One of the realities of fluid power is that more of it is being monitored and controlled by electronics. Two broad methods of control exist for hydraulics technology: valve control and pump control. In this context, the valve and the pump are the elements that connect the controlling electronics to the hydraulic system.
Factors too numerous to mention make electronics the control medium of choice, but one that must be reckoned with is the rush to electrify the world with communication buses and wireless communications—most recently wi-fi and smartphones. The ability of inanimate devices to freely communicate with one another is so compelling, and the advantages so enormous, that this juggernaut cannot be stopped.
Two-way communication allows centralized data-gatherers to interrogate the status of the system literally from anywhere in the world. Each instrument or device on the network can broadcast its presence and essential characteristics to any other instrument, controller, or data-gatherer. From an engineering and system design point of view, the enormous amount of information available will make initial startups of systems and even whole factories go much faster. And the same devices that are integral to the control system will broadcast diagnostic data that will either explicitly determine or be used to derive the general health of devices, components, and entire systems.
The fluid power industry is embracing these exciting new technologies. But before jumping in with both feet, we should review the challenges confronting our mature industry in order to link up—be it the serial bus of a machine or the Internet of Things (IoT). In pursuit of that aim, let us consider a pump architecture that is communication-read…and in the process expose at least some of the more important problems and vexing issues.
The aim here is not to provide all the answers, but rather, to establish a dialog by asking some of the right questions (at least from my perspective). Readers are encouraged to contribute, too, because there are many difficult decisions that must be made.
Comparing Pump Control to Valve Control
The use of valves to control hydraulic actuators is inherently inefficient, whereas pump control of actuators is very efficient. In spite of this reality, ample reasons exist why valve control is preferred. Primarily, valves can respond much faster than pumps can. Therefore, the most demanding applications—hoses requiring both rapid response and accuracy in controlling position, speed, pressure, force, or any combination of these—have relied on valves.
Changes taking place in industry (and those that will take place) reflect the need for the very best pumps that can be designed and built. In the text that follows, I’ll try to describe what’s needed from high-end pumps to meet industrial demands.
The Advantage of Pump Control
The main advantage of using a pump as the control element is that pumps can regenerate power, whereas valves cannot. For example, during the acceleration of a load mass or inertia, the pump displacement is increased, motivating the actuator to run at a higher speed. This transfers energy from the pump, through the actuator, and into the load. Upon deceleration, pump displacement is reduced, and the actuator pressure undergoes sign reversal. Essentially, the actuator then becomes a pump, taking energy from the load. The pressure reversal causes the pump to become a motor, overdriving the prime mover.
With an electric induction motor as the prime mover, it becomes a generator when it is forced into overspeed and puts power back onto the electrical grid to be used by other electrical consumers. Thus, power is not consumed during deceleration, but rather, is regenerated and put back into the electrical power bus. This technology has been highly developed by those dedicated to electrical actuation.
Now, contrast this with valve control. Upon acceleration, the valve opens, allowing power to transfer from the pump to the actuator and load. Upon a signal to decelerate, the valve reduces its opening, causing a large pressure drop at the same time flow occurs. The resulting pressure-flow product is power, which is dissipated in the valve lands by heating the fluid, and all the energy of deceleration is converted to heat and lost. Yes, valves can be very responsive in terms of changing flow direction and speed, but they cannot regenerate power.
Universal Pump Prototypes
The universal, electronically controlled, variable-displacement pump will serve as the prototype for the rest of this discussion. The pump is shown in analytical schematic form in the Fig. 1. But a word of explanation is in order in view of the earlier comments regarding valve vs. pump control. The interface between the hydraulic circuit and the electronic circuit can be only a valve or a pump. In reality, it is always done with a valve, even in the case of pump control. This interface occurs at the signal level, not power level. The device that accepts the electrical signal and produces a hydraulic reaction will be one of those two power devices.
1. The variable-displacement pump of the future will use sensors and microprocessors for electronic control rather than the fluid logic control used in today’s pumps.
Now, back to the pump. The conventional pressure-compensated pump uses internal pressure-sensing pistons, springs, and spools. The energy output of the pump itself effects a reduction in displacement as pressure rises above the compensator’s cracking pressure. Hydromechanical logic is used to establish the rate of displacement change and its sensitivity to the output-pressure change. This has led to the use of intricate spools, poppets, sleeves, and other hydromechanical pieces that are expensive and difficult to develop. The resulting machines represent well-developed technology and provide users with a variety of reliable pressure-compensated pumps with an even greater variety of performance characteristics.
Pumps will use a universal prototype as the kernel, and the logic to control them will be increasingly relegated to the computer—or some kind of so-called intelligent controller. This can be expected for several reasons:
- Requirements for the small, intricate parts used in the hydromechanical sensing and controlling will be all but eliminated, replaced by the small parts for the displacement variators for the kernel pump.
- The degree of control will be improved.
- Specific performance characteristics will be easily modifiable merely by entering a different parameter, say loop gain, as a digital input parameter.
- Electronic compensation methods can allow refining pump responses, especially when considering the tendency of some hydromechanical pumps to go unstable under certain loading conditions.
- Variations on integral control can make it possible to provide an electronically controlled, pressure-compensated pump with essentially perfect steady-state pressure control. That is, the output pressure will not have to change with changing load flow. The deadhead pressure can be the same as the average running pressure.
- Initial design will be expedited, because it is far easier to tune an electrohydraulic controller than it is to produce spools, poppets, and springs in almost endless combinations to achieve a specific response. Anyone who has tuned an electronic controller may disagree that it is easier. However, the process must be put into context. In the case of conventional hydromechanically regulated pumps, all the tuning is done by sizing the spools and sleeves and poppets and springs during product development in the laboratory—which can take weeks, if not months.
- Tuning of the electronic pump is done at application time, and even novices can fumble their way to acceptable tuning in a few hours or maybe days. And the important fact is that the electronic pump will be tuned to application conditions, not laboratory conditions. In the end, the net tuning time will be reduced for the electronic pump.
- More sensors are used in today’s electronically controlled pumps, and the cost they add to the pump is more than offset by the advantages derived from them. The greatest advantage can be that all the hydromechanical parts for a pressure-compensated pump will be the same as those for, say, a load-sensing pump. The only difference between these two machines will be in the control software. In fact, the control program for each machine will be loaded into the controller, and the operator will select the pump of choice based on the application circumstances at that instant.
Challenges of Software
Once the software has been developed, the cost to reproduce it will be almost trivial. Compare this to the cost of producing thousands of identical precision-manufactured parts for hundreds of identical pumps. As a result, a manufacturer’s ability to standardize on all the mechanical parts will result in substantial price reductions in products.
Now, anyone who has suffered the pains of developing software knows that this can lead to a significant expense. Software development is a technical specialty, and the quality of the control program will determine the ultimate degree of success of the electrified pump in the application. Debugging can be particularly frustrating to the uninitiated because usually only the programmer understands how difficult bugs can be to locate, and how challenging the corrections can be.
Most control-software programmers do not have the luxury of redundancy that designers do. A checker typically reviews each and every dimension and line drawn by the original designer before mechanical drawings are finalized. Programmers often work alone. And because most lack intimate quantitative fluid power skills, they are inclined to blame the hardware when a system behaves unacceptably. Of course, the hydraulic engineers are prone to blame the software. The real problem, most often, is on the system level.
Simulation and Real-Time Control
Learning simulation and mathematical modeling is an excellent preparation for developing programs for realtime control. Real-time control refers to the process where the program is written to execute with sufficient speed so that its calculations and logic always stay ahead of the machine it is controlling. It involves, among other things, the use of techniques that not only result in very efficient (fast-executing) code, but also synchronize the program with the controlled machine. Such is the purpose of wait loops, polling of input devices, and software and hardware interrupts.
In the industrial seminars I’ve conducted, control programmers often needed engineering details of servo and proportional control systems. More often than not, they were electrical engineers with programming experience, which provided them with a good background for modeling of hydraulic systems. Unfortunately, enough differences exist between electric and hydraulic circuits to make the quantitative choices difficult for someone not specifically schooled in digitally controlled hydraulic machinery.
It should almost be a prerequisite that anyone responsible for writing real-time control software be competent in simulation. Simulation would teach the control algorithm writer some of the nuances of hydraulic non-linearities and their expected dynamic responses.
The electrified pump, shown in combined schematic-block diagram form in Fig. 2, consists of the basic electronically stroked pump kernel, plus several sensors used for both control and diagnostics. We will examine the pump not as a component, but in the context of the system to which it must adapt. The elements in Fig. 2 form the framework for that look.
2. Modifications to the universal prototype pump allow greater Interaction between the pump and electronic controls and sensors. This configuration lends itself to two-way communication between devices and controls through the data bus.
The communications interface performs the connection between the pump assembly and the enterprisewide communications bus. The pump has a single input from the control computer to allow the pump to be configured in any of several ways. By selecting different control algorithms in the controller, the pump can act as a simple variable-displacement device, a conventional pressure-compensated pump, a speed-control loop, a position-control loop, a load-sensing pump, or any other desired performance—provided the performance can be programmed into the control software.
By periodically monitoring the sensors, other analytical software can compare the existing performance against original performance and assess any degradation in performance.
The Pump Kernel
The pump kernel is nothing more than the universal, electronically controlled, variable-displacement pump. The electromechanical interface will undoubtedly be a coil of some type (torque motor, force motor, or proportional solenoid) that requires a substantial current to provide appropriate mechanical motion of the valve, flapper, or whatever. That current will range between a low of 10 or 15 mA for certain torque motors, to a maximum of 2.5 to 3.5 A for certain proportional solenoids. This requires that the coil be driven by an analog servo or proportional power amplifier (S/P amp), shown as A in Fig. 3.
3. The prototype millennium pump is shown here with seven different sensors, which are responsible for monitoring critical pump variables. These variables Include outlet pressure, Input shaft speed, input torque, displacement, inlet pressure, case drain pressure, and case drain temperature.
Some Issues to Confront
There are several issues for the designer to consider. First, there is the issue of where the amplifier will be located. Most manufacturers have moved control electronics to the valve bodies of servo and proportional valves. This move has significantly reduced headaches with cabling and wiring, which are the least reliable components—mostly because they are external and subject to severe environments and physical abuse. The proximity of the electronic module to the valve allows delicate wiring and cabling to be safely tucked within rigid, sealed enclosures, in addition to reducing conductor length.
More of the electronic adjustments can be done at the factory, then set and locked, reducing the tuning needed at application time. Effects of severe environments are also minimized by eliminating exposure of the electronic circuit boards to such destructive conditions as mud, dirt, dust, steam cleaning, chemical cleaning, or perhaps worse.
However, experience with valves has also shown that in high-vibration applications, the mechanical integrity of the circuit board and its mounting are critical. Field reports of circuit board structure and mounting failures are rare, but in severe vibratory applications, they do occur. It is incumbent upon the electromechanical design staff that the electronic devices on board the pump be industrially hardened in order to withstand the rigors of the most abusive applications. This will require designing for structural integrity, the vibration and environmental testing, and finally qualification of the entire pump subsystem. Having done so, the advantages of placing the electronic circuits directly on the pump far outweigh the problems that accompany remote siting. The final package will have the S/P amp safely enclosed and sealed, as well as equipped with any necessary cooling fins.
The second issue that must be faced involves the need for DC power, which is required by the local electronics such as the servo-proportional amplifier. The dc power supply must also provide the power for the coil. Certain bus systems have been standardized to include two or three wires in the bus cable for carrying dc power to the devices. As long as the valve coil is at the low end of current requirements, (i.e., 10 to 20 mA), the bus can probably carry sufficient current for at least one of the millennium pumps.
If the pump is going to use a proportional solenoid requiring 2 or 3 A, then a special cable with dc power will have to strung for that purpose—it is unlikely that the bus will be able to supply that much current. Special tee connectors at the bus nodes permit a remote power supply to be added for special needs within a local area. Alternatively, low-voltage (probably 12- to 20-VRMS) ac power could be connected to the pump, but then, dc power would have to be generated on board the pump—requiring rectifier, filter capacitor and voltage regulator circuits. Note that this external cabling defeats a promised advantage of buses: that is, reducing the number of all those accursed cables!
The prototype millennium pump is drawn with seven different sensors to monitor the critical variables associated with the pump (Fig. 3). For any given application, this may be overkill, but the aim here is to look at possibilities and to discuss the issues, even in the extreme, if need be. Specific choices must be made with the application in mind. Refer to Fig. 3 regarding the following sensors.
- HPO is the pump outlet pressure sensor;
- HN is the pump input shaft speed sensor;
- Hr is the pump input torque sensor;
- HD is the pump displacement or cam angle sensor;
- HPI is the pump inlet pressure sensor;
- HP, CD is the pump case drain pressure sensor; and
- HCD is the case drain temperature sensor.
Each sensor has issues associated with it that will affect the success of the pump in the marketplace from the standpoints of utility, performance, and cost. Three pressure sensors should be as identical as possible in order to reduce inventory and related costs.
Jack Johnson is an electrohydraulic engineer with IDAS Electrohydraulics, Milwaukee.