Automated valve fitting

TMR004In 2002, TM Robotics (Europe) Ltd and one of its longest serving integrators completed a project in partnership with a major UK manufacturer.  The project saw three SCARA robots fitted as part of an automated system to increase output in pneumatic valve production.

It isn’t always easy putting one’s trust in a robot.  There are a lot of statistics to put into the equation: the cost of the robots, the cost of installation and maintenance and the training required for key staff to manage the operation.  All of this has to be weighed up against the cost of a manual alternative.  One must also bear in mind potential downtime if the automated system is replacing an existing manual one.

After weighing up the options, one significant UK manufacturer of fittings isolated its ‘push in fittings’ assembly as one process where the installation of an automated system could increase productivity and profitability.

However, conventional automation presented an unattractive proposition because of relatively small batch sizes (sometimes as few as 1000) and frequent changeover periods.  As a result, the decision was taken to automate the assembly process using Toshiba Machine SR-654-HSP SCARA robots.

The key factors in the success of the automation process were accuracy and flexibility.  Accuracy was provided by the ±0.02mm repeatability of the Toshiba Machine SCARAs and the degree of flexibility allowed the system to cope with 240 different product variants, all consisting of at least five component pieces.

The series benefits from parallel processing to offer smooth, fast motion combined with an extremely sophisticated set of control instructions to suit all applications. The SCARAs have traversing rates of 300mm in 0.63 seconds, carrying a 2kg payload. The robot’s reinforced arm can handle up to 20kg at half-speed, or 10kg at full speed.

One of the key factors in the installation process was in ensuring a quick changeover period between different product variants to minimise downtime.  This was where a manual process can be advantageous, the worker simply finishing a batch of one product type and collecting the components for another, with no long changeover period required.  In order to make the installation cost effective, the automated alternative had to be designed with optimum efficiency in mind.

The required changeover efficiency was achieved through the use of the Toshiba Machine SCARAs in conjunction with a PC containing all of the component information.  The PC linked to a PLC, which drove the inputs and outputs and automatically prompted the operator to choose the correct components.  The only information that had to be submitted manually was the fitting number.   Furthermore, in order to further decrease downtime, every non-automated tool required by the member of staff controlling the process was accurately positioned close to the place in which it was actually used.  The result of this was that the average ‘chip to chip’ changeover time was just 15 minutes, giving an impressive operating efficiency of 90%.

The process being automated was quite complex even before considering the product variations and changeover periods.  The manual process took twelve seconds, under optimal conditions, and consisted of a swivel pushing fitting being connected to a pre-lubricated nylon tube (either an air-ring or a gripper).  These were then inserted manually into a swivel end, which fitted snugly over a spigot.  Finally, a nut is fitted over the air ring and the twelve seconds are up.

To automate this process, three SCARA robots, each working in tandem with a vision system, were required.  When the process begins, the body part of the fitting is fed onto one of two conveyors where it is viewed by the first vision system. This conducts an automatic check on the size and type of the fitting body and generates co-ordinate information, which is then passed to SCARA 1. SCARA 1 inserts the pre-lubricated air-ring and passes the two parts across the second vision camera.  At this point, the parts are placed on a collet, which is facing upwards at the next staging point.  SCARA 1 then places the part on the transfer table ready for SCARA 2 to take over the process.

SCARA 2 takes a delicate swivel nut and places it into a ‘nest’ before positioning the air ring on top, ready for swaging.  A pneumatic ram cylinder, positioned above the nest, then drops down onto the two parts and swages them into the correct shape.  The swaging press then serves as a natural transfer point for SCARA 3 to take over the process.

SCARA 3 then performs a pressure decay test on the entire part and checks for the presence of the air ring.  It then runs the part across a fibre optic sensor, which verifies the presence of the collit.  The robot places the parts in one of four output trays at the end of the robot cell.  Furthermore, it automatically counts how many fittings it has already placed in the tray, so that when the first tray is full it moves on to the second, then the third, etc.  This further reduces the amount of man-hours involved by reducing the amount of times someone has to visit the cell to empty the trays.

The total cycle time for the three robots to assemble the fitting is just 7.8 seconds, 4.2 seconds faster then the manual method.  Furthermore, the automation has the obvious advantage of constant running.  Put bluntly, it doesn’t slow down when its tired, it doesn’t take coffee breaks and never takes long lunches. This improved cycle time and constant output delivers a very favourable payback.