At last week’s 2022 Maha Fluid Power Conference at Purdue University, Chris Williamson of Danfoss presented Digital Fluid Power Systems: Challenges, Opportunities and “Critical Mass.” In his talk, he discussed some of the basics and offered his thoughts on the industry’s readiness for digital systems.
Let’s start with the idea of critical mass, said Williamson. If you put enough uranium together in sufficient density, then you can get a self-sustaining chain reaction. In a metaphorical sense the same holds when you think about the development of a new technology, especially a disruptive technology. In its life cycle, there’s an inflection point where suddenly the volume of sales increases, the adoption rate increases, where it goes from requiring external investment to being self-sustaining, and that’s where the new technology really takes off. The question is, has digital fluid power reached that inflection point?
As a bit of background, digital control is based on on-off states, he explained. That could be with fast switching methods, sort of a PWM analogy. It could be with switching valves operating in parallel. There are a variety of different combinations, along with a lot of research into the details. “The important point is that today, fluid power is generally controlled in an analog way. We use proportional valves. We use continuously variable flow controls for pumps and motors. There’s generally an analog paradigm with a few exceptions, and digital fluid power is more about going to discrete states to control outputs.”
Two examples are the digital displacement pumps from Danfoss and NorrDigi multi-actuator cylinders, he said. Both of these components fall into the category of parallel technologies. With the DDP, each cylinder in the pump is connected to on-off valves — basically a parallel system. With the multi-actuator, cylinder, multiple piston volumes connect in parallel to generate different combinations of force.
“The main advantage is higher efficiency. Most fluid power systems today are controlled by resistances. Think about proportional valves and pressure-reducing valves and counterbalance valves. It’s all about metering the pressure. And that’s how we control outputs of force and velocity. Digital fluid power is trying to move in a different direction and to eliminate a lot of those power losses and facilitate energy recovery,” said Williamson.
The downside with those discrete states is that operation is not necessarily smooth. If those state changes happen slowly, that can feel like a bump or a noticeable jerk to the operator. As we go to higher frequency switching, the operator detects noise or vibration, which is undesirable. That is certainly a challenge, along with other hurdles like cost and complexity.
“But then on the other hand, digital hydraulics has some advantages in terms of redundancy and safety, having states that are very clearly defined and not having failure conditions where something gets stuck in the middle. A point that I want to make is that it’s not one single advantage that really is meaningful. In talking with customers, it’s usually a combination of advantages that makes a digital technology more desirable,” he said.
The NorrDigi cylinder from Norrhydro has four volumes which connect in various ways to produce 16 area combinations. That provides a stair-step modulation in force using constant pressures. Research on an excavator shows up to 50% fuel efficiency improvement in terms of tons of dirt per liter of fuel. Volvo is currently conducting field trials and has announced that they aim to begin production in 2024. So the digital hydraulic cylinder is now out of the lab and is accumulating real customer experience and development.
The digital displacement pump was created by Artemis in Scotland, a spinoff from the University of Edinburgh. The concept has been in development for more than 30 years, and Danfoss recently acquired the company.
“The basic idea behind digital displacement is using on-off valves to control each of the pistons individually,” said Williamson. “The advantage is you get fast response and high efficiency because only the pressurized cylinders are connected to the load. Everything else idles.” One DDP example involves an excavator. Because each of the pistons is controlled individually, that allows us to create groups of cylinders in the pump that are controlled separately from one another and create a multi-outlet pump. In one concept, a 24-piston pump is divided into eight pumplets of three pistons each, which with a ganging manifold on the top of a manifold of switching valves allows us to combine those in different ways to two service outlets.
“So it’s effectively a two-outlet pump with a variable capacity. You can change the maximum displacement from zero to 192 cc, connected to both of those outlets. Why would you do that? Well, rather than having to split the flow between two consumers that are at different pressure and flow demands, now you can split the capacity efficiently and match the power demand from the actuators with the power supply from the pump, and reduce the throttling losses associated with splitting the flow from one pump and multiple actuators.
“This works. I’ve operated this particular excavator myself. We’ve demonstrated up to 30% fuel savings based on this concept, and this is without energy recovery. If we look to the future of digital displacement pump-motors that can recover energy, then this number looks more like 50% based on the lab results that we’ve done,” he said.
DDP has been demonstrated in a wide variety of applications beyond mobile equipment, including in hydraulic power units for test equipment, waterjet cutting machines, and cable-pulling machines for installing utility cables through underground conduits. It even powers a hydraulic drone, the Flowcopter.
A system that is both efficient and controllable is attractive. Traditional hydraulic systems use resistance for control and a higher level of controllability equates to higher losses.
A major driver behind digital fluid power is electrification. “People have said digital hydraulics are too expensive. When you compare it to the cost of batteries and power electronics, the cost of the hydraulics is minuscule. So a benefit in terms of energy savings really pays back when you’re talking about resizing batteries or longer run time for batteries.”
Another area of interest is autonomous vehicles and mobile robots. They need smarter components that can be more predictive and provide more information to the control system. The intelligence that gets built into digital hydraulics is a benefit.
“We’re now reaching a point where the technology has matured to a point of being useful and where the market has matured to a point of wanting it and being ready for it. Twenty years ago, nobody wanted a pump or a cylinder that had lots of sensors built in. It wouldn’t have been reliable, but that’s now achievable today. And, also, the advances in embedded controllers mean much, much more capability today than there was 15 or 20 years ago,” he said.
“So a few closing thoughts. Digital hydraulics are out of the laboratory and into the field, accumulating tens of thousands of hours today. Have we reached that point of critical mass, that inflection point? I don’t think so. Are we getting close? I hope. There are certainly some interesting factors that are pushing in that direction. The world needs higher efficiency, better control and more intelligence. There are also many challenges to bring this to pass, noise and vibration among them, but there are also others. We are looking for bright people to do that, both within the industry and in academia, to join our team and help us move hydraulics into the digital age.”
Filed Under: Cylinders & Actuators, News