Design of a non-invasive blood pressure simulator based on oscillographic method

In this article, a non-invasive blood pressure simulation system based on the oscillographic method is proposed, and a piston-connecting rod-type variable air chamber is designed and installed in the system as the pulse waveform generation mechanism. It uses PWM constant current control and ultra-small subdivision technology to control the linear stepping motor to achieve stable and accurate displacement and to reproduce the pulse wave according to the blood pressure envelope, to simulate ambulatory blood pressure. In this article, a hardware circuit design scheme is also put forward, including a power supply circuit, pressure acquisition feedback circuit, stepping motor drive circuit, and so on. The experimental results show that the simulator can meet the requirements of JJF 1626-2017 Calibration Specification for NIBP Simulators and can be used as a calibration standard with efficiency and accuracy.


Introduction
Blood pressure is an important parameter of human vital signs and one of the most important indicators reflecting the hemodynamic state [1][2] .By measuring human blood pressure, which includes systolic and diastolic pressure, we can monitor the function of the human cardiovascular system and find abnormal conditions such as hypertension and hypotension.Blood pressure is an important index for the discovery, diagnosis, treatment, rehabilitation, and prevention of diseases [3][4] .Non-invasive blood pressure measurement is an indirect and non-invasive method using a cuff and pressure sensor [5] .Non-invasive automated sphygmomanometer based on oscillography which measures the oscillation waves in the blocked cuff, is widely used in medical institutions and families, because of its advantages such as easy operation and intuitive indication [6][7] .Non-invasive sphygmomanometer belongs to Class II medical devices, and the reliability of their metrological performance will directly affect the diagnosis results and treatment effects, while their malfunction can even endanger the health and life safety of patients [8] .JJG 692-2010 Non-invasive Automated Sphygmomanometers puts forward clear requirements for the measurement performance of both static and dynamic non-invasive sphygmomanometers.In this article, a non-invasive blood pressure simulation system is designed based on oscillography, which realizes the evaluation of measurement range, indication error, air tightness, overpressure protection, and other critical indexes of static pressure, through the standard pressure sensor and static pressure module.Meanwhile, the same system can also evaluate the dynamic blood pressure measurement performance of non-invasive sphygmomanometers by reproducing the human artery pulse wave through the blood pressure envelope.

Measurement principle
The oscillographic method is based on the principle of blood vessel unloading to realize blood pressure measurement, which means the extraction and analysis of human pulse waves.The inflatable cuff that reaches a certain pressure is used to block the blood vessel, and during the slow and uniform deflation process, certain means are used to record the pressure oscillation waves in the cuff.After the trend of gas pressure waves in the cuff is extracted and analyzed, systolic and diastolic blood pressure can be determined by a fixed ratio calculation method or mutation point criterion [9] .The principle of dynamic blood pressure simulation is derived from the process of measuring blood pressure using the oscillographic method, which involves reproducing the pulse wave corresponding to blood pressure through a certain executing mechanism.The shape and trend of pulse waves are closely related to the blood pressure envelope, and different blood pressure values have different blood pressure envelopes.The blood pressure envelope is a virtual line formed by connecting each pulse wave peak in a set of pulse waves.The blood pressure envelope can intuitively reflect the relationship between cuff pressure and pulse wave amplitude [10] .Dynamic blood pressure simulation requires the blood pressure simulator to restore the corresponding amplitude pulse wave at the corresponding cuff pressure based on the blood pressure envelope.The specific output waveform P (t) is represented by Equation ( 1): where Pc is the pressure coefficient of the cuff, is the pressure coefficient corresponding to the set systolic and diastolic blood pressure, T is the period of the pulse wave, and Y is the pulse waveform, which is represented by Equation ( 2): where A is the amplitude of the pulse wave, h is the position of the descending middle gorge, l is the position of the descending middle wave, and f is the frequency of the pulse wave.

System components
A linear actuator is used to compress the gas in the sealed chamber according to a certain pattern, generating a pressure pulse that is superimposed on the pressure waveform of the main chamber, to simulate the pulse wave picked up from the cuff.The blood pressure simulator is mainly composed of a microprocessor, pressure sensor, cylinder, piston, stepper motor, air pump, and solenoid valve.The microprocessor senses the static pressure of the pneumatic system and controls the stepper motor which is connected to the piston through a guide screw, and the piston is driven to move back and forth in the cylinder.The pressure of the entire gas circuit system including the cylinder will change because of the reciprocating motions of the piston, which will result in oscillation waves.The amplitude of the oscillation wave is determined by factors such as the amplitude of the piston motion, the static pressure of the gas circuit system, and the amount of gas.The cylinder piston group is equipped with a position sensor to indicate the actual position of the piston, to avoid errors caused by friction and position deviation.The pressure sensor is connected to the cylinder and the entire gas circuit to measure the gas pressure of the circuit.A direct current diaphragm pump is connected to the cylinder to inflate the entire gas circuit.The pump is equipped with additional components such as a filter with an inlet passage and a one-way valve with an outlet passage to improve inflation quality and gas tightness.There are two channels between the cylinder and the outside world.One is the front panel interface that connects to the non-invasive blood pressure device being tested, and the other is connected to the outside world through an electromagnetic valve as a vent.The block diagram of the noninvasive blood pressure simulation system is demonstrated in Figure 1.

Hardware design
The hardware design of the simulation system mainly includes the design of the hardware circuit, and mechanical structure of the pulse wave generation platform and pneumatic device, which is used to generate pressure pulse waves and provide pneumatic pressure in static pressure measurement.

Hardware circuit design
The hardware platform of the non-invasive blood pressure simulation system consists of 8 circuits, which include a power supply, core control circuit, LCD circuit, pressure acquisition circuit, A/D conversion circuit, stepper motor drive circuit, pneumatic device drive circuit, and matrix keyboard drive circuit.The block diagram of the system hardware platform is shown in Figure 2. The boost conversion circuit and buck conversion circuit of the power supply are both designed with switch regulators, to realize high efficiency and power density with smaller size and heat dissipation area.LM5122 boost chip has a maximum input voltage of +65 V and a maximum output voltage of +100 V.The power supply of the system has multiple power networks, whose structure is relatively complex.The input voltage DC+12 V is converted into the required voltage at all levels by switching the power supply chip, and the ancestor capacitor and ceramic capacitor are connected in parallel at the input and output ends of the power network at all levels respectively, to play the role of filtering and decoupling.The control unit of the core control circuit adopts the STM32F103xE series microcontroller, with a main frequency of up to 72 MHz.It has a large number of powerful I/O ports and peripheral resources and is packaged in LQFP100 with a size of 14 x 14 mm.It is a compact and powerful microprocessor with rich hardware resources, such as 512 KB FLASH storage, a considerable number of timers, A/D conversion, and various communication interfaces.In addition, the external main power supply voltage of this controller is up to 4.0 V, while both the port pull and fill current are up to 150 mA.

Mechanical structure design
The design principle of the pulse wave generation platform is to use a linear stepper motor as the rhythm generator of the system, simulating the work of the human heart.By regularly squeezing the gas in the gas chamber, pressure waves are generated to simulate the pulse waves of a human body, achieving the goal of simulating blood pressure.In this article, the pulse wave generation platform of the non-invasive blood pressure simulation system is designed based on the oscillographic method, and the platform mainly consists of an air chamber, an air chamber fixing device, a piston with a guide screw, a linear stepper motor, and a stepper motor driver.The connection diagram of various devices on the pulse wave generation platform is shown in Figure 3.The volume of the gas chamber is not yet specified.The scheme adopted in the article is 20 cc, and the maximum simulated pressure is designed to be 300 mmHg.According to Boyle Mariotte's law, the product of pressure P and volume V of the gas chamber is a constant, which can determine the relationship between the stroke of the stepper motor, piston displacement, and bottom radius.The system adopts Ametek 43H4-2.33-A05hybrid linear stepper motor, with a step angle of 1.8° and a step length of 0.05 mm.It is equipped with a high-performance subdivision driver and PWM constant current control to achieve small vibration and stable operation at low speed.The pneumatic device mainly consists of an air pump and an electromagnetic valve and can meet the requirements of static pressure testing: the pressure range is 0 to 40 kPa, and the air tightness is satisfying.The system uses a CJP37C series pressure pump manufactured by Xiamen Kunjin Electronics, which can provide a gas pressure of 50 kPa with air tightness of less than 3 mm Hg/min.

Experimental results and analysis
To evaluate the metrological performance (such as accuracy, repeatability, and stability) of the simulator, and to check whether the equipment can meet the requirement of JJF 1626-2017 Calibration Specification for NIBP Simulators, first, the simulator is connected to a standard pressure gauge and a metal container with a three-way valve to calibrate its static pressure simulation function, and the experimental results are shown in Table 1.

Conclusion
In the article, a non-invasive blood pressure simulation system is proposed based on the oscilloscope method, and a piston-connecting rod-type pulse blood pressure waveform generation mechanism with a variable air chamber is designed.PWM constant current control and ultra-small subdivision technology are used to control the direction and displacement of the linear stepper motor, and the pulse waves are replicated based on the blood pressure envelope.The experimental results show that the designed simulator can accurately simulate stable pulse waves, and metrological performance can fully meet the requirements of JJG 692-2010 Non-invasive Automated Sphygmomanometer, with high measurement accuracy and resolution.At the same time, it is easy to operate with excellent stability and repeatability.Further studies will focus on improving the anti-interference ability of the transmission line to avoid significant errors caused by signal interference during the working process.

Figure 1 .
Figure 1.Block diagram of noninvasive blood pressure simulation system.

Figure 2 .
Figure 2. Block diagram of system hardware platform.

Figure 3 .
Figure 3. Connection diagram of pulse wave generation platform.

Table 1 .
Experimental results of static pressure of blood pressure simulator.Secondly, the simulator is connected to a standard pressure gauge metal, a pressure converter, and a digital oscilloscope, then the simulator on adult mode is set, to calibrate its simulated pulse wave variability, and experimental results are shown in Table2.

Table 2 .
Experimental results of simulated pulse wave variability.