The John Deere assembly plant in Augusta, GA is a model of production efficiency. Automated guided vehicles (AGVs) move through the 3,500 steps required to make a Series 5000 tractor in an integrated and networked process that takes less than 4 hours.
The AGVs function as platforms that carry work-in-progress tractors through a 1,452 ft long assembly line with 106 workstations. Onboard computers route them through assembly steps, meshing with cross-trained workers using "smart tools."
Some workstations, for example, contain wrenches that count the number of bolts needed and provide feedback when the correct amount of torque is applied. The smart tools help the operators to do their jobs and monitor quality. AGVs do not move from specific assembly stations until the work is verified completed by the control systems and quality parameters have been achieved.
Air Conditioner Leak Detection
One of the most critical steps in the tractor assembly process is leak testing of the air conditioning system, and this is where the H2000 hydrogen probe from Sensistor Technologies has a critical role. A minuscule leak, as small as a bacteria cell, can cause the loss of 1 gram of refrigerant per year, potentially rendering an air conditioning system inoperable in a few years.
"Our customers depend on the reliability of our air conditioning systems," says Bill Tasch, assembly engineer at the Augusta plant. "They sit in the tractor''s glass cab for hours at a time, and it would not be easy to haul a tractor to a shop for repairs if the AC system had a slow leak. To maintain high customer satisfaction, we make sure we have a tight AC system before we charge it with refrigerant, and before it leaves the factory."
John Deere needed a "smart tool" to perform leak detection that could be integrated into the automated assembly process, being reliably and consistently performed within the cycle time allowed at each workstation along the AGVs'' path.
Traditional wet methods of air conditioner leak testing -- submersion and soap-bubble testing -- were not viable because they cannot be automated, and they are prone to operator error. In addition, it is virtually impossible to detect leaks as small as one gram of refrigerant per year using the wet methods.
On the other end of the spectrum, John Deere evaluated a leak-testing method using helium as a tracer gas, but this proved too expensive due to the price of the helium gas and the cost of the sophisticated helium mass spectrometry equipment. There were also concerns about the difficulty in maintaining the helium testing apparatus.
"Smart" Leak Detection
"We found the ideal ''smart tool'' for leak detection, the H2000 hydrogen probe from Sensistor Technologies," according to Tasch. "The instrument has been successfully used at a John Deere factory in Germany and it fits perfectly into our new automated assembly process with its mistake-proofing system. The test procedure is straightforward and the operator is guided through each step."
Once the cab is installed on a tractor chassis, the AGV brings the tractor to the leak-testing station. The AC system is automatically evacuated of air and then filled to the correct pressure with a gas mixture containing 5% hydrogen and 95% nitrogen by the TGC 28 Tracer Gas Controller from Sensistor Technologies.
The operator then positions the H2000 hydrogen probe at 7 joints. A touchscreen programmable logic controller (PLC) displays graphical images of each test location, and auditory signals lead an operator sequentially through a 10 second test of each joint.
An auditory alarm sounds if a leak is detected. The operator can usually repair the problem quickly by tightening a connection or replacing a seal. Since the hydrogen tracer gas dissipates readily, retests can be performed immediately -- a key advantage. Because hydrogen does not cling to surfaces like other tracer gases and refrigerants, false rejections on retests are eliminated.
After the H2000 probe verifies the tightness of the air conditioning system, the AGV moves the tractor to the next station, where the system is charged with refrigerant. "We test with hydrogen trace gas prior to filling the system with refrigerant, because it would be environmentally irresponsible to inject refrigerant into an untested system," explains Tasch. "It is also easier to perform repairs on systems that do not contain refrigerant. We simply perform the repair and then retest and send the tractor on its way to the next workstation."
The H2000 is a robust and maintenance-free microelectronic probe that is extremely sensitive and 100% selective to hydrogen. The simple calibration procedure is performed once each morning at the Augusta plant. The recommended 5% hydrogen mixture is significantly less expensive compared to any other trace gases. Additionally, hydrogen trace gas is completely safe, nonflammable, and environmentally friendly.