Optimization at the component level typically means balancing the performance gains of smaller series gaps with the cost of tighter tolerances of parts. Tightening the radial tolerances between the moving armature and stationary components of a magnetic circuit can improve the force output of the solenoid. It can also have the undesired effect of increasing the radial forces exerted on the armature, leading to increased side loading and friction losses in the device. The balancing act is between the cost increase of part tolerances, the cost reduction of decreased copper due to more efficient magnetic circuits and the cost increase of friction mitigation strategy.
On the system level, environmental contamination can also require increased series gaps between the armature and the remainder of the circuit to maintain device function. Particulate contamination can lodge between moving parts of a solenoid and easily seize the mechanical system. Moreover, the solenoid is an electromagnet, so any magnetic particulates in the system will be attracted to the solenoid. This means that appropriate strategies for dealing with these particles must be introduced. Larger radial gaps in the plunger, adding debris mitigation grooves on the plunger, sealing the mechanical system against debris or filtering debris out of the fluid are all strategies with side effects on solenoid performance.
Considering filtering on the solenoid valve itself raises concerns of lifetime durability and valve function. A small filter on the valve can clog rather quickly in a heavily contaminated environment such as a new engine cooling system. This is particularly the case when fluid flow through the valve is only in one direction. In a hydraulic power and control environment, the fluid could flow in both directions through the valve, helping a filter to effectively self-clean or back flush.
Using larger gaps for debris tolerance or debris mitigation grooves has two effects on solenoid performance. One is the reduction of magnetic material capable of performing work. The second is an increase of friction within the device due to asymmetric radial loading on the magnetic circuit. If larger radial gaps are employed, there is room for the plunger to move off the center axis of the working magnetic circuit. Once this happens, the effect will be compounded by the eccentricity of the working plunger in this extra space causing the plunger to be pulled even harder towards one side of the working bore.
In the case of mitigation grooves, the same effect happens with a slight difference in the mechanism. When machining the debris mitigation grooves into a solenoid plunger, there will be asymmetrical features due to tolerance and machining capability. Again, this will cause an eccentricity of magnetically influenced material resulting in increased friction in the mechanical system. In both cases, increasing the size of the plunger and increasing the mass of copper used in the system are the primary recourse from a component level to overcome these introduced inefficiencies.
The final strategy for debris mitigation is to isolate or seal the mechanical system from debris. This is not always practical for an application, particularly in a fluid control valve. However, for an actuator, this may be an effective strategy. Providing an effective isolation approach also increases friction in the case of a dynamic shaft seal or resistive elastic forces due to a diaphragm-type seal. On a component level, the seal itself adds cost and mass and the increase in resistive forces calls for more magnetic material and more magnetic energy in the form of increasing copper mass.
Integrating Engineering Teams
With these considerations, it is important that engineering teams from the component and system level cooperatively integrate early in a project. With the goal of developing the most beneficial strategy for the system, open conversation near the outset of the project can help designers explore options such as upstream filtering in fluid control, cleanliness of manufactured system and components or other potential strategies for increasing system efficiency. All of these design considerations become critically important to optimizing the mass of the system and the cost.
Working environment considerations need to be addressed as an integrated project team for the optimization of cost and mass to produce an efficient end product. Particularly in the case of fluid control, whether it be coolant control, fuel metering, hydraulic control, etc., a fundamental openness and cooperation in regards to system function is critical to valve design. Particularly in the realm of solenoid valves, design teams on the component level usually include engineers that specialize in electromagnetics, mechanical systems, electronic control strategy and fluid mechanics. Conversely, a typical engineering specification for a valve (especially in new product development) is based on models, simulations and assumptions.
All models are to some degree incorrect, but if used with discipline and team cooperation, models become vitally useful. In a CFD-modeling (computational fluid dynamics) environment, it is necessary for a valve design team to understand what is happening both upstream and downstream of the valve, as well as to have a firm understanding of the physical principle of the end-use system. Typically, a valve modeled in CFD to a component specification will use a “test manifold” or the planned validation manifold for modeling the performance of the valve. This strategy may be valid as a finished product benchmarking exercise for quality control in manufacturing, but the details of the real system’s working characteristics are fundamental to the design of an efficient valve.
This cooperation can expose opportunities for further cost and mass savings by utilizing the specialized talents of component and system engineering teams. For example, a “sharp” corner 90° direction change in a molded rigid-elbow fitting will carry a much more severe pressure drop and head loss than a smooth bend of a molded polymer flexible hose. Figure 4 shows how CFD can be used to not just mathematically, but visually identify losses or restrictions in a system using pressure contours and streamlines. Identifying these losses in a cooperative environment can easily identify unforeseen system inefficiencies, mitigate these inefficiencies and end up with a more efficient design leading to benefits in system durability, mass reduction and cost savings. These are three terms that everyone loves to hear together.