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.