Positive pressure sleep apnea machine hardware design plan

A sleep apnea machine is a device used to treat upper respiratory tract obstruction in patients during sleep . Upper respiratory tract obstruction during sleep can cause snoring, reduced blood oxygen, metabolic disorders, etc., and can lead to or aggravate various chronic diseases in the long term: such as hypertension, diabetes, cardiovascular and cerebrovascular diseases, neurological diseases, etc. The sleep apnea machine continuously delivers a certain pressure of airflow to the patient's respiratory tract, thereby forming a gas stent to support the patient's airway opening, ensuring the patient's breathing is smooth, and eliminating snoring and the negative effects of OSAHS.

This time, technology-based authorized agent Excelpoint invited Shen Gong, a senior engineer in the industry, to share his practical design experience. The positive pressure sleep apnea machine hardware solution designed by Shen Gong and his team consists of a motor control system, a pressure/flow measurement system, a gas circuit system, a heating/humidification system and a human-machine interface.

Most patients who use positive pressure sleep apnea machines use the device during sleep, so the ventilation comfort and silent performance of the ventilator are important indicators. In addition, one of the functions of a sleep apnea machine is to detect and immediately take appropriate protective measures when an abnormality occurs in the system. Its core point is the control of the ventilator fan.

This positive pressure sleep apnea machine hardware design has three important features. The first covers extremely dynamic motor speed range: in bi-level mode, the motor speed range will quickly switch within 10,000rpm and above 25,000rpm, and the speed switching time can be less than 0.5 seconds. Second, the noise can be kept as small as possible. When the ventilator pressure is 10cm water column, it is generally required to be lower than 30dB, and high-end products can achieve 23dB and below. Third, it has flexible and convenient system diagnosis and protection mechanism: it can conveniently combine driver information and related sensors to locate related abnormal events, including respiratory mask falling off, breathing tube leakage, fan blocking, etc.

Household positive pressure sleep apnea machines require small size and minimal operating noise. Therefore, the motor control needs to use the FOC algorithm. According to the load (system pressure and flow conditions), the fan is controlled to operate stably and the torque pulsation is reduced, thereby greatly reducing the motor own operating noise. Usually during product development, engineers will choose to directly purchase the driver board for the fan, which is relatively expensive and the interface control is not flexible enough. Shen Gong's solution can achieve better performance and flexibility at a lower cost.

1. Ventilator fan control system

Table 1 Ventilator fan parameters

Figure 1 Physical picture of the medical grade fan used

Based on TMC4671 and ventilator fan parameters, Shen Gong and his team developed a fan driver board. The brief block diagram of the motor control part is as follows:

Figure 2 Brief block diagram of motor control part

The motor control core FOC algorithm and control are completed by ADI Trinamic's TMC4671, and the power drive part and current sampling are completed by ADI Trinamic's TMC6200. TMC4671 greatly simplifies the algorithm of the motor control part, frees up the computing power of the MCU, and allows users to focus on system application-level design, such as pressure-flow curves that are more suitable for patients.

The pressure value is fed back through the pressure sensor, and after PI calculation by the MCU, its output is used as the speed given signal of TMC4671. TMC4671 outputs the fan control SVPWM signal to TMC6200. TMC6200 acts as a MOS tube gate-level driver to control the three-phase H bridge to drive the motor. , to maintain the ventilator pressure at the required value or change according to a given curve.

2. Introduction to core devices

Table 2 System core components

TMC4671 is the world's first fully integrated servo controller IC, integrated ADC, position sensor interface (increased Encoder, HALL signal, etc.) and position interpolator complete assembly.

Figure 3 TMC4671 structural block diagram

The chip adopts a cascaded closed-loop control architecture (position loop, speed loop and current loop), as shown in the figure below.

Figure 4 TMC4671 control loop block diagram

The FOC algorithm at the core of the torque loop integrates time-critical calculations such as Park and inverse Park changes included in the FOC algorithm into the hardware. As a result, developing a dynamic servo controller requires only a few lines of code, which not only relieves the processor from handling real-time critical tasks, but also optimizes the design cycle and shortens time to market.

Figure 5 TMC4671 internal FOC algorithm block diagram

Table 3 TMC4671 core functions

TMC6200 is a high-power gate driver designed for permanent magnet synchronous motor servo or brushless DC motor. It can drive motors from several watts to several kilowatts; it integrates the full high-voltage part of the FOC drive system and is suitable for 12V, 24V or 48V. system. With the TMC6200, a rugged drive diagnostic with full protection and protection can be built with a minimum number of external components.

In addition to driving the three-phase H-bridge, the TMC6200 also completes the collection and conversion of current signals. It has a built-in operational amplifier and PGA, which is suitable for motors of different powers. Sampling phase current is very suitable for the delta-sigma current measurement implemented in TMC4671, which can achieve low-noise sampling of current and improve the control accuracy of the system. In addition, TMC6200 provides complete power level diagnostic functions to facilitate system troubleshooting.

Figure 6 TMC4671+TMC6200 system architecture block diagram

Figure 7 TMC6200 block diagram

Table 4 Overview of TMC6200 alarm and protection functions

3. System connection and testing

Both TMC4671 and TMC6200 use SPI to communicate with MCU, and the official IDE can easily calibrate peripheral connections. After the calibration is completed, the user only needs to perform relevant configuration through SPI to control the motor, which is very convenient and efficient.

The picture below is the relevant schematic diagram of TMC4671 and TMC6200.

Figure 8 TMC4671 and TMC6200 related schematic diagram

Figure 9 TMC4671 and TMC6200 related schematic diagram

Figure 10 TMC4671 and TMC6200 related schematic diagram

4. Technical difficulties

PI adjustment

TMC4671 is also very convenient to adjust the motor control parameters. You can easily set the PI control parameters of the system through the official IDE. The official original RTMI debugger is not flexible enough to connect to your own target board due to interface problems. Shen Gong and his team designed a module with the same function, while achieving electrical isolation of debugging signals. In addition, power and signal transmission indicators were added. The interface was also changed to 2.54mm, which can also be expanded to 2.0mm or 1.27mm. It can be easily transferred to the target board.

Figure 11 TMC4671 debugger with isolation and signal indication

Figure 12 HALL identification

Figure 13 Motor parameter identification

Figure 14 PI parameter adjustment

Figure 15 Relationship between speed and current

Figure 16 Optimizing performance using built-in dual fourth-order filters

The positive pressure sleep apnea machine hardware design solution uses the combination of TMC4671+TMC6200, which greatly lowers the threshold for high-performance motor driver design, allowing users to build related high-performance motor applications very quickly without writing complex and time-consuming underlying drivers, saving money. A lot of debugging time; related diagnostic functions allow engineers to quickly locate system faults in the early stages of debugging; with TMC IDE, users can easily carry out system identification and control parameter optimization, speeding up product design finalization; users can focus on the design optimization of the product itself , which is the direction of future motor control applications. Excelpoint provides relevant technical guidance and support, which can help users design better and get products to market as soon as possible.

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