White Papers
Transense Technologies, in collaboration with the transmission and electric motor drive systems development experts at Drive System Design Limited (DSD), have developed a new approach that significantly improves the power density of electric motor drive systems.
By leveraging Drive System Design's unique simulation and development expertise and Transense's innovative torque sensor technology, we have unlocked a new electric motor control strategy to unlock more power density.
Increasing Electric Drive System Power Density
To increase the power density of electric drive systems, we need to make them smaller and lighter. This requires designing electric motor systems that operate at higher rotational speeds.
However, higher rotational speeds, require higher inverter switching speeds, reducing the available control algorithm execution times. Meaning motor maximum RPM is limited by algorithm execution speed, constraining motor speed and power density.
Our new approach addresses this limitation with faster control algorithm execution times. Control algorithms operate as sequential procedures, following a defined set of steps that cannot be skipped or removed. These steps determine the execution time, which for Field Oriented Control is 8.24 microseconds in the reference system. The motor speed dictates the window for performing these calculations, but in the most commonly used approach, Field Oriented Control, the entire window is consumed, limiting machine speed and powertrain size.
To overcome this, Transense and Drive System Design have employed Direct Torque Control (DTC). Although DTC has traditionally suffered from torque ripple error and observer interdependency, our new method mitigates these issues.
Improved Torque Control with Direct Torque Control (DTC)
The red trace in our tests shows the torque response with significant ripple over the blue torque command. With DTC, waveforms are forced onto the motor, but this does not always result in the requested torque, as shown by the green trace indicating actual torque. This discrepancy affects vehicle performance.
Torque ripple and error arise from the use of measured currents in the torque observer, which injects noise and creates interdependency with the flux observer, also affected by noise from voltage and current measurements. We eliminate these noise paths by replacing the torque observer with an actual measured torque.
Benefits of Measured Torque
On the image below the red line represents measured torque directly feeding the DTC control, unlike traditional DTC which relies on a torque observer. This new method eliminates noise paths and observer interdependence, aligning the delivered torque (green trace) with the torque command. The torque ripple on the green line is reduced by 50%, significantly improving electric motor performance.
Challenges and Solutions in Implementing Torque Sensing
Implementing torque sensing in electric motors presents challenges. Conventional torque sensors, typically used in dyno testing, are too large and costly for powertrain systems. They also require maintenance and are susceptible to electromagnetic radiation and DC magnetic fields. Consequently, measuring output torque in production motor systems is difficult and often overlooked in the development process.
Surface Acoustic Wave (SAW) Sensor Technology offers a robust solution for torque measurement in electric motors. This wireless, non-contact system comprises SAW sensing elements, passive strain and temperature measurement devices mounted on the rotating shaft, a simple antenna called an RF coupler, and an electronic interrogation unit known as a reader.
The reader generates an interrogation signal transmitted to the rotating shaft via the RF coupler. The SAW-sensing elements, which do not require an external power source, reflect the interrogation signal back to the reader as a passive backscatter. The frequency of this backscattered signal is influenced by physical measurements such as strain and temperature. The reader analyses the received signal and calculates the measured strain and temperature, providing an accurate measurement of shaft torque.
Cost Efficiency and Competitive Advantage
Cost is a critical factor in any powertrain design. This advanced direct torque control technology can potentially reduce overall system costs. By eliminating the need for a costly sine-cosine position sensor and simplifying the algorithm execution, we reduce the processing power required, enabling the use of lower-cost microcontrollers, which are a significant part of the control system's bill of materials.
The shift towards electric drive in automotive, aerospace, and robotics is driven by the need for competitive advantage through increased efficiencies. Transense's SAW Sensing Technology can help deliver this competitive edge.