In this article an introduction to digital hydraulics is presented. For those unfamiliar with the term digital hydraulics or digital valves, this is a great place to get familiar with the area and the special demands that are put on these components.
What is digital hydraulics ?
The core component of a digital hydraulic system is the digital valve. When used to replace a proportional valve, most typically a fast switching valve is used for PWM control or a set of digital valves are used in a digital valve block for pulse-code-mode. They can also be used to control a multi-chamber cylinder or a hydraulic motor. All applications depend on fast, reliable low cost ON/OFF switching valves.
A hydraulic system that utilizes discrete control of either the supply part, the control (valve) part, or the actuator part can be called a digital hydraulic system.
Special demands for digital valves
When parallel connecting digital valves the number of valves used is quite high. When using switching technologies and other modes of operation, the switching cycles is extremely high, and much higher than what traditional valves encounters.
For a permanent switching valve, a typical switching frequency of 50 Hz is to be considered in the low end, and yearly cycles are in the order of 10^9.
Digital valves used in motor application face the same challenges. The valves typically switch one time during each rotation. For a medium sized HTLS motor, e.g. rotation at an average speed of 150 rpm, 8 hours a day, 5 days a week, 52 weeks pr. year means 1.87x10^7 million shifts pr. year. The same number is a factor 10 higher for a high speed 1500 rpm unit.
In parallel systems with several valves a low cost and robust design does not only apply to the valve, but also the power electronics and power consumption and mean power and peak current also becomes an important aspect.
The higher number of valves poses a higher risk of valve failure, but also the possibility for more redundancy. Regardless of this, a low failure rate is wanted and/or good failure detection systems and redundancy methods that all must be judges in the light of overall cost.
For most applications the valves must be either leak tight ore almost leak tight. With may parallel valves this becomes more important as the leakages sums up.
The valves should be compact to limit the space requirement and enable compact builds with small chambers to avoid unnecessary hydraulic capacitance and inductance which might cause dynamics problems in the system.
The Digital Distributor Valve System developed by Diinef is designed with the extreme demands in mind. The valve block is very compact, designed for low pressure losses and is tested for more than 10^9 cycles at 275 bar.
The valve block is developed for controlling cylinders in HTLS machines, but can be utilized elsewhere, e.g. in cylinder control. To read more about the valve block and other possible use cases see the application page.