How many times have you thought: “There must be a better way!” Imagine a hydraulic system lowering a heavy load in a controlled manner. As the load forces hydraulic oil through the control valve, potential energy is converted into heat. The oil absorbs most of that heat, which can overheat very quickly. Overheating hydraulic oil can be detrimental to the hydraulic system, as we learned in blog no. 3. Therefore, we must remove this heat, which requires more energy via the cooling system. Now imagine harnessing this energy as electricity and putting it right back into your factory grid?

Every time a load is lowered, electricity can be generated and used by other machinery at your location.

In blog post number 4, I wrote about energy savings by using a variable frequency drive (VFD) with a pressure compensated pump. The VFD contains special software to run the motor only at the speed required to maintain the pressure setting.

DFEn Control – A Giant Leap Forward

Today, I’m writing about a pressure compensated and load sensed pump (DFR in Rexroth nomenclature) complete with a digital controller. That makes it a DFEn control. The digital controller contains the pump and electric motor efficiency data as well as some very intelligent software. Combining this with a variable frequency drive provide us with a giant leap forward in control of hydraulics at the pump level.

DFEn Sytronix package where “D” stands for pressure control (Druck), “F” is for flow control, “E” is for electronically controlled and “n” is for speed control.

Eliminate the Throttle Valve

In many cases, this allows us to eliminate the use of a proportional control (throttle) valve. Thus we eliminate the pressure losses and heat generation over the valve. Therefore, we can reduce the size of the electric motor, as well as the cooling capacity.

This time, the “secret sauce”, the software is in the digital pump controller, not the VFD. All the calculations are done in the pump controller, which then sends out two control signals:

  1. required speed the VFD
  2. the displacement setting (volume per revolution) to the pump.

In addition to the digital controller mounted onto the control valve, the following sensors are an integral part of the pump:

  1. position sensor on the control valve spool
  2. Swashplate position sensor

A separate pressure sensor completes the sensor package.

Precise Control of Flow, Pressure & Power

With this system, we can control pressure, flow and power in a precise manner. This is due to the closed loop control of the valve spool and swashplate position (flow control). The pressure sensor allows us to control the swashplate in order to limit the maximum pressure, which can vary throughout the machine cycle.

The flow control can be very precise, because in addition to the above mentioned closed loop controls, the control software uses volumetric efficiency data of the pump to compensate for the internal leakage. Thus, when you ask for 30 litres per minute, you actually get 30 litres per minute out of the pump, for example. Additionally, this can be re-calibrated at any time in order to compensate for pump wear.

The ability to control pressure and flow means that we can also limit the power with a simple input. Again, this can be varied as the available power varies throughout the machine cycle.

Two Quadrant Control = Energy Recovery

Coming back to our introduction of recovering energy from lowering heavy loads. The pump swashplate is able to go 100% over centre (two quadrant operation), which means that we can lower a load without a directional or proportional valve! With the swashplate in a negative position, oil is flowing from the load back into the tank at a controlled rate. During this function, the load is trying to drive the electric motor faster than its asynchronous speed, which turns the motor into a generator. To capture this energy, we need an “active front end” on the variable frequency drive.

This recovered energy is fed directly back into the factory grid to be consumed by other users. Imagine the energy savings now! In addition to only running the electric motor at the speed to produce the required flow, we are now also generating energy!

Additionally, the pump can be ordered with an Ethernet based bus system for easy communication of system parameters. This allows us to view operational data from the machine, as well as change the operating parameters remotely.

Control Features

So if you can use the following features:

  1. flow / speed control
  2. pressure control
  3. power / torque control
  4. the ability to switch between flow and pressure control at any time
  5. power limitation via flow control at any system pressure
  6. energy savings by producing only the flow & pressure required
  7. energy recovery when lowering loads
  8. less cooling required
  9. smaller tank size => less environmental risk due to oil leakage

Then this DFEn-controlled pump from Bosch Rexroth is one of the best solutions. It uses standard, robust components that have proven their reliability in many industries.

Your Turn

Now you know there IS a better way to lower heavy loads. Do you have some machinery, where this solution could provide you with a competitive advantage? Contact me, if you’d like to discuss this further.