An ASABE Meeting Presentation Paper Number: 1009603
Dynamic Performance Evaluation of a Low Cost Load Sensor Abhinaya Subedi University of Idaho, Department of Biological and Agricultural Engineering, email:
[email protected]
Dr. Dev Shrestha University of Idaho, Department of Biological and Agricultural Engineering, email:
[email protected]
Written for presentation at the 2010 ASABE Annual International Meeting Sponsored by ASABE David L. Lawrence Convention Center Pittsburgh, Pennsylvania June 20 – June 23, 2010 Abstract. A low cost load sensor was tested for its dynamic performance for vibrating field combine loading condition for an automated soil sampler that is being designed at the University of Idaho. The sampler requires a load sensor to continuously measure the weight of the soil on board, mounted on a potato combine harvester. Two types of sensors, a low-cost load sensor (Flexiforce) and a standard load sensor were tested and their performance was evaluated. Flexiforce sensor was found to be more susceptible to external electrical noise than its expensive counterpart. However, the performance of the flexiforce was comparable to the high end load sensor after filtering the noise. Keywords. Soil sampler, Flexiforce sensor, Standard load sensor. (The ASABE disclaimer is on a footer on this page, and will show in Print Preview or Page Layout view.)
The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessarily reflect the official position of the American Society of Agricultural and Biological Engineers (ASABE), and its printing and distribution does not constitute an endorsement of views which may be expressed. Technical presentations are not subject to the formal peer review process by ASABE editorial committees; therefore, they are not to be presented as refereed publications. Citation of this work should state that it is from an ASABE meeting paper. EXAMPLE: Author's Last Name, Initials. 2010. Title of Presentation. ASABE Paper No. 10----. St. Joseph, Mich.: ASABE. For information about securing permission to reprint or reproduce a technical presentation, please contact ASABE at
[email protected] or 269-429-0300 (2950 Niles Road, St. Joseph, MI 49085-9659 USA).
Introduction In 2006, officials of USDA’s Plant Protection and Quarantine (PPQ) and the Idaho State Department of Agriculture (ISDA) announced the detection of pale potato cyst nematodes (PCN), Globodera pallida, a major pest of potato crops (USDA, PPQ 2009). The potato industry then requested PPQ to devise a national survey for PCN on certified seed potato and commercial potato acreage. To date, the procedure to know the exact data of nematodes present in soil has not been so effective. The problem demanded the design of an automated soil sampler machine to know the exact field scenario. By knowing the exact field conditions, then field can be treated with proper amount of pesticides. The quantity of pesticides will certainly differ from the one field to the other depending on the density of the nematodes. An automated soil sampler is being designed at University of Idaho for efficient soil sample collection. The soil sampler can be easily mounted on the potato combine harvesters. Flexiforce sensor is one of the vital components of the soil sampler since it is used to monitor the weight of the soil sample. Standard soil sampling paper bags containing soil sample continuously move through the sampler, so the sensor needs to take the data continuously. Hence the dynamic loading condition of the flexiforce sensor needs to be assessed. Flexiforce sensor was chosen in design because it occupies very small space and its price is very less compared to the other sensors. The assessment of the sensor is necessary to examine its effectiveness. If the sensor proves to be effective, then the sensor can be considered for design whereas if it proves ineffective, then the flexiforce sensor should be discarded and another sensor should replace this. If the sensor seems effective, then it can save a huge amount of money. A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. Sensors have been traditionally used for industrial process control, measurement, and automation, often involving temperature, pressure, flow, and level measurement. In this paper, performance evaluation of one of the low-cost sensors, flexiforce sensor and expensive sensor, futek sensor was carried out. As precision and economy are the two fundamental aspects in the engineering design, if the low-cost reading is within an acceptable limit, it can replace the expensive counterparts. Force sensing resistors are thin isometric force sensors whose resistance decreases with the force applied in a non-linear way.
Figure 1. Flexiforce sensor The FlexiForce A201 force sensor is an ultra-thin, flexible printed circuit (Figure 1). The standard A201 force sensor is constructed of two layers of substrate (polyester) film. On each layer, a conductive material (silver) is applied, followed by a layer of pressure-sensitive ink. Adhesive is then used to laminate the two layers of substrate together to form the force sensor. The active sensing area is defined by the silver circle on top of the pressure-sensitive ink. Silver extends from the sensing area to the connectors at the other end of the sensor, forming the
2
conductive leads. A201 sensors are terminated with male square pins, allowing them to be easily incorporated into a circuit. The two outer pins of the connector are active and the center pin is inactive. Fabrizio Vecchi and colleagues have experimentally evaluated the Interlink FSR sensor and Flexiforce sensor through a series of measurements. They concluded that the Flexiforce sensors present better response in terms of linearity, repeatability, time drift, and dynamic accuracy, while the Interlink FSRs are more robust. A. Hollinger and M. Wanderley also evaluated three commercial force-sensing resistors including Flexiforce, Interlink FSR and LuSense PS3. They found that Flexiforce showed the highest precision if compared to both the FSR and the PS3, but with higher noise than the other two.
Price comparison of flexiforce and futek sensors Flexiforce sensor is a low-cost sensor. The current market value price of standard A201 flexiforce sensor is $ 65 per pack. Each pack contains 4 sensors, which means one flexiforce sensor costs only $ 16.25. Likewise, one futek sensor of 25 lb capacity, which is used in this experiment costs $ 475. A futek sensor thus costs 29 times more than a flexiforce sensor.
Objectives of the study The main objectives of the study are as follows: • To assess the performance of the flexiforce sensor under dynamic loading conditions •
To compare the flexiforce sensor with high tech futek load sensors
Instrumentation
Motor Variable frequency drive
Eccentric Rotor
Spring
Flexiforce sensor
Figure 2. Experimental set-up of the sensor In Figure 2, Dayton 3-phase electric motor was used for the instrumentation purpose. The frequency of the motor was controlled by programmable variable frequency drive. The variable frequency drive used in the experiment was manufactured by HITACHI. The flexiforce sensor A201 of capacity 0 – 25 pound was used for the experiment.
3
Procedure The rotor is connected to the shaft of the motor; hence the rotor rotates at the same speed as shaft. A hole was grooved at a distance x from the center of the rotor to adjust the shaft. The rotor compresses a spring and the spring resides on the sensing area of the flexiforce sensor. The reading is then taken from the flexiforce sensor. As the rotor is eccentric, the compression of the spring varies for a complete rotation and follows the same trend. The set-up was so adjusted that the rotor just touches the spring at an instant and the compression becomes maximum when the opposite end hits the spring. Then again, the compression reduces and becomes zero at the starting point.
A r
ф
θ O
a
O’
C
B x
r O – center of circle O’ – center of rotation r - radius a - eccentricity
Figure 3: Illustration of the working principle of rotor
In Figure 3, O is the center of the circle and O’ is the center of the rotation or center of the shaft. The distance from O to O’ is a, which is the eccentricity. We can construct a small circle with radius a in order to determine the displacement x. When the rotor rotates, OC line changes from r to r+2a. To determine the displacement x, we have, (1) (2)
4
(3)
We can examine that when ф = 0, θ = 0, then And when ф = 180 degree, θ = 180 degree, then Thus x varies from 0 to 2a. As the equation involves with the sine and cosine values, the graph plotted from different values of θ will be almost sinusoidal. In our experimental design, a was chosen as 1/8 inch and the radius of the rotor, r was chosen as 2 inch. Hence for a= 1/8, r=2, by choosing the values of θ from 0 to 360 degrees, the following plot was obtained:
Figure 4. Response for one complete cycle. The expected curve was compared with the pure sinusoidal curve and both the curve seemed quite similar. Hence for one complete cycle of the rotor, the curve should be like in Figure 4. It can be observed from Figure 4 that the load varies from 0 to 4.85 kilograms.
Spring selection The spring was chosen with the spring constant (k) as 42.8 lbs/ inch. Since the total displacement is 0.25 inch, the maximum load the spring can bear is 42.8 * 0.25 = 10.7 lbs, or equivalently 4.85 kg. The spring was designed for maximum 14.3 lbs load. Hence the spring should work for the design load.
5
Methodology Flexiforce sensor was evaluated on static and dynamic loading conditions. For static loading condition, five flexiforce sensors were chosen and each sensor was loaded from 0 to 3000 grams. Each sensor was replicated for 4 times. For dynamic loading condition, same flexiforce sensors were chosen and the sensors were loaded with varying load continuously. An experimental set-up was designed to obtain the dynamic load including motor, rotor, spring and other accessories. The frequency of the motor was varied from 1.5 Hz to 5 Hz. For each frequency, 512 samples per second were taken for 2 seconds. To record the data of flexiforce sensor, a circuitry was designed. A data acquisition device PMD1208LS was used to acquire the data from the sensor. With the help of labview program, the data was stored at desired sampling rate and period. Besides flexiforce sensor, futek sensors were also used for static and dynamic loading conditions respectively. The performance of the flexiforce sensors was compared with these sensors.
Calibration of the sensor Calibration of the sensor is necessary to convert the acquired data into the units of weight. Flexiforce sensor gives the reading in ohms whereas futek sensor provides readings in volt. Hence for flexiforce sensor, resistance versus load calibration curve is needed; similarly for futek sensor, voltage versus calibration curve is required. Figure 5 shows the load-conductance curve obtained from the static loading condition of the sensors. Five different curves were obtained from five different sensors. The ANOVA table suggested that there is significant variation from sensor to sensor, so each sensor needs to be calibrated separately. The load-resistance curve is exponential in nature. However, loadconductance curve is linear, which is easier to use for calibration purpose.
Figure 5. Calibration curve for the 5 Flexiforce sensors
6
Circuit used for the flexiforce sensor A very simple circuit was designed to obtain the sensor data. The flexiforce sensor works as a variable resistor. It can be observed from the calibration chart that under no-loading condition, resistance value is more than 6 MΩ, whereas when the load increases, the resistance decreases exponentially.
5V R1 (flexiforce)
R
2
5
3
8
4
1
7
6
C1 (.1 µF) MAX 7400
Data Acquisition Device
Computer
C3 C2 (.1 µF)
Figure 6.Circuitry for flexiforce sensor The output voltage which is read by the data acquisition device is noted as V. Similarly R1 is symbolized for the flexiforce sensor and R is for the other resistor. The value of R should be so chosen that the value of V should be in between 0 and 5V for any given load. The equation for V becomes: (4)
Resistance R needs to be so chosen that the output voltage will be in between 0 and 5v for any given load for all the sensors. It can be observed from the calibration curves that sensor 2 and sensor 5 are the two extremes. Hence if the chosen R is fit for these two sensors, it should be fit for all the other sensors. So if R is chosen as 50K, then for sensor 2, for 0 kg load, R1 = 6 MΩ and corresponding V= 0.041 V. Similarly for maximum load (4.85 kg), R1 = 226.75 KΩ and corresponding V = 0.9 V. Hence the condition is satisfied for sensor 2. For sensor 5, for 0 kg load, R1 = 6 MΩ and corresponding V = 0.041 V. Similarly for maximum load (4.85 kg), R1 = 82 KΩ and corresponding V = 1.89 V. Hence the value of R as 50 K should work for all the sensors. Then from equation (5), R1 was calculated as: (5)
7
Dynamic loading condition Flexiforce sensor was tested to evaluate its dynamic performance. Many experimental trials were conducted to get the expected response. Huge noise was detected when the spring directly touched the rotor(figure 9). This means that the flexiforce sensor is highly sensitive to the external noise. Then another experiment was conducted to observe the response of the sensor after applying low pass analog filter in the same condition. The low pass filter was designed as in Figure 8. The filter cut down the noise from the higher frequencies. Still there was some noise from the metallic collision (Figure 10). Then finally another experiment was carried out by placing a plastic sheet in between the rotor and the spring along with the low pass filter. As a result, a very fine sinusoidal curve was obtained (figure 11). The low pass filter was designed by setting the corner frequency as 38 Hz. Then Clock frequency = Corner frequency*100 = 38 * 100 Hz = 3.8 kHz Then C3 = (38 * 103 /3.8) pF = 10000 pF = 10 nF Hence the value of C3 was chosen as 10 nF. After designing this low pass filter, the noise from the 60 Hz electrical signal was filtered out, which was considered to be the main source of noise. The following plot was obtained at 1.5 Hz frequency of the rotor.
Figure 7. Unfiltered response of the flexiforce with direct metallic contact. This is the data recorded when the spring directly touched the rotor and without the application of filter.
8
Figure 8. Filtered response of the flexiforce with direct metallic contact. This is the data recorded when the spring directly touched the rotor but with the application of low pass filter. The filter used was low-pass analog filter (MAX 7400).
Figure 9. Filtered response of the flexiforce preventing direct metallic contact. This fine sinusoid was obtained after the application of low pass filter and also placing a plastic sheet in between the rotor and spring, thus avoiding the direct metallic contact. The following graph was obtained from the experiment for the flexiforce sensor with the application of filter preventing direct metallic contact.
9
Figure 10. Response of flexiforce sensor Figure 12 is the plot obtained from 1.5 Hz frequency. The data were also taken for higher frequencies up to 5 Hz. Previously it was observed that the higher the frequency, the higher the values for all the sensors when there was direct metallic contact between the spring and the rotor. This might be because of the error obtained from the external noise. However, when the direct metallic contact was avoided, then no evidence was found to support the above fact. The ANOVA table shows that the frequency effect on the sensor reading is insignificant. Hence the sensor readings are same regardless of the frequency values.
Futek sensor Futek sensor was also experimented in different conditions. No significant difference was observed for futek sensor, whether the sensor readings were taken in direct metallic contact or preventing the direct metallic contact. This means that the futek sensor is unaffected by the external noise. 2 futek sensors were experimented by preventing the direct metallic contact between the spring and the sensor. Figure 13 shows the calibration of the futek 1 sensor; likewise Figure 14 shows the response of the futek 1 sensor for 1.5 Hz frequency.
10
Figure 11. Futek sensor calibration curve
Figure 12. Response of futek sensor All of the experimented sensors, whether they were flexiforce or futek, produced excellent sinusoidal curves. The peaks obtained from these sensors were compared with the expected curve in Table 1. The root mean square error value for these sensors was 0.067 which is quite considerable. Table 1. Ratio of peak observed to peak expected for all sensors Sensors Flexiforce Flexiforce Flexiforce Flexiforce Flexiforce Futek 1 2 3 4 5 1 Peak(0bs)/Peak(exp) 0.89 0.98 0.98 1.03 0.89 0.93
Futek 2 1.03
The slight error in peak reading was observed because of the following possible reasons:
11
•
Calibration curve might be slightly inaccurate.
•
Wear and tear of the rotor because of the friction.
Conclusion After many trial and errors with the flexiforce sensor, finally smooth sinusoidal curve was obtained in dynamic loading condition. The flexiforce sensor performed quite satisfactorily with the application of a low pass filter. Since the flexiforce is one of the low cost sensors, it will save huge amount of money. The following conclusions were drawn from this experiment: • Flexiforce sensor is more susceptible to the external noise than futek sensor. •
All flexiforce sensors need to be individually calibrated to get the precise reading.
•
Both flexiforce and futek sensors are acceptable for dynamic load measurement purposes, however, while using flexiforce sensor, a low-pass filter should be used to obtain the more precise data.
•
Flexiforce sensor can be used in the soil sampler with some noise precautions.
References Fraden, J. 2004. Handbook of Modern Sensors. Physics, Design and Applications. 3rd ed. Springler Verlag. Futek Advanced Sensor Technology Inc. 2010. Available at: http://www.futek.com/product.aspx?t=load&m=llb350. Accessed 18 May 2010. Hollinger, A., and M.M. Wanderley. 2006. Evaluation of Commercial Force-Sensing Resistors. NIME06. Tekscan Inc. 2009. Flexiforce Sensors User Manual. USDA. 2009. Pale Potato Cyst Nematode National Survey and Diagnostic Cyst Sample Forwarding Protocols. Vecchi, F., C. Freschi, S. Micera, A. Sabatini, P. Dario, and R. Sacchetti. 2000. Experimental Evaluation of Two Commercial Force Sensors for Applications in Biomechanics and Motor Control. In Proceedings of the 5th Annual Conference of the International Functional Electrical Stimulation Society, Aalborg, Danemark., pages 44-44. Webster, J.G. 1999. The Measurement, Instrumentation, and Sensors Handbook. CRC Press.
12
