Semiconductor and other equipment manufacturers often build machines that incorporate an isolated automated inspection station and a non-isolated robotic parts handling system. The robot moves the parts onto and off of the isolated inspection station during the inspection cycle. In general, the robots are programmed to place the parts in a specific location and the required part location accuracy is less than ±0.005 in. in three axes. Inspection cycle throughput requirements place a burden on the isolation system to re-level and settle very quickly.
Pneumatic Isolator Repositioning Accuracy
Leveling accuracy of Newport’s IPV Series leveling valves is ±0.010 in. — probably the best standard offering in the industry. We also make higher precision valves for tighter customer position requirements but these re-level and settle too slowly for high throughput applications.
Newport’s pendulum horizontal isolation systems are gravity driven and re-position a “free” isolated platform within about 0.001 in. However, when cabling is routed from the isolated platform to the tool frame, hysteresis is introduced that makes the horizontal re-positioning accuracy non-repeatable and outside the allowable range.
OEM Docking Systems
To meet the isolated platform position requirements of robotic parts handling systems, Newport supplies “docking systems” on a number of OEM products. Docking systems lower the isolated platform onto three semi-kinematic mounts. The precision position interface is typically made of three tooling balls mounted on the frame that contact three mating locations on the isolated platform. The mating locations consist of: 1) a conical socket, 2) a vee groove socket, and 3) a flat plate.
Un-Regulated Gravity Docking Systems
The simplest docking system incorporates a three-way pneumatic solenoid valve between each leveling valve and the isolators that it controls. On the “dock” command from the customer’s system controller, the solenoid closes the output from the leveling valve and bleeds air from the isolators. No longer floating, gravity brings the platform down and causes the tooling balls to engage the mating locations. On the “un-dock” command, the bleed port is closed, the leveling valve output port opens, and the systems re-float normally.
Unfortunately, unregulated gravity docking systems are slow and limited by the rate at which mechanical leveling valve systems can fill the isolators. The re-leveling times are usually 3 to 5 seconds — longer than acceptable in many high throughput inspection applications. If the maximum airflow rate is exceeded, the isolators will oscillate while floating. Therefore, changing to high flow valves won’t make the system refloat and re-level faster.
Kinematic Mounts
Any platforms' position can be defined uniquely in terms of six independent coordinates: three translations and three rotations, with respect to some arbitrary fixed coordinate system. A mount is said to be kinematic when the number of degrees of freedom (axes of free motion) and the number of physical constraints applied to the mount total six. This is equivalent to saying that any physical constraints applied are independent (non-redundant). A kinematic mount, therefore, has six independent constraints.
The most common type of kinematic mount is the “cone, groove, and flat” mount schematically illustrated in Figure 1. Consider the object as being attached to the coordinate system of the three spheres in the figure, and its corresponding mount having the cone, vee, and flat. If the platform is first seated in the cone, three degrees of freedom (x, y, and z translations) are eliminated without redundancy.
At this point, the platform can still rotate freely about all axes. Next, the second sphere is seated in the groove that is aligned towards the cone. This constrains or eliminates two more degrees of freedom — pitch and yaw — as shown in the figure. The alignment of the groove towards the cone is important in order not to over-constrain one or more of the translation degrees of freedom. Roll about the x-axis is the final degree of freedom to constrain. This is accomplished by seating the third sphere on the flat. Six non-redundant constraints make this a kinematic mount.
Figure 1—Kinematic Constraints
Regulated Gravity Docking Systems
One approach to speedup the re-floating process is to regulate the pressure to which the isolators are bled down when the payload is lowered. For example, if 70 psi is required to float the payload, then lowering the pressure to 50 psi may bring the payload onto the tooling balls. Obviously, it takes less time to refill the isolators from 50 psi than from 0 psi. The figure below diagrams the regulated gravity approach.
Settling time and leveling accuracy of the system can be further improved by incorporating Damping configuration. Damping uses high gain leveling valves to make the system respond faster and constricted damping orifices to restrict airflow in the hybrid chamber.
Full Pressure Docking
Full pressure docking systems never exhaust the isolator but use a mechanical system to pull the platform onto the tooling balls (against the pressure of the isolator). The system adds a two-way solenoid valve between the leveling valve and the isolator(s). On the “dock command”, the valve closes and holds the isolator at the existing pressure required to float the isolated platform. After the isolator solenoid valves are closed, the mechanical system pulls the isolated platform down until the platform is firmly seated. Typically, a set of three (or four) pneumatic cylinders is used to pull the platform down. Some systems use motors connected to a spool and wire system to pull the platform onto the tooling balls.
On the “un-dock command”, the platform is released and the isolator solenoid valves are opened. Because the isolators were held at the floating pressure during the docking operation, they do not need to be brought up to pressure by the leveling valve. As a result, re-leveling and settling times are very short. Using a Damping system, settling time can be further reduced by increasing the amount of damping in the isolators.
Summary
The docking system chosen depends primarily on the throughput needs of the machines. Unregulated gravity docking is a simple solution for applications with long operations where docking time is a fraction of the overall cycle time. Regulated gravity docking provides significant improvement for very little increase in cost and system complexity. Full pressure systems offer the fastest recovery time, especially when used with Damping, but at a definite increase in parts count and cost.
In general, docking systems and Damping Isolator configurations are unique to each customer machine design. In some applications heavier duty diaphragms must be chosen or a special valve setup must be used to optimize the system. Each machine will have its own loads, cycle time requirement, CG, and other issues to contend with. VC Engineering is available to help your customer develop the most appropriate system for their machine.
Both docking system types are commonly used. Both versions of the “bleed down” docking are suitable for customer operation with long cycle times where the docking time is a small fraction of the overall cycle time. For applications with short cycle times, the “pull down” system is far superior. Contact the OEM Engineering or Vibration Control Custom Products Group to choose the best docking system for your application.
