Servo-proportional valves and servo valves are considered iconic in the field of hydraulics. However, even in this sector, new technologies such as 3D printing are driving significant changes. Exciting new possibilities are emerging to counter the rise of electric actuation. Let’s explore the case of Domin, a dynamic and highly innovative UK-based company.
The hydraulic sector is often seen as an area where technological innovations are slow to take hold. Hydraulics are not electronics, and the term is frequently synonymous with reliability and durability.
In industrial plants, energy-inefficient but highly reliable solutions are often favoured over more energy-efficient but less dependable alternatives. However, the mobile sector tells a different story, where energy efficiency and control take center stage.
Recently, several manufacturers have introduced innovative technologies in the industrial sector as well, driven by the growing competitive pressure from electric systems.
One of the most advanced solutions to hit the market recently is from Domin Fluid Power Limited, based in Bristol, UK. I was introduced to the company by my friend Alessandro D’Angelo, their Sales Director, whom I would like to thank for providing the material for this article.
Let’s begin by reviewing how a traditional servo valve works and the operating principle behind Domin’s new valves.
The Servo-Proportional Valve
The simplest way to control the movement of hydraulic actuators is through servo-proportional valves. These control devices use a spool, featuring intricate mechanical designs, which is driven by a high-dynamic proportional solenoid. Typically, these valves are controlled in a closed loop using an LVDT linear transducer.
A distinctive feature of these valves, as previously mentioned, is the presence of notches on the spool. These notches determine the relationship between the spool displacement and the flow through the valve, depending on their shape. An example of this flow characteristic is shown in figure 2.
There are two key characteristics of proportional valves: (i) the deadband, which is the zone where, despite current being supplied to the solenoid, no flow occurs; and (ii) the nonlinear flow characteristic.
The width of the deadband and the proportionality between current and flow depend on the shape of the notches.
The ring-shaped grooves on the spool serve a stabilizing function, preventing jamming during valve operation. The asymmetry along the Y-axis, as shown in figure 2, is often intentional for functional reasons tied to the application.
Despite the complex and costly machining required on the spool, servo-proportional valves are simpler to build than two-stage servo valves due to the absence of a pilot stage and fewer machining requirements on the valve body.
Other typical characteristics of these valves include long actuation strokes, lower precision compared to servo valves, and slower dynamic response. This is illustrated in the logarithmic diagram in figure 3, where the valve’s response drops by over 3 dB beyond 4 Hz, meaning the signal amplitude is halved at higher frequencies.
In practical terms, this means that control precision is only reliable for actuation frequencies below 4 Hz. Traditionally, proportional servo valves are bulkier than two-stage servo valves, making them less suitable for applications where space and weight are critical.
The Two-Stage Servo Valve
A two-stage servo valve consists of two separate components: the pilot stage and the power stage, as shown in figure 4. The pilot stage uses system pressure (usually no more than 30 bar) to generate the actuation force needed to move the power stage.
In the pilot stage, flow rates are low, minimizing pressure losses. The pilot generates a pressure signal that is used to drive the power stage.
By controlling the pilot stage with solenoids, which requires minimal power due to the low flow and pressure, the movement of the power stage’s spool can be managed to handle much higher flows and pressures.
The difference in areas between the pilot and power stages creates a signal amplification effect.
Depending on the pilot system used, dynamic responses can be significantly higher than those of servo-proportional valves, as shown qualitatively in the logarithmic diagram in figure 5.
Minimizing parasitic losses is crucial for valve users. Two-stage valves require a small but constant actuation power to keep the pilot stage active.
Rather than addressing the root cause of parasitic losses, traditional manufacturers often reduce the size of the pilot stage control opening, increasing the risk of oil debris blocking the valve.
For this reason, highly efficient filtration is required to prevent pilot stage clogging.
The main advantage of traditional two-stage valves is their compact size and light weight, making them ideal for sectors like aviation, automotive, and robotics, where space and weight are critical. Moreover, they offer exceptional dynamic response, remaining stable beyond 200 Hz.
Domin’s Rotary Actuator Servo Valve
Domin has pioneered a new approach to servo-proportional valves, combining their simplicity with the speed and precision of two-stage servo valves.
The key innovation lies in leveraging 3D printing to machine the valve body precisely and harness the benefits of brushless motors for both precision and stability.
In figure 7, we see the functional diagram of Domin’s S4 Pro valve. Unlike standard servo-proportional valves, where the spool moves linearly, the spool in this valve rotates, opening and closing the flow paths within a 3D-printed valve body.
This patented solution allows the S4 Pro valve to handle flow rates from 0.5 to 18 l/min, with a bandwidth of 200 Hz. Additionally, unlike traditional servo-proportional valves, the S4 Pro provides a perfectly linear relationship between command and flow.
The S4 Pro consumes 2W in standby mode and less than 100W at full flow, weighing only 290 grams, including the integrated control electronics.
The benefits of 3D printing extend beyond this. In traditional valves, expensive machining is required on the spool to adjust the flow characteristic. In Domin’s design, the tuning is done on the valve body, allowing for customization according to customer requirements.
In the S6 Pro model, instead of a rotating spool, the spool moves axially, similar to traditional servo-proportional valves. However, in this case, the actuation is still powered by a brushless motor connected to the spool via an eccentric.
The S6 Pro (NG6) processes flows of up to 60 l/min and offers a bandwidth exceeding 200 Hz, with a weight of less than 600 grams.
The control electronics are integrated into the valve, which can be operated via a simple voltage or current signal, like traditional servo-proportional valves on the market.
This adds another advantage: full compatibility with traditional solutions. The innovation is all inside the valve, so customers don’t need to make any special adjustments to install Domin’s products.
For higher flow requirements, Domin also offers the S10 Pro (NG10), which handles up to 250 l/min, with a bandwidth of over 100 Hz.
By the end of the year, Domin will release two more models: the S12 Pro (NG16) and the S14 Pro (NG25), extending the range to flows of up to 1500 l/min.
Domin is a dynamic and fast-growing company, quickly making a name for itself in the global fluid power industry. The company is expanding rapidly, as evident by several job openings currently available on their website.
3D printing is bringing a revolution to even the most established areas of hydraulics, like valves. The challenge is on, and it will be interesting to see how competitors respond.
