[Popular Solution] Electronic Fuel Pump Reference Solution Based on S12ZVM

The Electronic Fuel Pump (EFP) is one of the core components of the vehicle power control system, and is mainly responsible for transporting fuel from the fuel tank to the engine.

At present, most of the EFPs on the market are based on brushed DC motors, but DC motors use a mechanical brush commutation structure, which has disadvantages such as mechanical wear, noise, sparks, and short life during use, resulting in reduced EFP life and reliability. Compared with DC motors, brushless DC motors (hereinafter referred to as BLDC) use electronic commutation instead of mechanical commutation, solving the above DC motor shortcomings while retaining its good speed regulation performance. Therefore, BLDC motors tend to gradually replace DC motors in EFP applications.

In order to prevent some corrosive chemicals in the fuel from corroding the sensor, the BLDC motor in the EFP basically adopts position sensorless control. Starting technology has always been a challenge for position sensorless control technology. Since EFP has higher requirements for fast and stable starting, solving this problem is the key to the entire reference design.

Based on this background and market demand, NXP has developed an EFP reference design based on S12ZVM , which provides a total solution for EFP applications based on BLDC and PMSM in 12V automotive systems to help customers shorten development cycles and reduce development costs. This reference design is not only suitable for EFP, but can also be applied to other BLDC and PMSM control systems with power below 250W, such as blowers.

The chip used in the EFP reference design is the MC9S12ZVM from the NXP MagniV family, which has all the advantages of a 16-bit MCU with a maximum frequency of 100MHz, while retaining the efficiency advantages of low cost, low power consumption, EMC and code size currently enjoyed by existing S12(X) series users.

The MC9S12ZVM series offers different pin-out options, using 80-pin, 64-pin and 48-pin LQFP-EP packages to accommodate LIN, CAN and external PWM-based application interfaces. This chip provides several key peripheral modules required for motor control, such as ADC, PTU, PMF, GDU. This highly integrated MCU can optimize the system architecture and save a lot of space. It can achieve a fully integrated single-chip solution and drive up to 6 external power MOSFETs for BLDC or PMSM motor drives. The block diagram of the MC9S12ZVM series is shown in Figure 1:

Figure 1: MC9S12ZVM series block diagram

The technical indicators of this solution mainly include the design goals of the reference design and the required performance requirements. The details are listed as follows:

• Supply voltage range: 8V to 16V, the maximum speed is allowed to be reduced when the supply voltage is below 12V;

• Rated voltage: 12V;

• Maximum power: up to 250W at 12V supply voltage;

• Start-up time: reliable and fast start-up, from standstill to rated speed, the start-up time is less than 150ms;

• PMSM sensorless FOC control, supporting dual-resistance and single-resistance current sampling;

• Integrated initial position detection and current staggered start algorithm to ensure fast and reliable start;

• The S12ZVM-EFP system passed the power supply voltage fluctuation, load fluctuation, speed command jump and temperature rise tests;

• Complete protection mechanism, supporting over-voltage and under-voltage protection, over-temperature protection, stall protection, hardware over-current and current limiting protection, etc.

• Support LIN/PWM to send speed commands.

The hardware part of this reference design adopts the design of S12ZVML128 + 7*NMOSFED. The simple design of the hardware part is due to the high integration of S12ZVM. The driver chip, power conversion chip and analog circuits such as op amps required for conventional motor control are all integrated internally.

This reference design supports two current sampling schemes: dual resistor and single resistor current sampling, and provides jumper cap settings, allowing users to select any control scheme through the jumper cap. Although this reference design uses position sensorless control, a Hall sensor interface is also reserved on the reference design board to provide users with more options.

For detailed information on the hardware design, please refer to the S12ZVM EFP RDB Hardware User Guide . The hardware system block diagram and PCB physical diagram are shown in Figure 2 and Figure 3 respectively.

Figure 2: Hardware block diagram of the EWP reference design

Figure 3: EWP PCB physical picture

The PCB of this reference design has passed FCC certification and is implemented using a 4-layer board structure, as shown in Figure 4.

Figure 4: PCB of the EWP reference design

The overall software architecture of this solution uses NXP's PMSM dual/single resistor sample program based on S12ZVM, optimizes the startup strategy, and integrates current staggered startup and initial position detection. You can refer to the software user manual S12ZVM-EFP RDB Software User Manual and the corresponding sample code on NXP's official website. The following software section describes the overall software architecture and initial position detection, current staggered startup, motor parameter measurement, and protection strategy.

The software architecture of this reference design is shown in Figure 5. The bottom layer is the hardware part, the middle layer is the related peripheral drivers and middleware components, and the top layer is the user APP and motor control module. The peripheral driver module consists of CPMU, ADC, PMF, GDU and PTU modules.

• The CPMU module is used to configure the system clock;

• The ADC module is used to sample motor phase current, bus voltage and chip temperature;

• PMF is used to generate six PWM signals;

• PTU is used to trigger ADC sampling at a specific PMF reload time point;

• GDU is the gate driver unit, which amplifies the PMF signal to turn the MOSFET on or off, and also implements charge pump management, fault mechanism configuration, etc.

Figure 5: System software architecture

The initial position detection of this reference design uses the salient pole characteristics of the PMSM. If the salient pole characteristics of a permanent magnet synchronous motor are obvious, the inductance and reluctance of the motor will vary with the rotor position. In particular, if the reluctance of the winding changes sinusoidally, its inductance will also show a sinusoidal change pattern.

As shown in Figure 6, the change period of the inductance is twice the rotor electrical period. For example, when the voltage applied to the motor is in the direction of VW, W->U, U->V, the inductance of the floating phase will change with the rotor position, so the voltage on the phase will also change. For example, when the U phase voltage is equal to half of the bus voltage, the V phase voltage is less than half of the bus voltage, and the W phase voltage is greater than half of the bus voltage, then it can be considered that the rotor is at -180° or 0°, so the rotor position is located within the range of 180°.

Figure 6: Inductance variation for different rotor positions

In order to obtain the precise rotor position, it is necessary to determine the direction of the rotor's N pole, that is, to determine whether the rotor is facing -180° or 0°. In general, for inductance, the smaller the magnetic resistance, the greater the inductance; the greater the magnetic resistance, the smaller the inductance. If it is an air-core inductor, then its magnetic resistance is certain, and therefore the inductance is also certain.

Therefore, an appropriate voltage vector can be applied to the motor to distinguish the N pole from the S pole using the magnetic saturation effect. As shown in Figure 7, the magnetic field generated by the rotor permanent magnet strengthens the magnetic field generated by the stator coil. Before reaching magnetic saturation, the stator inductance change rate is negative and the magnetic resistance increases. If the rotor permanent magnet is rotated 180°, the inductance change rate of the stator winding becomes positive.

Figure 7: Relationship between magnetic field and inductance

The relationship between the permanent magnet synchronous motor rotor position and the three-phase winding is shown in Figure 8. The U-phase stator winding is closer to the rotor's d-axis, and the V-phase winding is closer to the rotor's q-axis. Therefore, the U and V phases are magnetized to different degrees by the rotor, and the U phase is magnetized to a higher degree than the V phase. When a voltage vector is applied to the motor (U phase connected to VDC, V phase connected to GND), Lu is smaller than Lv at the beginning, but the gap between the two gradually narrows and finally becomes equal. That is, the direction of the rotor's N pole can be distinguished by the change law of the W-phase voltage.

Figure 8: Relationship between rotor position and three-phase winding

The DQ axis current staggered start algorithm used in this reference design is the key to ensure fast and reliable start of the EFP, so that the EFP can reach the rated speed from standstill within 150ms. The setting of two speed thresholds is the key to the DQ axis staggered algorithm. The first is to set the critical value when the DQ axis starts to change. When the speed reaches this threshold, the D axis current decreases and the Q axis current increases. The second is to determine the critical point when the motor enters the sensorless closed-loop operation. At this time, the D axis is 0 and the motor is forced to enter the sensorless closed-loop operation state. This algorithm can better improve the starting performance.

The protection strategy of EFP is similar to that of conventional motor controllers, including overvoltage and undervoltage protection, overcurrent protection, short circuit protection, stall protection, and overtemperature protection. I will not go into details here. For details, please refer to another article of this public account, " Electronic Water Pump Reference Design Based on S12ZVM " , which introduces various protection strategies in detail.

If the user does not have the motor specified by this reference design, or uses his own motor according to his project requirements, he first needs to measure the parameters of the motor used. To measure the motor parameters, you need to prepare some general equipment, such as a multimeter, oscilloscope, LCR meter, and power supply. For specific methods on how to measure the parameters, please refer to the document AN4680 on the NXP official website. We will not go into details here, and you can refer to the software user manual S12ZVM-EFP RDB Software User Manual on the NXP official website for details .

In combination with the actual needs of the fuel pump, basic tests such as fast and reliable start-up test, load fluctuation test, power supply voltage fluctuation test, PWM sending jump speed command and temperature rise test were performed on the project.

The oil pump system built in the environment is shown in Figure 9. The test equipment includes a 4-channel oscilloscope, a current probe, an adjustable DC power supply, and a signal generator.

Figure 9: Test environment setup: oil pump system

By frequently opening and closing the S12ZVM-EFP, a total of 100 experiments were conducted, and the result was 100% successful startup. The initial position detection and current staggered startup make the startup fast and stable. One of the startup current waveforms is shown in Figure 10. The rated speed is up to 8000rpm, and the time is 107.6ms, which is less than 150ms.

Figure 10: Fast start phase current waveform

The S12ZVM-EFP was operated at 13V and 8000rpm. By changing the load of the oil pump, the phase current and speed changes when the load changes were checked. As shown in Figure 12, the speed remains stable and does not change even from light load to heavy load, or from heavy load to light load.

Figure 11: Phase current variation during load jitter

The working state of EFP was tested in 8V, 13V and 16V, the power supply voltage was changed dynamically and the speed change was checked. The test results showed that the speed did not fluctuate much under different power supplies. Figure 12 shows the speed fluctuation under the power supply voltage of 13V and 8000rpm. This speed fluctuation is actually the fluctuation of instantaneous speed. If we look at the average value, the speed fluctuation is very small.

Figure 12: Speed ​​fluctuation at 13V 8000rpm

By using different duty cycles of input PWM to send speed commands and dynamically input jumping speed commands, the speed value and dynamic performance of S12ZVM-EFP under this condition are tested. As can be seen from the figure below, the speed drop slope is less than the speed increase time, because the deceleration acceleration is less than the acceleration acceleration, that is, the speed increase we set is faster than the speed decrease.

Figure 13: Phase current under PWM speed command

Figure 14: Speed ​​displayed in FreeMASTER

The EFPMOSFET was operated at room temperature with a supply voltage of 13V and a rated power of 250W. The temperature of the MOSFET on the development board was tested. The temperature range was from 18°C ​​to 52°C. When the running time was 5 minutes, the temperature was stable and close to 52°C. It can be seen that the temperature rise was 34°C. When running for a long time, its temperature change was basically stable at 52 degrees Celsius, as shown in Figure 16.

Figure 15: Temperature rise test results

The S12ZVM-EFP reference design provides users with software and hardware reference designs. In terms of software, it optimizes the algorithm by integrating initial position detection and current staggered startup, so that the EFP can start quickly and reliably. This is the most critical technical difficulty and challenge of the entire reference design.

At the same time, our reference design has a wide range of applications. It is not only suitable for EFP applications of BLDC/PMSM, but can also provide reference for other automotive small-node motor applications (such as oil pumps, water pumps, fans, and cooling systems).