The overall goal of this experiment is to investigate the biodynamic response and vibration transmissibility of the hand-arm system during the operation for hand tractor. This study provides a protocol for hand transmitted vibration measurement of the tractor-hand-arm system with consideration of changes in the grip force and vibration frequency. This method will be helpful for the understanding of the vibration transmission characteristics of the hand-arm system.
Begin, place the hand tractor in a test site with dry, firm and a level ground surface. Remove the elastomeric materials of the handles. After obtaining informed consent from all the subjects, measure each subject's body dimensions including standing height, mass, forearm lengths, upper arm length and hand length.
Wrap the accelerometer adapters tightly on the hand and arm of each subject. Each adapter was a fabricated using a nylon strap and a piece of galvanized iron sheet to provide a rigid and light attachment. The accelerometer adapters were wrapped at the locations of the back of the hand, distal end of the forearm, proximal end of the forearm, distal end of the upper arm, proximal end of the upper arm and the acromion.
Before initiating a measurement, gather all of the components of the measurement system, including accelerometers, data acquisition system, thin-film pressure sensing system, tachometer, digital goniometer, accelerometer adapters and other relevant components. To set up the acceleration measurement system, first, using the accelerometer cables, connect the accelerometers with the data acquisition card. using an Ethernet cable, connect the chassis with the computer.
Then, attach one tri-axial accelerometer on the left handle or the hand tractor, and attach the other one on the accelerometer adapter of the subject's left hand. Attach single-axis accelerometers one by one, on the accelerometer adapters of the subject's arm and shoulder. Adjust orientations of the tri-axial accelerometers to be consistent with the specified coordinate system.
Finally, using adhesive tape, secure the accelerometer cables on the skin surface of the subject's arm and the tractor's handlebar. To set up the grip force measurement system, first, using double-sided adhesive tape, attach two thin-film sensors on the left handle of the hand tractor. Then, place the screen system at a convenient height.
So that the subject could monitor and adjust the grip force to the specified level during the operation of the hand tractor. After that, instruct the subject to hold and raise the handle to a horizontal position. Measure the subject's hand and arm posture using a digital goniometer, including the shoulder horizontal abduction, the shoulder vertical abduction, the elbow extension, the wrist extension and the wrist deviation.
Ask the subject to maintain the posture until the end of the trial. Start the hand tractor in neutral, and keep running at a low engine speed until it is stabilized. Turn on the tachometer, thin-film pressure sensing device, laptop computer, and the acceleration data acquisition system.
Set the parameters of acceleration, acquisition mode and sampling rate for data collection. Click Run and wait about ten seconds until the system stabilized, then click Record to start recording the acceleration data. During the testing, ask the subject to monitor the tachometer and adjust the engine speed to 1, 500 revolutions per minute until it is stabilized.
Then, instruct the subject adjust the grip force carefully to 20 Newtons, and keep this grip force level for about 30 seconds. Next, adjust the grip force to 30 Newtons and keep about 30 seconds. And then, adjust the grip force to 40 Newtons and keep about 30 seconds.
After that, adjust the engine speed to 2, 500 revolutions per minute and repeat the above procedure of grip force adjustment. Last, adjust the engine speed to 3, 500 revolutions per minute and repeat the above procedures of grip force adjustment. At the end of the trial, instruct the subject to put down the handle and shut down the engine of the hand tractor.
Save the vibration acceleration data, remove and place the accelerometers on the next subject. Repeat the above steps until the end of the data collections of all the subjects. And export the acceleration time-series data for further analysis.
These plots display samples of the time domain and frequency domain accelerations on the handle for half a second time duration. The maximum acceleration of the X and Z directions occurred at the frequency of 58 Hertz. Most of the vibration energy was found to be centralized in the frequency range from 50 to 200 hertz.
The mean relationship between the grip force and root-mean square acceleration of the hand-arm system was obtained. It was observed that the increase in the grip force increased the vibration acceleration notably at frequencies between 20 and 100 Hertz. And the three resonance frequencies were found to increase almost linearly with the increase of the grip force.
Meanwhile, the relationship between the engine speed and root-mean-square acceleration was obtained. As shown in the plots, lower frequencies were transmitted relatively unattenuated. While, attenuation was quite marked for higher frequencies.
Thus, it is reasonable to conclude that, most of the vibrational energy was dissipated in the hand and the forearm. This figure presents the averaged transmissibility at different locations of the hand-arm system. It was found that the transmissibility of the hand-arm system decreased with the increase of the distance from the vibration source.
The highest transmissibility was observed at the back of the hand. The resonant frequency of the wrist and elbow transmissibility was around 20 hertz. It was also found that only vibrations of less than 25 Hertz were effectively transmitted to the forearm, upper arm and the shoulder.
This method was established based on hand transmitted vibration standards, and was developed as a standard protocol for the measurement of the hand transmitted vibration of the human hand-arm system during the operation of a hand tractor in a stationary condition. The total weight of the accelerometer and adapter should be as light as possible to reduce measurement errors. The test trial of each subject should be completed without interruption to reduce the effect of the operating posture.
The main potential applications of the proposed methodology are the estimation of human-tractor interaction phenomena, the ergonomics development of hand tractors, and the development of protective devices such as isolators and gloves.
