor angle with respect to the stator is estimated every 11 microseconds to provide a smooth operating drive with minimal torque ripple.
The sensorless control algorithm operates over a continuous speed range of 10 to 100% of full speed without overload for greater performance. Removing the Hall sensors reduces the motor’s cost while significantly improving reliability.
The module is designed to shut down during an overcurrent, overtemperature, or an over- and undervoltage event. Phase currents are monitored. If the safe condition is exceeded, a gate kill signal is applied and a fault occurrence is indicated. At this point, a restart sequence can be initiated which includes clearing the fault.
A thermistor is mounted on the IMS substrate board close to the power silicon. If the substrate temperature exceeds the safe limit of 105 degrees C, a gate-kill signal is asserted. The DC bus is also monitored for under-voltage and over-voltage conditions and minimum and maximum speed conditions. Operation outside these preset limits will also produce a fault to protect the load. To clear the fault condition, a fault clear pin is provided to initiate restart. Communication to the device is done via an RS-232 serial interface and can be used for longer cable lengths or noisy environments. Additionally, the module uses an error detecting protocol to maintain the integrity of the host registers.
Optimized Power Stage
Based on selected parameters, the proprietary algorithm within the digital controller ultimately controls the gate driver in the motor control module and the switching of the power semiconductors during commutation.
The optimized chipset—including the control, gate drive and power MOSFETs and intellectual property—is combined with rugged packaging to meet vibration profiles in a small lightweight package ready to mount on a cold plate or heatsink.
This integrated package incorporates a fully qualified power MOSFET mounted on an insulated metal substrate (IMS) close to the gate drivers for the lowest possible inductance. Each lot is screened to military specification MIL-PRF-38534 to ensure the highest level of reliability and rated at the full operating temperature range of -40 to +85 degrees Celsius.
Since the control algorithm is a hardware-based IC dedicated to motor control, certification requirements are simpler than those designed for a software-based algorithm programmed into a microcontroller or FPGA, resulting in a rapid, cost effective time-to- market solution for electric aircraft system integrators.
A companion development tool expedites software development by providing a simple menu-based selection process where motor design and user defined parameters such as ramp time or maximum speed are organized a drive parameter spreadsheet. Upon completing motor design, the final design parameters and the user defined parameters are entered into the development spreadsheet. The development tool then translates the motor parameters and stores them within the module, enabling the motor designer to quickly evaluate the motor’s performance. Diagnostic tools are also available to aid this process.
This article was contributed by Michael Toland, Product Marketing Manager, International Rectifier, El Segundo, CA. For more information, Click Here