Biomedical Engineering, Vol. 38, No. 2, 2004, pp. 96-100. Translated from Meditsinskaya Tekhnika, Vol. 38, No. 2, 2004, pp. 39-42. Original article submitted November 12, 2003.
Exchange of Experience A Controller for Medical Devices A. E. Vasilev and G. N. Leonov Introduction
etc.
Medical life-support systems play an important role in resuscitation and life support medicine. To meet constantly increasing requirements for the quality of such systems, it has become necessary to implement their control units based on up-to-date computer technology. This makes it possible to provide multimode flexible control, easy interaction with the user, and high reliability. Among many types of computer aids, built-in microcontrollers can be especially effectively used as a basis for control units of life-support systems. A builtin microcontroller is a full-featured single-chip computer with a set of built-in hardware modules supporting standard control algorithms. A typical built-in control system (Fig. 1) includes the following components: controlled object: execution units (EU), object or process, and a system of sensors providing information about the object; microcontroller executive system providing object control according to control algorithms and information obtained from sensors and control panel; object interface correlating control and information signals from the microcontroller system and control object; control panel. Control systems should solve the following problems: generation of complex pulse patterns; detection of time intervals; analog-to-digital and digital-to-analog conversion;
input, conversion, and output of digital codes,
Medical systems should also meet the requirements for patient safety, reliability of operation, and possibility of self-recovery after failures. All these problems can be solved using appropriate software and microcontrollers. Structure and Characteristics of General-Purpose Controller Modules A general-purpose microcontroller-based module for control systems of medical devices was developed at St. Petersburg State Polytechnic University in collaboration with Krasnogvardeets, Ltd. This module is compatible with different types of medical devices [1, 2]. The algorithms for control of executive units, interaction with user, and reception of information from sensors are implemented as a control program stored in the module memory. The control program can be modified to make the control module compatible with different types of medical devices. The controller module (Fig. 2) is based on the Infineon MCS-51 80C535 microcontroller. This microcontroller enjoys wide popularity among developers of built-in applications because of its elaborate architecture, availability from a number of leading manufacturers of such equipment (Siemens, Intel, Dallas Semiconductor, Atmel, etc.), and effective price policy of developers and providers. The following characteristics of the 80C535 microcontroller made it the optimal choice for the control system under consideration: 8-bit Harvard architecture; 111 basic commands; 1 µsec work cycle at reference pulse frequency of 12 MHz;
St. Petersburg State Polytechnic University. Krasnogvardeets, Ltd., St. Petersburg, Russia; E-mail:
[email protected]
96 0006-3398/04/3802-0096 2004 Plenum Publishing Corporation
Controller for Medical Devices
256-byte internal data memory; possibility of access to peripheral program memory (up to 64 Kb); six 8-bit inputoutput ports;
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8-bit input port integrated with 8-channel analog-to-digital converter with programmable measurement range; three 16-byte timers;
Fig. 1. Structure of microcontroller-based controlrot system.
He
Fig. 2. Structure of general-purpose controller module.
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fast inputoutput unit; 12-vector four-level on-chip interrupt controller; 16-byte watch timer; idling and reduced power consumption modes. The Shell51 custom-made software, a powerful CAD system, was used to develop the control software. The Shell51 system makes it possible to implement multifile projects, search contextual errors, and debug the executive program in interactive mode. Example of Use of the General-Purpose Controller Consider the use of the developed controller module as a control system of a UMG-1 medical gas humidifier (Fig. 3). The main function of this device is to provide necessary temperature and humidity of gas mixture flowing through it. The required parameters of the control procedure are inputted from the control panel. Sensors provide feedback information about the current values of these parameters. The controller implements the control algorithm providing gradual approach of the current parameters to the required values. The control signals are sent to the executive organs of the humidifier. The current values of the parameters of the control procedure are indicated on the control panel. The control system consists of two control loops and a single monitoring loop. The input and output parameters of these loops are listed below. 1. Loop of temperature control. Input parameters: required temperature (inputted from control panel),
current temperature (detected by sensor). Output parameters: heater control (by executive organ). 2. Loop of humidity control: sub-loop of evaporator heating control. Input parameters: required temperature (constant), current temperature (detected by sensor). Output parameters: heater control (by executive organ); sub-loop of control of liquid supply to evaporator. Input parameters: humidified air flow rate (inputted from control panel), current temperature (detected by sensor). Output parameters: control of stepping motor (SM) of water supply pump (by executive organ). 3. Loop for monitoring of permissible parameter values: sub-loop for air temperature monitoring. Input parameters: admissible error in temperature (constant), range of permissible values of the current temperature (constant), required temperature (inputted from control panel), current temperature (detected by sensor). Output parameters: heater control (by executive organ), control of sound and light alarm (on control panel); sub-loop for evaporator heating monitoring. Input parameters: range of permissible values of the current temperature (constant), required temperature (inputted from control panel), current temperature (detected by sensor). Output parameters: heater control (by executive organ), control of sound and light alarm (on control panel); sub-loop for monitoring of liquid level in the evaporator. Input parameters: level of liquid (detected
Fig. 3. Functional and logical structure of control system of UMG-1 humidifier.
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Fig. 4. Structure of control system of humidifier.
by sensor). Output parameters: control of sound and light alarm (on control panel); sub-loop for monitoring of supply voltage. Input parameters: voltage level of primary power source
(detected by sensor). Output parameters: control of power consumption mode of the power source. The following binary command signals are transmitted from the control panel to the controller: required
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temperature increase/decrease, change of the flow rate control mode, required flow rate increase/decrease, change of sound alarm lock mode. The controller sends the following binary signals to the control panel: selected flow rate control mode, water and air heating parameters, selected sound alarm lock mode, open-circuit failure, temperature overrun, and lack of liquid in evaporator. The controller also sends the following digital signals: required air temperature (two decades in binary-coded decimal representation) and selected flow rate (three decades in binarycoded decimal representation). The following analog signals are transmitted from sensors to the controller: air temperature in a range of 0 to 5 V, evaporator temperature in a range of 0 to 5 V, and damage to primary power source (0-15 V). The sensor of minimal liquid level in the evaporator produces a binary signal. The controller sends the following signals to the executive organs: air heater control (digital, 1-bit PDM); evaporator heater control (digital, 1-bit PDM);
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control of liquid supply to evaporator (digital, tetrad). The structure of the control system of the humidifier is shown in Fig. 4. Conclusion The experience in development and use of the generalpurpose controller module showed such advantages of microcontroller-based control systems as high reliability, reasonable price of component parts, and possibility of modification. Thus, the developed controller module can be recommended for use with a wide range of medical devices. REFERENCES 1. 2.
A. E. Vasilev and G. N. Leonov, In: Abst. Conf. Fundamental Studies in Technical Universities [in Russian], St. Petersburg (2003), pp. 273-274. E. Vasilev, Microcontrollers: Development of Built-in Applications [in Russian], St. Petersburg (2003).
