Compact wheel loaders are one of many applications benefiting from electronically controlled hydrostatic transmissions. Operators enjoy the advantages of simplified vehicle operation for increased safety and productivity.
Diagram of ETAC system, which provides smoother power transmission, more accurate speed control, and reliable performance. Fuel efficiency and lower operating costs are also achieved when using fully integrated electrohydraulic systems.
By Mike Kanitz, Eaton Hydraulics Operations Div., Eden Prairie, Minn.
Electrical controls for proportional hydraulic circuits are nothing new, but thanks to several recent advances, electronic controls for hydrostatic drives are setting new standards in the mobile industry.
Electronic Transmission Automotive Control (ETAC) systems, once limited to use in Europe, are beginning to appear in North America, due to many cost and operating benefits. Applications can include lift trucks, telehandlers, railway maintenance equipment, utility vehicles. and compact wheel loaders. Within the ETAC system, the electronic controller is integrated with the engine throttle management system, as well as a closed circuit pump. It drives high performance proportional valves and uses electronic swashplate feedback to provide precise, dynamic system control.
Automotive control of a hydrostatic drive allows large vehicles to be operated in a way similar to a standard automobile with an automatic transmission. A single throttle pedal controls the engine and transmission, giving output speed and torque as needed for vehicle operating systems. Following are some of the main benefits that an ETAC system provides.
Adjustable performance — Within the capabilities of the engine, vehicle responsiveness (aggressive vs less-aggressive) can be adjusted relative to changes in engine speed. The settings affect both acceleration and deceleration models of operation.
Anti-stall — ETAC's scalable anti-stall sensitivity enables maximum engine loading and allows use of maximum available engine power over the full speed range. The response rate and load profile is adjustable to specific engine characteristics.
Operator interfaces — specifications and recommendations are provided for all system components, including the foot pedal, forward/neutral/reverse selector switch, inching pedal, and interlock switches (parking brake and operator presence).
Hot shifting — This feature allows the transmission to be rapidly shifted from forward to neutral to reverse (or vice versa) without damage to the vehicle powertrain system. Hot shifting improves productivity compared to mechanical systems by simplifying operation and reducing operator fatigue.
Hydrostatic braking — ETAC uses full capacity of the hydrostatic system for slowing vehicles, which reduces the demands on service brake systems. Hydrostatic-braking greatly reduces the need for frequent brake servicing and associated vehicle downtime. It is initiated by simply releasing the travel/accelerator pedal. More aggressive braking action can also be obtained by depressing the inching pedal.
Inching control — With this option, a second foot pedal permits the operator to temporarily control the relationship between engine speed and vehicle speed for more productive work cycle times. Inching control is useful when precise positional control is required or when engine speed must be increased independent of travel speed to support other machine work functions such as raising a load on a forklift.
Performance — The ETAC system offers more consistent performance compared to hydromechanical ( automotive drive) systems. Performance is not impacted by temperature effects on oil viscosity because engine speed is measured electronically, and charge pressures can be greatly reducedto improve overall hydrostatic-transmission efficiency. The use of electronic control offers the capability to more precisely match transmission performance relative to engine power. This delivers greater maximum vehicle performance, reduced fuel consumption and faster cycle times resulting in reduced overall vehicle operating cost.
Safety — The safety features ensure pump displacement is maintained at mechanical neutral by interrupting power to the pump control. Other integrated features include parking brake release and an operator seat presence switch.
Threshold stability — The system's electronic swashplate position (ESP) technology makes system control insensitive to temperature and/or oil viscosity. This ensures positive neutral — even under extreme cold start conditions — without-the complexities of extra valves. The control in turn, provides smooth, precise, reliable, and repeatable operation with predictable performance.
Setup — The operator must size the HST transmission, including pumps and motors, when setting up the system design software. From those calculations, many details can be tailored for the needed system applications, including:
- vehicle parameter settings
- vehicle speed vs. travel pedal profile
- engine speed vs. travel pedal profile
- pump displacement vs. engine speed, and
- engine torque curve.
Mike Kanitz is product manager for the Eaton Hydraulics Operations Div. He can be reached at [email protected].