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S. Gopal

In the rapidly changing world of machine control, machine designers may select one of three drive technologies for their motion control applications — electrohydraulic drives, electromechanical drives, electropneumatic drives.

While each technology uses similar control schemes in them, there is a motion controller that is the brain of the system, and a feedback device that is the `eyes' of the system. There are many features and design factors that will influence the selection of a specific type of drive for a rotary or linear application, based on operation, performance, environmental and cost factors.

Operation

Linear motion in an electrohydraulic system is achieved with a hydraulic cylinder that one moving member, a piston and rod assembly that converts the pressurised oil flow into a linear motion. Air cylinders also have a piston and rod assembly to create linear motion in electropneumatic system.

Any electromechanical system must use many parts to provide linear motion because the source of power is an electric motor. Mechanical gearing components such as ballscrews, gears, or belts are employed to convert the electric motor output into linear motion. Ball screws make up over 90 per cent of the electromechanical linear motion market.

Power sources

Electrohydraulic drives require a source of pressurised hydraulic flow from a hydraulic power supply. This power source may be a centrally located pumping unit or a power unit that is dedicated for the purpose of one application. Electropneumatic drives also require a source of pressurised air and most often the existing compressed air system in the plant is used.

Electromechanical drives are self-contained units but for the need of electrical power to the point. These electromechanical drives are the simplest to install.

Performance

Electrohydraulic systems excel when the application calls for the high speeds with heavy loads. Electro hydraulic systems are also equal to the task when applications call for lower speed and accurate position or force control.

Electromechanical systems provide excellent control with lighter loads (less than 2500 pounds) and lower speeds, and the position accuracy requirement is less than +/- 0.001.

Electrohydraulic servo motor drives consist of a hydraulic motor coupled to a servo valve or a servo solenoid control valve.Electrohydraulic rotary actuators provide a single rotation up to 360 degrees of motion and are controlled with a servo valve or solenoid servo valves. Electric servo motor drives consist of a special motor with encoder feedback.

On rotary system variable speed electric motor drives are lower in cost in horsepower ranges under 5 HP. Whereas, variable speeds, electrohydraulic motor drives are the cost effective solution above 5 HP.

On linear systems, once again electromechanical drives are cost effective when the thrust force requirement is below 2,500 pounds. Electropneumatic servo actuators are best used below 1000 of thrust and they are more cost effective when thrust requirements are above 2,000 pounds.

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