Engineering meets MacGyver: Unconventional vibration testing for Sioux City Helistop

Transcription

1 Engineering meets MacGyver: Unconventional vibration testing for Sioux City Helistop By Jon Mooney, PE SAV engineers constantly face adversaries in the field which test our battle plans. Cold temperatures make batteries fail. Computer operating systems reboot at inconvenient times. People walk obliviously through and in front of test areas. To be successful, we must have several contingency plans in mind. The real world requires us to think on our feet, take stock of what we have at hand, and sometimes MacGyver a workable solution. This is the story of one of my MacGyvered projects. Mercy Medical Center in Sioux City, Iowa, is the designated Level II trauma center for western Iowa, eastern Nebraska, and southeastern South Dakota. The Heart Center tower (Figure 1, left side) is home to the region's leading cardiac care program. It has received numerous national honors. Mercy Air Care flies in patients from 33 sparsely populated rural counties in the tristate area. Mercy s landing pad is one block to the north and patients must then be wheeled from there to the cath labs located on the top floor of the Heart Center. Hospital administrators were interested in moving the helipad to the roof of the Heart Center and adding two more cath labs to the operating floor. But would the cath labs still function with helicopter operations directly above them?

2 Figure 1: Mercy Medical Center Sioux City Planning As illustrated in Figure 2, the problem can be broken down into three questions: 1. What is the forcing function produced by the helicopter operations? Certainly someone must have already done this work. 2. How much of that forcing function gets transferred to the labs? We can discover this by placing accelerometers throughout the upper floor while we shake the roof. 3. How high can the vibration level be before the doctors would stop the procedure? Cath procedures rely on X-ray imaging to guide the catheter during an operation. Based on image resolution, we anticipated 8000 uin/sec as the operational limit. Figure 2: Questions to be answered by the study

3 What is the forcing function? A quick web search brought me to an article written by Jeff Zapfe. ACENTECH had characterized the dynamic loading of a helicopter caused by its downwash as 2% of the helicopter s weight and a frequency equal to the main rotor spin of 200 rpm to 500 rpm. So let s do the math. With a typical medevac helicopter weighing 8,000 pounds, 2% is 160 pounds at 3 Hz to 8 Hz. Before committing to a test using a helicopter, we placed 16 accelerometers throughout the fifth floor. The accelerometers recorded the vibration of the floor for about three hours. Ambient floor vibration was measured to be relatively broadband with a level of 6000 microinches/second already above the 4000 uin/sec goal for an OR. What is the transfer function? Since the calculated helicopter loading is not much higher than that used to calculate floor vibration from human activity, while the accelerometers were recording, about an hour into the test my associates Rob and Kelly and I located the top of four columns on the roof where the helipad would mount and, as the head of the structural department edited my test report, imposed a dynamic loading at these locations as shown in Figure 3. Figure 3: Imposing a convenient dynamic load on the roof Ambient vibration of the fifth floor was measured as 6000 microinches/second. Based on image resolution, we estimated an equipment limit of 8000 uin/sec. Vibration of the fifth floor during loading of 80 pounds at 3 Hz (running) was measured to be 7000 microinches/second. Therefore, we extrapolated the vibration level of the fifth floor during helicopter operations on the roof would be 14,000 microinches/second. This was not a good-enough test to design to, but good enough to recommend a repeat of the measurements with an actual helicopter. Measurements with the helicopter matched the predictions as shown in Figure 4. For this test we not only measured vibration in the cath labs, but also sound and vibration levels throughout the hospital.

4 Figure 4: Vibration measured in cath lab during helicopter hover What is the sensitivity? The final task was to shake the floor of the operating rooms during a simulated catheterization to determine how much vibration it would take before the surgeon saw image blur. I had previously MacGyvered a low-cost floor shaker that would do the job, with a mini-lathe serving as an adjustable, low-speed, high-torque motor. Add a 6-inch diameter x 7-inch long naval brass bar for the inertial driver. A rolling camper cooler keeps everything contained. And you have one low-cost floor shaker as shown in Figure 5.

5 Figure 5: A low-cost floor shaker A couple days before the test, it hit me that the motor and power cord of my old floor shaker might produce enough EMI to make the X-ray image fuzzy, even without shaking the floor to do it. A quick look at the specs, and a few calculations gave me an estimate of the minimum EMI which would be a problem. Without quick access to a meter, I wound a 500-turn search coil and hooked it up to an oscilloscope to discover just enough noise to make any test results questionable as to whether vibration, electromagnetic radiation, or a combination of both from the floor shaker would be the cause. The mini-lathe shaker couldn t be used, and I thought there would be little chance that shielding would work. So one day before the scheduled test, I cancelled and promised to test it a week later. If I couldn t rely on electrical power, maybe there was something out there that used a pneumatic shaker. An internet search on that phrase resulted in a pneumatic paint shaker selling at a hardware store. Now we were in business. Replace the paint can with a few free weights. Add some air power. That should do it. Except, cath labs are trying to maintain ISO Class 7 air quality, and compressed air contains lubricating oil which would contaminate the room. So compressed air was out. No problem; I could use medical air. Code, however, doesn t allow the use of the cath lab s medical air, so we d need to use a portable tank. One medium tank has the capacity for several minutes of power. There would be no oil to contaminate the room, but the pneumatic cylinder would probably seize up in a few seconds. This one had me stumped. Nothing seemed to want to play together. At the dinner table my family shares the events of the day and I explained my current dilemma. Most of the time I just assume everyone is just letting me ramble on, but this time, my youngest son offered a solution: Why not just hook up an exhaust hose to remove the spent compressed air? And, of course, the answer is there s no place to hook up that hose. But take the cylinder out of the paint shaker. It bolts right on to a 10-pound weight. Four 5-pound

6 weights on a short piece of iron pipe with end caps serve as a 20-pound drive weight. Another 10-pound weight strapped to the base to balance the reaction and the whole assembly fits neatly inside a fivegallon bucket (Figure 6). Figure 6: A low-cost pneumatic floor shaker A quarter turn valve on the side serves as the on/off switch. An in-line oiler keeps the piston from seizing up. Compressed air powers the reciprocating weight assembly. Exhaust air escapes from the vent holes but is trapped inside the bucket. Exhaust passes through an in-line dessicator and out the exhaust hose on top. As long as the bucket remains tightly sealed, no compressed air is released into the OR. Figure 7 is a zoomed-in image of X-ray video of a practice target taken at a vibration level where the head of the department just noticed blurring of the edges. As illustrated in Figure 8, we re way above the recommended envelope, but the image is fairly steady.

7 Figure 7: X-ray image showing nominal blurring during floor vibration Figure 8: Cath Lab floor vibration level causing nominal X-ray image blur

8 Conclusions From our testing, we concluded 1. Ambient cath lab floor vibration level is 6000 uin/sec. 2. Cath lab floor vibration during helicopter operations would be 14,000 uin/sec. 3. The cath lab image sensitivity is 100,000 uin/sec. We also found that 20 engineers jumping on a roof equals 10 helicopters hovering over the roof equals a 50-pound bucket jumping on the cath lab floor. Of course, the hospital has other things to consider, such as FGI Guidelines, which recommends 4000 microinches/second for cath labs. During helicopter operations over the Heart Center roof: 1. Noise levels within the patient rooms in the adjacent bed tower rose to 75 db(a). FGI Guidelines recommends db(a). 2. Noise levels in 4 th floor offices in the Heart Center rose to 85 db(a). FGI Guidelines recommends db(a). 3. Noise levels within the cath labs rose to 64 db(a). FGI Guidelines recommends db(a). Jon Mooney, PE, is a senior engineer and acoustics specialist at KJWW Engineering Consultants. him at mooneyjw@kjww.com or visit his website.