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Designing Positioners for Robotics
Understanding a few principles of positioner design will help your robot and positioner become an integrated whole


Fig. 1 - Example of multiaxis positioner used with robots.
Part positioners for robotic welding have many things in common with traditional welding (manual or hard automation) positioners. Both must be able to support the tooling fixture and parts (load) for welding, and provide a suitable current return path to the welding power supply. They might also need to manipulate or turn the load to provide optimum welding orientation. Additional positioner features required for robotic welding applications might include operator safety interlocks, equipment safety interlocks, and coordinated motion with the robotic manipulator. If multiple welding power supplies are used, separate current paths for each power supply are recommended.

Choose from Many
The spectrum of robotic welding positioners runs the gamut from the simple flat table to very complex multiaxis, servo-driven devices. The servo-driven head- and tailstock system might be the most common, versatile robotic welding positioner in use today. The application challenges of properly supporting and manipulating the load, providing suitable safety interlocks and weld ground return paths with this system, are representative of most robotic positioners.

Types of Positioners
Simple positioners include stationary tables, manual indexing tables, and pneumatic- or motor-driven indexing tables. Single-axis servo positioners include the servo-driven rotary table, cantilever-type headstock, and the headstock/tailstock (HS/TS) configurations. Multiaxis servo positioners come in a wide variety of configurations, including the popular ferris-wheel type headstock/tailstock, the tilt and rotate (skyhook), and table/headstock combinations. Also in this category are custom positioners with as many axes as required by the application - Fig. 1

Process Requirements for Robotic Welding Positioners
Robotic welding power sources are sophisticated pieces of equipment that require suitable weld ground circuits to ensure proper feedback and optimal performance. Noise and interference on the feedback circuits can cause poor quality or inconsistent welds in addition to other process problems.

Single welding gun systems must provide a well-defined, low-resistance return current path. Some manufacturers return weld current through preloaded bearings. However, dedicated contact brushes with prescribed current paths are the preferred method.

Multigun systems also must provide a low-resistance weld current return path. However, it is recommended that the individual welding gun circuits avoid shared current paths. This can cause cross talk between the power source feedback systems, resulting in poor weld control and inconsistent quality. Therefore, each weld system should have its own independent weld ground system - Fig. 2.

Fig. 2 - Schematic showing the progression from good to bad for current path systems.
Production tooling might require electrical I/O control signals, pneumatics, cooling water, and other peripherals. When reciprocating motion is required, managing the interconnecting cables and/or hoses gets more challenging. Cables and hoses can tolerate a limited amount of reciprocating motion if they are properly managed, protected, and terminated. Common ways to manage reciprocating motion include using a through-hole on the rotational axis; energy chains (flexible cable trays); slip rings (rotary connectors); or overhead suspension. Slip rings are required for continuous motion applications.

A high-quality robotic positioner should have features that allow it to repeatably identify the "zero" or "home" position. This will help minimize robot program touch-ups after collisions or repairs. The homing feature should be easy to use and repeatable. It can be accomplished by using a metal or plastic pin, gauge, alignment tab, or other physical feature.

Safety Requirements - Operator and Equipment Protection
The ANSI/RIA R15/06-1999 safety standard applies to robotic workcells and contains specific safety guidelines that must be followed to protect the operator and the robot. The equipment also must meet all applicable local, state, and federal codes. Safety-related features of the positioner must integrate with the cell controller architecture and should include dual-channel compatible switches or sensors, and emergency stopping (E-stop) capability.

Fig. 3 - Method for determining bearing capacity of headstock.
The E-stop time and distance traveled is a performance characteristic of the positioner. This should be tested under prescribed load and speed conditions. The documented results can be used to design the operator safety plan for the production cell.

Part Support and Manipulation
Robotic welding positioners generally are rated by their capacity to support and manipulate the load, which includes both the weld tooling and the part. For table-type positioners, this represents the thrust capacity of the table bearing system. For headstock, HS/TS, and other multiaxis positioners, this represents the moment and radial capacity of the drive and free-end bearing systems. Single- or multiaxis positioners all must have the ability to manipulate the load in a smooth and controlled fashion. Excess vibration or settling time at the end of motion might adversely affect cycle time and overall weld quality.

Headstock and headstock/tailstock positioners are commonly used in robotic cells due to their versatility and simplicity. When used in tandem, the operator can load one positioner while the robot is welding on the other one, thus improving throughput. A detailed study of this configuration provides insight into evaluation of the other positioner types.

Calculating Load Capacity
The stand-alone headstock must support the load in a cantilevered fashion. This creates high moment loads on the bearing system, which, in turn, defines the load capacity of the headstock. The moment load (M) equals the load (W) multiplied by the distance (D) from the bearing center and should not exceed the limits set by the bearing manufacturer - Fig. 3. Many positioner manufacturers rate their headstocks at 50% of this bearing capacity value, providing a safety margin for overload conditions.

Fig. 4 - Load-bearing capacity of a stand-alone headstock can be increased with a headstock/tailstock setup.
The capacity of the headstock can be significantly increased with the addition of a tailstock (free-bearing support) because the load is no longer cantilevered. Traditionally, the tooling has been rigidly mounted between the headstock and tailstock bearing systems. This capacity can be modeled and the bearing loads (moments) can be calculated using fixed-beam theory. The bearing moments are a function of the load distribution on the beam and the distance between the support bearings. This limits the allowable span between the head and tailstocks. Additional disadvantages of this rigid tool mounting approach include the need for precise alignment between the headstock and tailstock (including precision machine bases) and precision tooling. These combine to increase cost and limit the bearing capacity of the positioner system.

An alternative to rigid tool mounting is a simply supported, or flexible, tool mounting system. The primary advantages of this approach include more controlled and predictable moment loads on the support bearings with no limitation on the span between the head and tailstocks. Additional advantages include less stringent alignment and tooling precision requirements - Fig. 4.

The challenge of this approach is to allow the beam to flex while still controlling the rotational motion. Generally this has been achieved with custom designs that might include "dog and pins," clevis pins, or other types of flexible rotational limiting devices. However, at least one robot manufacturer1 now provides a cost-effective standard design that meets the challenges of simply supported beam motion. This system allows up to two degrees of total misalignment and eliminates the need for precise alignment, costly machined bases, and high-precision tooling.

Manipulating the Load
The headstock output torque is required to manipulate and hold the load in orientations required by the welding application. The available headstock output torque (Tr) can be calculated by multiplying the motor torque (Tm) by the total gear reduction ratio (R) - Fig. 5.

Fig. 5 - Method to calculate headstock output torque.
Most positioner manufacturers rate their headstocks by holding torque, or the torque required to hold the load (W) in a horizontal orientation, at a prescribed distance (r) from the turning axis. However, no standard rating system exists, so when evaluating different positioners, it is very important to understand how the positioner is rated to ensure it is capable of moving and controlling the intended load.

As an example, one manufacturer might rate its 450-kg headstock at 150-mm turning radius and require 50% rated motor torque under those conditions. Another manufacturer might call the same motor-reducer combination a 500-kg headstock rated at 216-mm turning radius and require 80% rated motor torque. While the headstocks might appear to have different ratings, they would be expected to have identical performance characteristics.

The motor-reducer torque is also required to accelerate and decelerate the application load about the rotational axis. This torque is equal to the rotation mass moment of inertia (inertia, J) of the load multiplied by the angular acceleration and should not exceed the peak torque rating of the headstock.

Inertia is a property of the load and describes the distribution of mass about the rotational axis. While crude estimates for the inertia can be calculated based upon the load material and geometry, today's 3-D modeling packages used to design tooling can provide very accurate estimates and should be used whenever possible.

The total inertia is also a significant factor in the control stability of the servo-driven headstock. This is generally evaluated as the ratio of the reflected inertia (Jr) divided by the motor inertia (Jm). The reflected inertia is the sum of the reducer inertia (Ji) and the load inertia (Jl) divided by ratio squared, Jr = Ji + Jl/R2 - Fig. 6. Most headstocks have a maximum recommended reflected ratio (Jr/Jm) of 5 to 10, depending upon the total mechanical stiffness of the headstock drive system. Applications with reflected ratios approaching or exceeding the recommended limit may demonstrate poor control stability, undesirable vibrations, or motor overheating.

Fig. 6 - Calculation for determining total inertia.
The root mean square (RMS) duty cycle torque is an average of the total torque requirements (holding and motion) for a given application duty cycle. This value should be calculated and compared with the performance specifications of the proposed headstock. Root mean square torque requirements exceeding the headstock ratings may cause servomotor overheating and reduced headstock life. High RMS values due to excessive load imbalance can sometimes be corrected with the addition of counterbalances, provided this does not result in excessive load inertia.

Robotic welding positioners come in many styles as required by different welding applications. Properly designed and implemented positioners have a number of common characteristics. They must be able to support and present the part in an orientation for optimal welding. They must support all required tooling and process control functions including I/O and weld current isolation. Finally, the positioner must have features and capabilities that can be used in the robotic cell control and safety architecture. The user who considers these items when designing a system for a specific welding application will be well on the way to a successful project implementation.

ZANE MICHAEL is Director, Standard Engineering and Development, and GEORGE SUTTON is Associate Chief Engineer, Motoman Inc., West Carrollton, Ohio.

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