Introduction: Tire Pressure Sensor
Newer cars typically feature a tire pressure monitoring systems (TPMS) that will warn the driver when a tire is significantly under-inflated. Tire pressure affects a vehicle's fuel economy, handling, and possible catastrophic tire failure.
This Instructable describes the design of an easy to make Tire Pressure sensor using GreenPAK. The system may also be used for anti-theft, warning when one of the sensors is not sending information (in that case, one of the tires may have been stolen).
In this implementation, the Direct Tire Pressure Measurement System will be used. It is based on installing a pressure sensor in each wheel to directly measure the pressure in each tire, sending the information to the vehicle's on-board computer which warns drivers when the air pressure in any of their tires drops at least 25% below the recommended cold tire inflation pressure, or if the tire has 25% over the recommended inflation pressure. These systems are typically more accurate and reliable, and most are able to indicate which tire is under-inflated.
Step 1: Pressure Sensor
The pressure will be measured and analyzed locally on each tire with the Silego GreenPAK SLG46620V, sending the information of under-pressure, over-pressure or correct pressure to the central system (onboard computer or dedicated system) via the communication system (Figure 1). With this implementation, a TPMS may also be retrofitted on older cars by adding a small central system to the console of the car.
Choosing the correct sensor is one of the most important stages in the design. There are two options about the type of sensor: Differential Pressure Sensors and Absolute Pressure Sensors. Since absolute pressure uses absolute zero as a definitive reference point, absolute pressure remains precise and accurate regardless of changes in ambient or process temperature. This is the main reason for choosing Absolute Pressure Sensors. The selected pressure sensor for this application is the SM5420C-060 from SMI Pressure Sensors. It is an absolute pressure sensor with an operating pressure range of 0 to 60 PSI. The power supply must be 5V (compatible with SLG46620V) and with a current low consumption of 1mA. The selected pressure sensor has a differential output proportional to the measured pressure and it can be modelled as: V0[mV]=(100/60PSI)xP, P= Absolute Measured Pressure.
The output circuit can be thought as a Wheatstone bridge, as it can be seen in Figure 2. Most important characteristics of the sensor are shown in table 1.
Step 2: Schematic Diagram
Due to the differential outputs of the sensor, and considering the simpler way for conditioning the signal for being acquired by SLG46620V’s ADC, the external implemented circuit is shown in Figure 3.
The signal conditioning circuit can be divided in two parts to describe it.
First of all, the differential output of the sensor (out+ and out–) is converted to a single ended signal with the operational amplifier U1. This is done by a typical differential configuration with unity gain. With this circuit, the signal obtained at the output of U1 is: Vu1="out+" - "out-"
It’s important to mention that, if pressure is zero, the output voltage is zero.
That’s why the operational amplifier must be a rail-to-rail operational amplifier. The other part of the design is the second operational amplifier (U2). It is required to condition the level of the signal to meet the input specifications of the SLG46620V analog-to-digital converter. SLG46620V, in the electrical specifications of the datasheet, specifies that the ADC with the single ended configuration, must have a minimum voltage input of 30mV/G (where G is the gain of the Programmable Gain Amplifier of the ADC) to acquire the signal. To obtain this minimum level voltage, the second operational amplifier adds Vmin voltage to the single ended signal coming from the sensor and the first operational amplifier. With this configuration, the output signal (Vout) can be directly connected to the analog input of the SLG46620V. Vout = Vu1 + Vmin
When the specifications of the ADC are considered, the maximum input level voltage in single-ended mode is 1030mv/G. If the worst case is analyzed, the maximum differential output level of the sensor can be 135mV. In this case, it can be risky if the Gain is configured to 8 (because the maximum input level is 137mV) because low pressures may not be compatible with the minimum input voltage of the ADC.
For this reason, the ADC and the PGA are configured with a gain of 4. With this configuration, Vmin must be between 23mV and 99mV. The selected value is 60mV, so the output range of the conditioned signal is 60mV to 195m. The Vmin voltage is obtained from the SLG46620V DAC, connecting its output to one of the GPIOs.
Step 3: Implementation
Two main benefits of using Silego GreenPAK for this implementation:
- small size and low current consumption of the SLG46620V, allowing to measure and process the pressure locally;
- and speed of processing. Considering the timing requirement of the NHTSA standard, the SLG46620V processes the sensor data very fast, so the onboard computer is free to make all the necessary verifications before reporting low or high pressure.
You can go through this step to understand how the GreenPAK chip has been programmed. However, if you just want to easily create the Tire Pressure Sensor without understanding all the inner circuitry, download GreenPAK software to view the already completed Tire Pressure sensor design file. Plug your computer to the GreenPAK Development Kit and hit program to create the custom IC to control your Tire Pressure Sensor.
This step will discuss the logic that is inside the GreenPAK Tire Pressure Sensor design file for those that are interested in understand the internal circuitry.
The GreenPAK™ circuit design implementation is shown in Figure 4.
The single ended signal from the sensor is obtained from PIN 8, which connects to the input of the PGA. The PGA configuration is shown in Figure 5. It shows the PGA configured in Single-Ended Mode with a Gain of 4 and it’s always powered on.
The output of the PGA is connected to the Analog to Digital Converter. The configuration of the ADC is single-ended mode, with the RC oscillator as the ADC clock as shown in Figure 6. With this clock configuration, the ADC sample rate is 1.56 ksps.
The ADC conversion is analyzed with the DCMP/PWM blocks. DCMP0 compares the pressure with the low limit, indicating when the pressure is lower than the configured value with a low level on its OUT+ output. The DCMP/PWM 0 block is configured as DCMP, comparing the positive input with the value stored in Register 0.
DCMP2 compares the pressure with the high limit, indicating when the pressure is higher than the configured value with a high level on its OUT+ output. The PWM/DCMP 2 block is configured as DCMP, comparing the positive input with the value stored in Register 2. DCMP0 configuration is shown in Figure 7. Configuration of DCMP2 is the same as the configuration of DCMP0. To determine the outputs of the system, 2-bit LUT4, LUT5 and LUT6 are used. LUT4 output is high only when a low pressure is detected (low level at OUT+ of DCMP0 and at OUT+ of DCMP2). LUT5 output is high only when the correct pressure is detected (high level at OUT+ of DCMP0 and low level at OUT+ of DCMP2). LUT6 output is high only when a high pressure is detected (high level at OUT+ of DCMP0 and high level at OUT+ of DCMP2). Figures 8 to 10 show the configurations of 2-bit LUT4, LUT5 and LUT6.
The output of 2-bit LUT4 (Low Pressure output) is connected to Pin 16, the output of 2-bit LUT5 (Correct Pressure output) is connected to Pin 17 and the output of 2-bit LUT6 (High Pressure output) is connected to Pin 18.
DAC0 is included in the design as the voltage reference Vmin. It is configured to generate 60mV and is connected to GPIO19 via the VREF0 block. Its configuration is shown in Figure 11.
Step 4: Test and Conclusions
To test the implementation, a linear ramp of pressure was applied to the sensor, from a low pressure to a high pressure along the analyzed range. To analyze the results, pins 16 to 18 (in this order) were logged with a logic analyzer. These outputs can be seen in Figure 12.
It can be seen that the system is tested for the three possible states, obtaining a high level on the corresponding output pin of the SLG46620V.
We have shown how to condition the signal to meet the ADC and PGA specifications of the Silego GreenPAK and the entire implementation is described. It is important to mention that the values used to compare the ADC conversion can be changed for different car and tire models, without changing the logic of the system.
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