Some typical proportional valve applications in the medical segment include:
- Anesthesia and ventilator gas mixing
- Ventilator patient “delivery” valve
- Leak testing — Medical components and packaging
- Positive End-Expiratory Pressure (PEEP) control
- Oxygen concentrators
- Patient simulators
- Shock wave therapy
- Surgical instruments and patient monitoring
- Clinical diagnostic equipment
When selecting a proportional valve, the medical device design engineer needs to consider both the medium and variable to be controlled. Gas (air, oxygen, CO2 or others) is usually the medium, and a proportional valve can control many variables, including force, position, temperature or level. In this article, however, we will focus upon the two major variables of pressure and flow.
After the design engineer has established what variables need control, determining other application criteria for the valve(s) comes next, such as:
- Medium, which will influence required valve construction or materials
- Inlet pressure and maximum controlled output pressure
- Required flow range
- Media and ambient temperature
- Other environmental factors, such as location (hospital, clinical or portable) and any weight or space restrictions
- Power consumption, especially for portable/mobile applications
- Special requirements related to medical devices to comply with FDA or other regulatory bodies’ requirements
Constantly Changing Pressure and Flow
Generally speaking, each of the above applications requires a constantly changing control of flow or pressure, except for leak testing, which usually demands a set pressure be maintained at all times (and then quickly changed to a new setpoint for subsequent testing or for a new component/package to be tested).
Constantly changing flow and pressure require the setpoint to change frequently, causing the valve to operate by varying its opening size continually in response to an often-changing command signal. An example of this situation is a ventilator or anesthesia gas-mixing circuit, where the flow output of two or three valves is constantly adjusted to produce a desired mix of gases. The blended gas is then delivered to the patient, according to the design of the device. Other characteristics might include frequently altered pressure changes in a shock wave therapy device. In order to change the “strength” of the shock wave being generated by the device, the pressure in the system is altered. These dynamic devices demand robust proportional valves to meet ever-changing pressure and flow requirements within their respective systems.
Types of Proportional Valves
There are several operating methods used in proportional valve technology. The most popular valves serving the medical device sector are direct-acting proportional solenoids because they fall into the right range of capabilities and serve most medical device applications. However, other methods of actuation may be used to perform the control required in some applications, including pulsed, air-piloted or even motor control.
One form of direct-operating proportional valves is the piezoelectric version. This valve deforms a piezoelectric element to vary the opening of the valve when voltage is applied. Piezoelectric operators are incorporated where extremely low power consumption is necessary (portable devices) or in cases where the application demands very low heat generation, as in many clinical diagnostic machines.
Direct-acting proportional coil valves act directly on pistons or spools to adjust valve opening positions based on a varying current across the coil. These valves offer advantages over piloted valves as they may fit a considerably wider range of applications and provide simpler construction with fewer mechanical parts, simpler principles of operation and more dependable performance capabilities. Direct-acting valves last longer in dynamic operations where valves are cycled more often. Their robust construction reduces the concern about wearable component failure, and they are generally less sensitive to contamination in the air or gas media.
Other technical advantages include the ability to reduce overshoot, which is the tendency for a valve to accelerate toward a setpoint and then go past it, only to have it reverse back toward the setpoint — possibly overshooting again. This is a common occurrence in systems tuned for quick response times. The oscillations typically take less time to settle in direct-acting proportional coil designs because the valve directly moves the piston by varying current to the coil.
Direct-acting proportional coil designs also typically provide a precise control of pressure. This can be critical in applications like leak testing, which demands a very stable pressure throughout the testing phase. The valves are able to react to very small setpoint changes to make tightly controlled pressure adjustments.
Overall, the direct-acting design possesses greater speed, responsiveness, and resolution that make it superior to air-piloted or other designs for many applications.
Tuning to Achieve Proper Control — Electronic and Mechanical Interface
As previously described, the proportional control valve will adjust its opening in response to a variable current input. To make this feature work in a system, though, feedback and control are necessary. The control is used to set the parameter (flow, pressure or other variable), and the feedback is used to measure the progress toward achieving that goal. Then, electronic means are used to compare the setpoint to the result and make changes to the command signal to achieve the desired setpoint.
In the case of proportional pressure control, a typical “system” includes two valves, a pressure transducer and an electronic controller. The electronic controller serves as the brains of the unit, allowing a setpoint to be provided by the user and comparing/controlling the results such that the valves act in a way to quickly achieve the desired result. The transducer serves to provide needed information to the controller. The valves, of course, form the working parts of the system and alternately increase or decrease the pressure in the target location by allowing air (or other gas) to be introduced or expelled. The valves may be constantly actuated, or reach a steady state, depending upon the system dynamics. A constantly changing demand will keep the system active, and a changing pressure input to the valves can be overcome by the controller, as it constantly adjusts control parameters to meet the demand.
Emerson’s Aventics brand Sentronic series of electronic pressure controllers can be configured for either single-loop or double-loop (cascaded) control, which means that multiple feedback components can be incorporated. Each feedback loop would be involved in monitoring a different variable, such as pressure, flow, force, speed or temperature. The only additional component needed would be the sensor that provides the feedback. With this feature, the OEM design engineer may select to use the pressure controller as is, or to add (loop) an additional variable into the controller to provide more unique control to the overall system. Incorporating the Sentronic series into a machine’s overall control system can save the design engineer time and also ease the tasks of the production technician. With this strategy, the engineer may use the electronic pressure controller’s built-in features, linked with the control variables unique to the medical device or diagnostic machine.
Emerson’s design engineers often partner with the OEM design engineer to combine the Sentronic control features to address the customer’s unique needs for the medical device. Working with a team of engineers that are fluent in valve control and also knowledgeable about medical and clinical devices can significantly improve the design engineer’s overall task; this combination can make a better device for the market in a greatly reduced time period.
Individual Proportional Valves — Building Blocks for the Design Engineer
If the OEM design engineer does not need the combined elements of the electronic pressure controller, Emerson’s ASCO brand Series 202 Preciflow may very well suit the need. This valve series can be combined with the OEM’s device control to achieve excellent results. In this case, Emerson provides the valve as well as the team needed to advise the customer how to incorporate the valve into the medical device’s overall system architecture.
The ASCO fluid control proportional valve series includes Piezotronic, Posiflow, Preciflow, Preciflow IPC (Inlet Pressure Compensated) and Flapper Proportional versions, to meet virtually every control need in analytical and medical device applications. For gas and liquids, this series provides many levels of control to meet identified needs in a variety of devices.
Various electronic control devices can also be provided, both for development and for production. With this hardware, the design engineer can utilize known control or customize to their level of need.
Finding the Right Fit
If necessary, the OEM design engineer can call upon the technical resources of Emerson to collaborate for real-time adjustments to meet demanding application needs, either on the phone, online or at the manufacturer’s location. This teamwork increases speed to market by allowing an OEM to control or collaborate to modify the proportional valve as needed during design revisions.
The Aventics Sentronic electronic pressure controllers and the broad ASCO fluid control of proportional valves can be applied to a broad range of applications, while Emerson’s technical resources can help customize products or manifolds. That ensures proper fit, cost-effectiveness and inclusion of all the features needed for the application.
Paul Gant is director of business development, Analytical & Medical, for Emerson Automation Solutions. He previously served as director of global life sciences for Aventics.