Driving modes: motor power mapping
One of the most notorious advantages of an electric powertrain is its capability to offer a fast response to the user's input, as the nature of electric motors allows immediate variations in performance with minimum latency, being able to reach max torque in an instant.
However, from this positive trait arises the issue of power delivery adjustment and parameter definition for a correct power map development.
Numerous commercial control units offer a user-friendly autotune tool to facilitate the task of controller parameter definition. This tuning however, being a standardised process adapted to a wide range of different electric motors, usually leaves a margin for improvement. To solve this matter, a field expert is needed to perform a manual tuning that ensures the optimal interaction between controller and motor.
Before discussing the tuning process itself, a look must be taken at the different optimization options that a power map can face:
- Max range
Also known as eco mode, this setting reduces power and max speed to save battery charge. The throttle response behaves exponentially, limiting power access during a wide range of travel while reserving maximum acceleration for the last push. This dynamic is meant to support a smooth, slow and efficient riding style.
- Max power
In this configuration, response times are shortened and the electric motor delivers maximum torque from initial low revolutions. Battery monitoring becomes especially relevant as high discharge currents demanded from the motor could lead to cell degrading.
- City mode
In this setting, speed is limited to the urban environment and the acceleration curve shows a linear progression. Regenerative braking, if included, is configured to its maximum setting to take advantage of the speed changes and frequent stops.
Once the mapping possibilities have been seen, the dynamic changing parameter types can be reviewed.
Battery settings need to be included into the controller parameters to allow a correct power delivery towards the motor. Some of the most relevant aspects are: Battery discharge index (BDI), nominal voltage values, cell voltages or charge and discharge rates. For battery integrity, factors like over and undervoltage threshold as well as voltage protection techniques need to be included.
Motor thermal monitoring requires also input for sensor type, operational temperature values and current cut-off temperature.
- Power delivery
Another important aspect is power transmission to the motor and effective delivery. The throttle response graph is crafted from several setpoints that can shape different settings, depending on the operation mode. If the vehicle includes a boost mode, its operating parameters like maximum phase voltage and current need to be defined and tested to ensure safety.
Just as in the other managing tasks where the control unit is involved, signal correction techniques need to be established through the definition of integral and proportional parameters in control loops. This step is especially important to counter field weakening effects at higher speeds which would cause detrimental performance losses.
- Braking dynamics
Deceleration processes are of great significance in electric vehicles as they offer the opportunity to recoup energy through regenerative braking. To achieve this, the braking models action needs to be defined between off-throttle braking, manual braking or other iterations. Stopping progression and resistance also offer a wide array of parameters that play a key role for a smooth motor-to-caliper braking sequence.
- Additional features
Apart from functionalities involved directly to vehicle traction, some ECUs enable the implementation of additional features related to safety and user experience. Some of these modifications are: ability to turn off traction control under certain circumstances, reverse mode for motorcycles or cruise control. If the controller does not support these features, an additional ECU could be fitted to expand the capabilities of the system.
Related to parametric modelling of an electric powertrain, some of the beforementioned variables are defined with a theoretical study, others require a sensitivity analysis where parameter dependency and correlations are established.
As a final and closing step for the tuning procedures, EEVAM Technologies works hand in hand with users and test riders to adjust the vehicle dynamics to their feedback and preferences. Track days are essencial for validation and fine tuning to achieve the customer's desired power map.