Welcome to part 1 of a series of blog posts.

What in the world is a TPS?⌗

TPS_Sensor

For some context a TPS is a throttle position sensor. This is used by car manufactures to tell exactly how far down you’ve pressed the throttle pedal.

The specific part I’m working on designing a replacement for is a TPS sensor for a Honda Civic with a D-series engine.

The OEM sensor itself is a very simple design, housing a analog potentiometer inside of a plastic housing.

The sensor itself has only three connections, VCC at 5v, GND, and a tps signal that goes between 0.45v-4.5v during operation.

Why is a whole new design needed?⌗

This is where things get a bit more complicated. The original design used decent quality components and this meant that the potentiometer itself stayed in a usable state for some 20 years.

Issue is that almost no-one manufactures the part in a decent quality, with existing manufactures offerings falling apart or wearing through the resistive material internally so quickly that they will often last less than a year.

Design Criteria⌗

With all this in mind I came up with a list of requirements for the replacement.

Requirements:

  • No critical mechanical wear surfaces.
  • Reliable and tunable system.
  • Low cost of materials.
  • Powered off existing connection.

Component selection⌗

Breaking down the requirements as simply as possible, I needed a way to measure the angle or position of a shaft, map that to an appropriate voltage and send the signal to the ECU.

This list gave me the following basic outline:

  1. micro-controller
  2. magnetic rotary encoder
  3. DAC(Digital To Analog Converter)

My original plan was to try and create the system using as few components as possible. As the design progressed however I found myself realizing that the cost of a microcontroller that could preform the DAC functions needed would cost more than a lower cost micro-controller and stand alone DAC.

Magnetic Encoder⌗

For the magnetic encoder I only needed to actually track a small range of approximately ninety degrees. This also didn’t have a very strict requirement for how fast the sample rate needed to be given the low speeds or rotation.

For this I selected the AS5600 IC.

Spec Summary:

  • 12bit resolution, 4096 values.
  • I2C interface.
  • 5v/3.3v power input.
  • -40C-125C temperature range.

Given the specifications it’s more than adequate for working inside of a hot engine bay.

DAC(Digital To Analog Converter)⌗

The MCP4725 IC from MicroChip was an easy choice given it’s availability and low cost.

Spec Summary:

  • 12bit resolution.
  • I2C interface.
  • Single channel.
  • 5v/3.3v output.
  • -40C-125C temperature range.

Micro-Controller⌗

The selection of micro-controller is a bit different than the other components.

I selected the ch32v203 series of controllers for their low cost and interesting set of peripherals. I also considered the ch32v003 and stm32103 but ultimately decided I wanted to explore using a different architecture.

Spec Summary:

  • RISC-V QingKe V4 Core
  • 144MHZ clock rate.
  • 64KB Flash
  • 2x I2C
  • 4x USART
  • 2x SPI
  • 1x CAN
  • 1x RTC
  • ….and more.

Honestly the micro-controller is complete over-kill for this application In this case it’s a reasonable given it’s lower cost compared to the RP2040 and STM32 controllers with equivalent features.

I say equivalent but the RP2040 isn’t usually a controller I want to select or work with given the ADC issues, lack of built in flash as well as having no USB peripherals(PIO isn’t a substitute). Having a higher unit cost doesn’t help either although I find the dual core architecture interesting.

Schematic Design⌗

For the schematic and PCB design I used KiCAD.

Creating new Libraries⌗

New symbol and footprint libraries had to be made for the new components as the encoder and the micro-controller didn’t have any existing ones.

Continuation⌗

TO BE CONTINUED