Control Design of Heart Simulator Based on CompactRIO
challenge:
Develop a realistic, reliable and reconfigurable test environment to help the latest heart assist devices improve and improve without the need for animal testing.
solution:
Use NI CompactRIO to create an independent hardware-in-the-loop (HIL) test environment. This test environment can combine an artificial mechanical heart with a circulating blood flow model to create a vivid solution that contains a real hemodynamic environment.
Deaths caused by heart disease account for nearly half of all deaths in developed countries. Heart transplantation is still the most effective way to treat heart disease, but the donated organs are far from being in demand. In order to solve this imbalance, people are currently studying and using it. A novel mechanical artificial heart assist device being developed by the University of Leeds is named the intelligent ventricular assist device (iVAD). The device can be used as an artificial muscle to cover the heart, and can provide assistance to the failed heart by applying pressure synchronized with the natural rhythm around the outer surface of the heart ventricle. This periodic "squeeze" effect can increase myocardial motility and increase the blood output of the diseased heart.
We need to actually apply iVAD to a simulated heart in order to measure the effect of pressure on it, so a realistic in vitro test environment is imperative for development. In the past, other cardiac assist device testing systems generally used a huge mechanical simulation circulatory system, or used an isolated heart supported by the blood circulation of other animals. These two methods are not practical for us, so we created a unique HIL (Hardware-in-the-Loop) heart simulator, which can combine real-time software blood flow models with a solid 3D artificial heart. We use NI LabVIEW graphical programming environment and CompactRIO to further enhance the test environment, so the heart simulator can work like a stand-alone system and run reliably for a longer duration.
Principle of Heart Simulator
We need a heart simulator that can be reconfigured to replicate the real blood environment of different patient types, disease types, and animal models. This adjustment can reduce the reliance on animal experiments, because the heart simulator can extend the experiment using the iVAD prototype and provide information about the physiological effects of iVAD.
For auxiliary devices such as iVAD, the interaction between the auxiliary device and the surface of the heart is crucial. This interaction is likely to depend on human characteristics that are difficult to simulate, such as gaps and non-linear friction; therefore, it is important for a heart simulator to have a physical object that can interact with iVAD, and we can monitor the compression process Raw data.
Heart simulator design
In the process of designing the heart simulator, we adopted the principle of HIL simulation. This is a common test technique in industry. HIL simulates some components of the system in software, and connects them to specific real hardware in the same system that needs to be tested through I / O. In order to meet the requirements of the heart simulator, we used a mechanical heart as the hardware part of the HIL simulation and placed it in a simulated blood circulation model. And use the continuous interaction between the two to evaluate the circuit to understand how to assist when iVAD is transplanted into the human body and affect the heart and blood flow.
The shape of the artificial heart is determined by two deformable semi-circular structures. They are composed of curved spring steel bars. The steel bars are fixed at both ends, and the boundary shape can be adjusted. We have also developed a customized NI vision program to determine the necessary boundary shapes to match the outline of each steel bar to the reference heart model. We use two linear actuators to achieve the cyclic control of the curved steel strips to realistically show the dynamic motion of the left and right ventricles of the heart. We control the actuator in the blood flow model to move to simulate the movement of the simulated heart, so any volume change in the simulated heart will directly affect the artificial heart. In addition to being able to match the shape of the heart, this design also allows us to change the local hardness of the artificial heart by individually changing the mechanical properties (such as thickness) of the steel bar. Finally, we wrapped a thin layer of elastic band around the steel strip to realize iVAD.
Heart simulator implementation
As mentioned above, we use a loop with feedback to evaluate the help of iVAD to the cardiovascular system. Four similar pressure sensors are placed at equal intervals around the artificial heart to provide data in the iVAD assisted process (compression process). In the model, these data are converted into auxiliary pressure for each ventricle, and the subsequent effect on blood flow is calculated in real time, and finally output to the hardware and the motion of the artificial heart is changed accordingly.
The blood flow model works similarly to the closed-loop centralized parameter model of the electrical network. Because each region of the heart is simulated separately, we can achieve local control of the heart and adjust for special heart conditions or heart diseases. To meet our main goal, the blood flow model can be automatically adjusted to characterize physiological data by using a nonlinear least squares parameter estimation method (which can be implemented as a state in LabVIEW code). This means that the heart simulator can accurately reflect the hemodynamic characteristics of most pathologies and in vivo models, helping to improve our understanding of the potential effects of the device.
We use CompactRIO to control the artificial heart, run the simulation and send the data to the Windows host via TCP for display and storage. The real-time controller can execute two parallel running loops: a high priority control loop is used to control the blood flow model, and a low priority communication loop can send and receive TCP data in the queue to the Windows host. The high-priority blood flow model loop operates at 500 Hz and converts the two ventricular volumes to the calibrated positioning voltage. The positioning voltage is sent to the field programmable gate array (FPGA) I / O to control all linear actuators to execute. After being compiled, FPGA can process all I / O of CompactRIO, and provide proportional integral (PI) control of heater (used to keep the heart simulator shell temperature at 37 ° C. (Body temperature)).
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