Status | IN PROGRESS |
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On this page |
🌍 Overview
This is the design document for the Amperes Board. As 1 out of the 4 important boards within our system, we need to be able to track the current within the car so that it is in a safe and manageable level. We also want to think about how we can record the current to apply the result to a State of Charge (SoC) algorithm.
❓ Problem statement
BPS is tasked with securing the safety and monitoring of the battery, such as current monitoring. Over-current of the battery can damage internal components and pose a risk to the driver.
💡 Research insights
There are 2 ways to measure current, either using a shunt resistor connected to the load or using a hall-effect sensor. They have their own pros and cons but they solve the same problem.
📊 Solution hypothesis
The solution is successful if we are able to track current reliably and transmit that data to the leaderboard.
🪛 Design options
There are only 2 ways to measure current. Either measure the voltage across a resistor in series with the load or we can measure the magnetic field of the wire. Invasive vs non-invasive
Option 1 - Voltage Across a Resistor | Option 2 - Magnetic Field of Wire | |
|---|---|---|
Overview | Current Sense Amplifier or Isolated Modulators/ADC | Hall-Effect Current Sensing |
Screenshot |  |  |
Pros and cons | Accuracy Simplicity and cost Affected by electric noise Power loss and voltage drop | Electric isolation and no power loss Measures both AC and DC current Affected by magnetic noise and temperature Zero-drift problems |
Decision 1
- Option 1 - Making the PCB though a shunt resistor
In the future, members can look into the second option for a hall-effect version
Ideas
Figure 1 is a common method to measure current using a shunt-resistor. There are 3 steps to read the current:
The differential voltage is fed into the Current Sense Amplifier and converted to a single-ended signal.
This single-ended signal is connected to an ADC, digitizing the signal.
The signal is sent to a microcontroller for processing.
⚡ High Side vs Low Side
For reading the current, there are 2 different configurations you can have your device hooked up to.
See Introduction to Current Sense Amplifiers to learn more
Figure 2 shows an example of the current sense amplifier connected to the shunt resistor in a high-side sensing configuration.
Advantages:
Able to detect load short to ground
Current is monitored directly from the source
Disadvantages:
High voltage can limit the variety of devices
Figure 3 shows an example of the current sense amplifier connected to the shunt resistor in a low-side sensing configuration.
Advantages:
Wide range of available options
Does not need an advanced sensor
Disadvantages:
Difficult to detect load short to ground
Decision 2
- Both?!: use high and low-side sensing
We will get the advantages of both and no disadvantages except cost. Plus, built in fault monitoring.
Fault monitoring:
By comparing the readings from both high-side and low-side sensors, you can detect faults such as open circuits, short circuits, or unexpected current paths. Discrepancies between the two readings might indicate issues in the circuit that need attention.
🖼️ Chips to pick
There are different chips that you can choose to make your life easier. The current sense amplifier is great, but you could also use an isolated modulator (an integrated ADC) to track the voltage differential. Below is the selected shunt resistor given as a requirement.
Current Shunt Resistor: WSBM8518L5000JK
Resistance: 500 µOhms
Requirements
Below is the specs that the sensor should be able to meet. There needs to be two different ones as there is some better for low-side sensing and some better for high-side sensing.
High-Side Pick - INA226
Electrical Specifications:
Supply Voltage: +3.3V
Common Mode Voltage Range: -2.7V to +120V
Shunt Voltage Range:
Power Consumption: 36 W
Accuracy:
Gain:
Gain error: ±0.1% (maximum)
Gain drift: ±5 ppm/°C (maximum)
Offset:
Offset voltage: ±12 µV (maximum)
Offset drift: ±0.2 µV/°C (maximum)
Link to TI: https://www.ti.com/product/INA226
✅ Follow up
# | Decision | Status | Next steps |
|---|---|---|---|
1 | Decision 1 - Shunt Resistor | COMPLETED |
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2 | Decision 2 - Both Sided Current Sensor | COMPLETED |
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🔤 Vocab
Current Sense Amplifier: Amplifies the small voltage drop across a shunt resistor to measure current accurately in circuits.
Isolated Modulator: Converts analog signals to digital while maintaining electrical isolation, often used in high-voltage or noisy environments.
Hall-Effect Sensor: Detects magnetic fields to measure position, speed, or current without direct electrical contact.
Zero-drift: the phenomenon where a sensor's output signal shifts away from its baseline (or zero) value when there is no actual current flowing through the sensor; affects Hall-Effect Sensor
💎 Resource files
How to Sense Current | Hall-Effect White Paper | Shunt vs Hall-Effect | Isolated Amplifiers vs Isolated Modulators |
Introduction to Current Sense Amplifiers |