1. Field of the Invention
The present invention relates to an apparatus and methods for controlling a hydraulic actuator for use in an injection molding machine, and more particularly to controlling such a hydraulic actuator (both linear and rotary) with a processor which is disposed locally near the actuator and/or the hydraulic manifold.
2. Related Art
Injection molding machines produce great quantities of products at high speed. For example, the widely-used PET plastic drink containers are made at production rates of thousands per hour. During these high speed operations, various injection molding machine devices (such as the molding clamp assembly, the injectors, various control switches, and other machine components) are moved using a number of hydraulic actuators. Such hydraulic actuators are supplied with pressurized hydraulic fluid that causes movement of an internal diaphragm or piston which, in turn, drives the molding device. A control valve controls the flow of hydraulic fluid to the actuator to control movement of the diaphragm. FIG. 1 depicts a typical control valve. In FIG. 1, the hydraulic actuator 2 includes a piston 4 which divides a chamber into two halves 6 and 8. Movement of the piston 4 drives a load 10, which, for example, may comprise a mold and clamp mechanism. A position sensor 12 senses the position of the load 10 and provides a feedback signal to a system controller (to be discussed below).
The hydraulic actuator 2 has two hydraulic fluid lines 64 and 84 which allow hydraulic fluid to enter and escape from the chamber halves 6 and 8, respectively. Pressure transducers 66 and 86 respectively monitor the pressure in line 64 and 84 and provide output signals to the system controller.
Hydraulic fluid from a pressure source (typically a hydraulic fluid pump; not shown in FIG. 1) is provided to valve 14 through hydraulic line 74, while hydraulic fluid may be returned from valve 14 to hydraulic fluid storage tank (also not shown in FIG. 1) through hydraulic line 94. Pressure transducers 76 and 96 respectively monitor the pressure in lines 74 and 94 and provide output signals to the system controller.
Valve 14 controls the flow of hydraulic fluid through the chambers of hydraulic actuator 2 to move the piston 4 back and forth thus driving the load 10. Valve 14 has fluid ports A, B, P, and T which are respectively coupled to the hydraulic lines 64, 84, 74, and 94, as shown. The valve 14 has a straight flow section 142 and a cross-flow section 144 which are respectively driven by solenoids 146 and 148 in order to control the flow of fluid within the valve. For example, when the straight flow section 142 is driven to the A, B, P, and T ports, pressurized fluid will flow through lines 74 and 64 into chamber 6, driving the piston 4 toward the load 10. On the other hand, if cross-flow section 144 is driven to the ports A, B, P, and T, then pressurized hydraulic fluid will be provided through lines 74 and 84 to the chamber 8, driving the piston 4 away from the load 10.
In the related art, control of the hydraulic actuator 2 through the valve 14 was a relatively straightforward process. For example, U.S. Pat. No. 5,062,052 (incorporated herein by reference) discloses that such actuators may be controlled with an analog signal processor and/or a programmable logic controller which are disposed at a location remote from the injection molding actuators so that the processing circuitry is not damaged by machine heat and vibration. Typically, the analog signal processor and/or the programmable logic controller will perform closed-loop control of the actuator 2 through valve 14 in order to keep load 10 moving within the prescribed operational ranges. The analog signal processor and/or programmable logic controller will receive feedback signals from the pressure transducer units 66, 76, 86, and 96, and position information from position sensor 12 in order to control valve 14 according to a predetermined control program. The analog signal processor can also effect operational changes in the operation of the actuator 2 through command signals received through the programmable logic controller, for example to change the molding and clamping times used by load 10.
The programmable logic controller stores a plurality of predetermined control programs which cause the analog signal processor to control the analog devices of the injection molding machine. The programmable logic controller may also include circuitry for controlling the digital devices in the injection molding machine, for example, digital solenoid valves and proximity switches. The programmable logic controller thus controls the elements of the injection molding machine either through the analog signal processor or directly through the digital devices.
In the control scheme of the '052 Patent, however, the analog signal processor and the programmable logic controller are required to perform command and control operations for all of the various devices in the machine. This imposes a processing bottleneck. For example, the programmable logic controller may attempt to execute closed-loop control of multiple different analog devices at the same time. Typically, faster and more powerful processors have been used in an attempt to overcome such problems. Such expensive solutions have, nevertheless, been unable to solve the control timing problems experienced in known actuator control architectures.
Another problem with the known control architecture is the reliability of the analog signal processor and the programmable logic controller. If either one of these components fails, the entire machine must be stopped until a replacement is located, installed, and programmed to operate in the specified machine. Since each actuator in every machine has unique operating characteristics, the newly-installed processor(s) must be re-programed and/or re-parameterized with the operating characteristics of the corresponding actuator(s) before full-scale production can be resumed.
Furthermore, the dedicated wiring inter-connections used for communication between the analog signal processor and the programmable logic controller to each actuator results in a plurality of wires which are difficult to install, maintain, and repair.
Thus, a need exists for a control architecture for hydraulic actuators in an injection molding machine which provides fast, flexible, and reliable control of the actuators.