Measuring vehicle energy and power distribution for components, systems and certification

Importance of energy management in EVs

Understanding how vehicles use and distribute energy is crucial for electric vehicle development and certification. Electrical powertrain, including motors and inverters, heating, air conditioning, infotainment, and other sub-systems, all consume energy that is supplied by a battery pack. Any energy usage or inefficiency can result in a shorter range of the vehicle. By mapping out the energy usage of all the subcomponents, automotive engineers can make decisions about vehicle control, and component selection to maximise the range of the vehicle. Vehicle manufacturers must make important decisions between weight, cost, range, and performance to make vehicles that have a desirable range for customers and meet green energy requirements.

Figure 1: Excerpts of various drive cycles showing speed vs time

The three main groups interested in vehicle energy consumption are – certifying bodies, system engineers, and component engineers.

– Certifying bodies need accurate power measurements to give the electrical mile per gallon (MPGe) efficiency certification of a vehicle, allowing consumers and governing bodies to make decisions.

– Vehicle and system-level engineers need to understand how the different components and subsystems perform so they can optimise the range of a vehicle, not just for the powertrain but also for all the subsystems that are also consuming energy.

– Lastly, component-level engineers need to be able to look at where losses occur and how to minimise them.

These types of vehicle range tests require a standardised torque and speed profile so that it can be a true standardisation between vehicle ranges. The profiles, or drive cycles (Fig. 1), are a profile of speed vs time. Drive cycles have different speeds, accelerations, and decelerations to simulate ‘city’ and ‘highway’ miles.

The profiles will be played back on a chassis dynamometer that has been tuned for the vehicle rolling dynamics. The driver of the vehicle will then control the speed while energy usage and distance are measured. The energy can be measured for a whole vehicle, sections of the vehicle, or individual components depending on what engineers want to optimise.

Certification testing

When a manufacturer brings a vehicle to market, it needs to have its range and fuel efficiency certified by the governing bodies of that country. These groups run the drive cycles from full charge until the vehicle runs out of battery charge and record the energy used and the distance travelled. Measuring the energy usage is accomplished by putting a current clamp around the main DC cable and measuring the DC voltage and current into a power analyzer, which then calculates the electrical power and energy being passed through the cable. If a vehicle has multiple DC batteries, the energy would be measured from those as well and added up.

Measuring the DC bus voltage can be challenging. Voltage and current access points are often hard to access and require some vehicle modification to get voltage probes and current sensors in place. Some auto manufacturers will have currents running through their DC cable shield. In this instance, the OEM may need to route the shield around the sensor or have a clever method of compensation. Another challenge with certified tests is that the data and equations need to be traceable for auditability. Having recorded data, known measurement periods, and clearly defined equations will ensure that any discrepancy in the test is understood and resolvable. Some issues may include sensor drop out, electrical noise, misunderstood behaviour, and others.

The tests will often have different segments that need to have their energy separated so that governing bodies can assign not only a total fuel efficiency but also a city and highway fuel efficiency. The drive cycles will be segmented, and the energy will be determined for each segment. This will be used to determine the city or highway fuel efficiency.

The instrumentation for measuring energy and power during a range test can directly affect the accuracy and complexity of the test. Engineers often look for systems that simplify testing by selecting a measurement system that records electrical data and gives easy access to equations. Having the ability to audit and edit a test can make testing complex systems with multiple DC buses or shields a much simpler task. Some tests will need to be executed over many hours, in which case engineers will want a system that can store, and reduce a significant amount of data, while giving transparency to the results. Having the options to trigger, segment, and feedback data to a control room are also features that can help raise the reliability and quality of certified range tests. The HBM eDrive system is often used for certified range testing because it is designed to store data, receive triggers, edit equations, time align data and offer feedback to control systems.

Figure 2: Drive cycle power, energy, and segment reset signals

Engineering range test

An engineering range test is similar to a certification test but with more measurements and potential configurations. Engineers run the tests to optimise their vehicle energy usage so that they can maximise their range for certification. The test is still a full vehicle running through drive cycles on a chassis dyno; there is still limited access to voltages and currents, and the collection of data is even more important because now engineers will want to understand the details to make changes. The tests require more measurements because, in addition to the DC bus measurements, they will also include all the subcomponents and sub systems. This can include as many as 15 power/energy measurements, which can create measurement challenges since many power analyzers only offer 3, 6, or 7 channels. Having multiple systems to measure a test will result in time alignment issues and increased capital cost. Ideally, an engineer will measure and record all the energies so that they can have a detailed understanding of how power is distributed throughout a vehicle. Once the engineers collect the data from each of the power consuming units, they can begin to make decisions on how to control the vehicle and how to operate the subcomponents. To fully understand the system operation, this often needs to be aligned with CAN bus or vehicle information so that changes can be made to timing and operations. It also may be of interest to incorporate other vehicle level signals or temperatures to get a full vehicle understanding. Once the full vehicle power flow and signals are understood, changes to the vehicle level control, or subcomponents can be made to increase the range.

Engineers often try to measure the AC signals in the engineering range test to understand the inverter and motor losses during the test. However, this can prove to be difficult because measuring AC power requires an exact measurement of the fundamental frequency. Tracking the fundamental frequency requires advanced algorithms to be executed in real time to get an accurate power and energy measurement. An example of a cycle detect algorithm can be seen in Fig.3. When selecting instrumentation, the method of frequency synchronization needs to be considered to ensure an accurate measurement.

Engineering range tests will also be done on competitor vehicles for benchmarking or to understand how other companies manage energy distribution. This can present added challenges due to not always having access to measurement points. However, this can be overcome by taking voltage or current measurements off the vehicle bus. While not a perfect solution, it is better than no measurement and can provide added insights into how the vehicle is running. For the best estimate of the operation, the engineer will want to have the vehicle bus and measurements closely time aligned.

Engineering range tests have many of the same instrument requirements as the certification test. They need to have recorded data and transparent equations. Engineers need to be able to track the energy usage for different sections of the test and need to visualise and communicate measured values back to a control room. The difference is that they need to do more measurements and incorporate a wider variety of measurements. There is also a possibility that there will be AC measurements. The eDrive system can easily accommodate this type of testing by allowing for >51 power measurements of DC or AC in a single measurement system and include CAN and temperature measurements into the data file. The eDrive measures power with a digital cycle detect that can easily measure AC power during frequency changes.

Figure 3: Cycle detects for dynamic power measurements

Component range testing

Component range testing can also be done on a chassis dyno, but is often done on a direct drive dynamometer for the best quality measurements on components. The test involves mounting a motor and inverter to a precision dynamometer (Fig. 4) and then running the torque and speed profile of the drive cycle. The DC bus, AC phase measurements, torque, and speed will typically be measured for these profiles with high-precision instrumentation.

By taking high-accuracy measurements, engineers can start to understand the power losses of the components in detail. If the engineers choose to incorporate temperatures of both the rotor and the stator, they can start to look at iron, copper, and other losses to understand how energy losses are distributed throughout a drive cycle. Once the losses of the component are understood, they can start to be controlled and minimised.

Another goal of component-level testing is to calibrate the motor and inverter to be as efficient as possible while hitting performance objectives. To do this, engineers must understand the inverter control by reading the CAN bus, temperatures by measuring thermocouples, and efficiency by running efficiency maps and drive cycles.

Figure 4. Mounting a motor and inverter to a precision dynamometer

Testing for the component test has many of the same challenges as the two previous test types, but now includes torque and speed to characterise the motor, and motor losses in detail. Rather than optimising a system-level control, these engineers will be looking at fine details and making control or design changes for the future. The eDrive system can benefit these engineers by taking in a wide variety of measurements at a high accuracy and recording the data. This will allow for a better understanding of the details of the machine and make changes.

Conclusion

Vehicle energy management is a detailed process that includes many steps and considerations. It presents challenges because of potentially high channel counts and the dynamic nature of the signals. Vehicle energy management is potentially handled by a variety of groups, all with the goal of minimising their losses and maximising their range for real-world driving scenarios. The need to accurately understand the energy distribution throughout the vehicle has made measurement an important topic when discussing these types of tests. The eDrive system can help simplify testing of vehicle energy management and drive cycles by accurately measuring high channel counts of power and mechanical signals, including power during frequency changes. eDrive provides transparency to tests by presenting all the equations to the user and allowing them to change equations if necessary. Engineers can segment their tests into different drive cycles and visualise the data in a control room. The eDrive system can also be used for all three tests to give consistent data across a company, simplifying testing and saving money.

About the Author

Mitch Marks has been involved in electric motor development since 2015. His expertise lies in the test and measurement of traction motors and drives. His focus is on developing new testing techniques and accelerating the development process. Since 2017, he has been an integral part of the electric power testing team at HBK. Mitch holds both a bachelor’s and a master’s degree in electrical engineering from the University of Wisconsin – Madison WEMPEC program.

HBK (Hottinger, Brüel & Kjær) accelerates product innovation with solutions in virtual, physical and in-process testing. From the electrification of mobility to the advancement of smart manufacturing, HBK supports customers throughout the entire product life cycle, sharing their mission for a cleaner, healthier, and more productive world.

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