A hydraulic actuator assembly for a push-type die closing unit of an injection molding machine which has a central, internally bypassable power piston, lateral cylinders for rapid opening travel and a rearward stationary plunger for rapid closing travel, the lateral travel cylinders and the pressure spaces of the power cylinder creating a differential effect under which a small external flow volume of pressurized fluid is required from the pump to produce rapid opening and closing travel while a much larger internal flow volume passes through a flow connection between the cylinders. A program-controlled proportional-response throttle valve in this flow connection gives an accurate and steady control over the travel speeds, especially during deceleration. The throttle valve also makes it possible to obtain a forcible initial die opening movement.

A piston and cylinder arrangement for opening and closing entrance or exit doors or both in a milking compartment of a fully automated milking system. The piston and cylinder arrangement comprises an elongated cylinder having two pistons connected in tandem to an elongated piston rod. The cylinder is hermetically divided into two cylindrical portions each containing one of the pistons. The piston acts against a liquid in one portion and in the other portion the piston is actuated by pneumatic pressure. The piston rod is extended through the end of the cylinder and is connected to open and close an entrance door. The cylindrical portion containing the liquid has the liquid on both sides of the piston connected by a check-valve and a by-pass valve. To block movement of the pistons, the by-pass valve is retained in a closed position by pneumatic pressure from a supply of pressurized gas for actuating the piston in the pneumatic side of the cylinder. The by-pass valve is a sliding valve urged into an open position by a compression spring when the valve is not closed by pneumatic pressure. Both the by-pass valve and the check valve are disposed external to the cylindrical housing.

The present invention provides a pneumatically powered actuator having hydraulic control for both locking and controlling the velocity of an output rod without any sponginess. The invention includes a double-acting pneumatic actuator having a bore, a piston slidably engaged within the bore, and a control rod connected to the piston. The double-acting pneumatic actuator is mounted to a frame. A first double-acting hydraulic actuator having a bore, a piston slidably engaged within the bore, and a follower rod mounted to the piston is mounted to the frame such that the follower rod is fixedly connected to the control rod. The maximum translation of the piston within the bore of the first double-acting hydraulic actuator provides a volumetric displacement V.sub.1. The present invention also includes a second double-acting hydraulic actuator having a bore, a piston slidably engaged within the bore, and an output rod mounted to the piston. The maximum translation of the piston within the bore of the second double-acting hydraulic actuator provides a volumetric displacement V.sub.2, where V.sub.2 =V.sub.1. A pair of fluid ports in each of the first and second double-acting hydraulic cylinders are operably connected by fluid conduits, one of which includes a valve circuit which may be used to control the velocity of the output rod or to lock the output rod in a static position by regulating the flow of hydraulic fluid between the double-acting cylinders.

A hydraulic damper for a high speed actuator is provided to bring the actuator to rest with minimum oscillation around a target position. The hydraulic damper comprises a hydraulic circuit to communicate with the actuator over its entire positional range for the purpose of dampening a sudden stop of the actuator about a target position. The actuator is connected by a cable to a reel, the reel being connected to a shaft movement of the actuator produces rotation of the shaft. The shaft is also connected to a hydraulic motor which pumps hydraulic fluid through a hydraulic damper too, in effect, dampen the movement of the actuator.

A digitally controlled air-over-hydraulic actuator includes a pneumatic cylinder and a hydraulic cylinder each having a separate piston and a common connecting rod. The pneumatic piston applies force to the connecting rod urging movement of the hydraulic piston. Bleeding of oil from one side of the hydraulic piston to the other is controlled by means of digital pulses applied to a digital valve that allows oil on either side of the hydraulic piston to flow to the other. The digital valve includes a moving piston having an oil passage that is momentarily aligned with ports to opposite sides of the hydraulic cylinder. A control system senses the differential pressure between opposite sides of the pneumatic piston to produce pulses that accelerate the digital valve piston past the ports, precisely controlling forced movement of the hydraulic piston.