Hydrostatic transmission (HST) is a term referring to a closed-circuit pump and motor system where the pump is generally of variable-displacement, axial-piston design. The motor, or motors, can be fixed or variable displacement design. Machine propulsion is the traditional application for HSTs, but various work functions also benefit equally from the drive advantages of an HST.
The primary function of the HST is to accept rotary power from the prime mover (usually an internal combustion engine), and transmit that energy to a load via a hydraulic motor. In the process, the HST generally regulates speed, torque, power, and direction of movement. The typical HST drives a load from full speed in one direction to full speed in the opposite direction, with infinite variation of speed between the two maximums. Input speed to the pump is fixed for most off-road machinery, but sometimes is controlled via the engine throttle.
HSTs offer some important advantages over other forms of power transmission. Depending on its configuration, a hydrostatic transmission is capable of:
- transmitting high power in a compact package
- exhibiting low inertia
- operating efficiently over a wide range of torque-to-speed ratios
- maintaining controlled speed regardless of load, within design limits
- accurately maintaining preset speed against driving or running loads
- transmitting power to multiple locations, even if position and orientation of the location changes
- providing faster response than mechanical or electromechanical transmissions of comparable rating, and
- providing dynamic braking.
Whatever its task, the HST must be sized, designed, and controlled for an optimum match between the engine and the load. This allows the engine to operate at its most efficient speed and the HST to make adjustments to operating conditions. The better the match between input and output characteristics, the more efficient the entire power system.
While equipment manufacturers are continually being challenged to improve the operating efficiency of their machines, end-users of equipment are seeking better fuel economy, longer life, and higher productivity. Federal regulations regarding emissions are further restricting the horsepower available on equipment. The impact is limited displacement options for engines, and increased costs due to cleaner fuel blends, more fuel usage, and greater heat rejection. To meet these challenges, off-road HST drive systems must more efficiently use the horsepower available. The horsepower delivered will have to come from better-designed components and intelligent systems.
Control technology for HSTs has evolved far from simple lever, cable, or hydraulic pilot-operated pumps, where feedback links, four-bar levers, or inline valves and orifices limited any intelligence built into the system. Although some or all of these early control technologies are still being used, the trend is toward more sophisticated controls. This evolution has led equipment manufacturers to abandon conventional mechanical systems and invest in electro-hydraulic (EH) controls.
The past decade has seen a rapid transition from simple EH control to heavily integrated microprocessor controlled machines and work functions. Today's EH system is part of a network of functions built upon the premise of communication and decision-making. The advantage of an EH control is that it allows the equipment manufacturer to adjust power requirements around the machine in an infinite number of scenarios.
Machines are becoming more versatile — one machine platform is being used for multiple tasks around the jobsite. A single machine might at one point perform digging operations, then become a transport machine for material, and could eventually perform a wide array of other attachment functions during one working day.
An EH system allows the machine to adjust between these states. For example, during a digoverging operation, an EH drive system may adjust according to demand of an auxiliary function, thereby allowing power to be transmitted to the primary work function. A machine that uses various attachments can adjust to optimize the appropriate function. This versatility allows the machine to boost the overall operating efficiency of the machine while maximizing the output of the engine.
The EH multiple personality
Today's EH vehicles meet advanced requirements with surprising agility. Their key advantage is the ability to operate under multiple sets of algorithms and situations based on operator input, environmental conditions, and vehicle setup. These vehicles essentially have personalities that can be changed and adapted to provide increased productivity and safety.
Consider a wheel loader equipped with a full EH system. For normal haul-and-carry work, the propel system can operate in an automotive mode, where the operator uses the accelerator pedal to control ground speed by simultaneously ramping engine speed and pump and motor displacement ratios. This same vehicle, when equipped with a cold planer attachment, can switch to a displacement control mode and provide precise ground speed independent of system pressure. The change in personality for the wheel loader can be initiated by the operator, or even by the work tool via a Control Area Network (CAN) signal.
This same wheel loader can be equipped with a multi-function joystick that changes operation depending on attachment and work mode. It can also accomplish variable ratio EH steering that is speed dependent. Algorithms for all these scenarios reside in a microcontroller, which allows equipment manufacturers to quickly modify and update personalities based on new attachments, or changes in operator preferences in the field. Previous time spent changing orifices and springs within the hydraulic system is now spent in the cab, running calibration loops in the software and adjusting parameters on the fly.
Flexibility from the algorithm
Algorithms allow equipment manufacturers to gain market share. These manufacturers are able to create full product lines that share proprietary characteristics in performance or response to operator command. For example, a large excavator and small skid steer loader can share the same joystick control patterns, significantly reducing the learning curve for professional operators switching vehicles on the jobsite.
Additionally, equipment manufacturers can mimic the control of a competitor's machine. For instance, dual path machines can switch easily and quickly from ISO to H pattern, and back with a flip of an operator-controlled switch. Learning modes allow new operators to learn the behavior of a machine with less stress and fewer safety risks.
Better and safer
The responsibility to design a safe machine is heightened as demand on machine systems continues to expand and machines subsequently become more complex. To meet this market demand, equipment manufacturers and hydraulic suppliers are jointly using various risk analysis tools designed to highlight the "what if" scenarios associated with potential failures or malfunctions. EH controls are the ideal response to the complicated expectation of more productive machines with improved reliability and operational safety.
HSTs incorporating electrical pump control are especially suited for software and microprocessor control. A vehicle using some means of intelligent control can incorporate an infinite array of software algorithms that are designed to safely control and also monitor vehicle operation. HST acceleration and deceleration profiles are easily tuned to match desired requirements for safe vehicle operation.
Less obvious EH opportunities include self-diagnostics and system monitoring, such that if a failure were to occur it can be instantly detected and the machine placed in a fail-safe or de-energized state. Perhaps a limp home mode is desired if a particular fault is detected.
In addition to software control, integration of sensors and circuit valving is also becoming more accepted. Integrated or discrete sensors can be used to monitor pressure, speed, temperature, and pump displacement, so any number of safety-related circuits or controls can be achieved.
Integrated over-ride valves are rapidly becoming a system requirement where a vehicle can be held in a safe state until certain operator inputs are received. A vehicle can also be placed in a safe state when a fault is detected or an out-of-range output is recorded. The use of sensors and system monitoring, most importantly, enables the drive system to take preventive action prior to a disabling drive failure.
Pressure, speed, and temperature data logging for preventive maintenance or warranty administration are also easily provided with the capabilities of the onboard microprocessor intelligence and EH system monitoring.
Plug and perform
On-board EH intelligence in offroad HST machines plays a key role in improving drive performance and efficiency, as well as expanding vehicle capabilities and productivity. Additionally, an EH controlled HST can also shorten equipment assembly line and installation setup cycle times.
A smart controller can significantly reduce the setup time by eliminating the often time-consuming manual vehicle calibration and adjustment process of traditional mechanical or hydraulic pilot operated controls. An EH drive can quickly enter into a self calibrate mode where all thresholds and settings can be determined and stored in the controller memory. A secondary benefit exists within the aftermarket environment, where similar cycle time reductions can be found when a vehicle requires service or maintenance calibration.
Randy Rodgers and Branko Horvat are product managers, and Andy Sturtz and Darren Magner are application engineers for Sauer-Danfoss. Contact the company at 515-956-5750 or email [email protected]. Visit their web site at www.sauer-danfoss.com.