Electro-hydraulic stepping motors

An electro-hydraulic stepper motor (EHSM) is a device, which uses a small electrical stepper motor to control the huge power available from a hydraulic motor (Figure 4.10).

It consists of three components:
1. Electrical stepper motor
2. Hydraulic servo valve
3. Hydraulic motor.

These three independent components when integrated in a particular fashion provide a higher torque output, which is several hundred times greater than that of an electrical stepper motor.

The electric stepper motor undergoes a precise, fixed amount of rotation for each electrical pulse received. This motor is directly coupled to the rotary liner translator of the servo valve. The output torque of the electric motor must be capable of overcoming the flow forces in the servo valve. The flow forces in the servo valve are directly proportional to the rate of flow through the valve. The torque required to operate the rotary linear translator against this axial force is dependent on the flow gain in the servo valve.

The hydraulic motor is the most important component of the EHSM system. The performance characteristics of the hydraulic motor determine the performance of the EHSM. These are typically used for precision control of position and speed. These motors are available with displacements ranging from 0.4 cubic in. (6.5 cm3) to 7 cubic in. (roughly 115 cm3). Their horsepower capabilities range between 3.5 hp (2.6 kW) and 35 hp (26 kW). Typical applications include textile drives, paper mills, roll feeds, automatic storage systems, machine tools, conveyor drives, hoists and elevators.


Limited rotation hydraulic motor

A limited rotation hydraulic motor provides a rotary output motion over a finite angle. This device produces a high instantaneous torque in either direction and requires only a small amount of space and simple mountings.

Rotary motors consist of a chamber or chambers containing the working fluid and a movable surface against which the fluid acts. The movable surface is connected to an output shaft to produce the output motion.

Figure 4.1 shows a direct acting vane-type actuator. In this type, fluid under pressure is directed to one side of the moving vane, causing it to rotate. This type of motor provides about 280° rotation.


Rotary actuators are available with working pressures up to 350 kg/cm3 (4978 psi). They are typically foot mounted, flanged or end mounted. Most designs provide cushioning devices. In a double vane design similar to the one depicted in the figure above, the maximum angle of rotation is reduced to about 100°. However in this case, the torque-carrying capacity is twice that obtained by a single vane design.