A. Field of the Invention
This invention relates to electrical equipment used with farm animals and, more particularly, to methods for measuring the electric current flowing through cows.
B. History of the Prior Art
1. Stray Voltages in Farms
a. In General PA1 b. Causes of Stray Voltages in Farms PA1 c. Cattle Responses to Stray Current PA1 d. Prior Stray Voltage Detection Methods PA1 e. Other Cow Measurement Schemes
Researchers agree that extraneous electrical voltages and their resulting currents found on dairy farms have physiological and behavioral effects on cows. Although animals are not necessarily effected by voltages per se, animals are affected by the currents that are created from the respective voltages. When compared with humans, cows have been found to be more sensitive to electric currents. The concept of electric current flowing through a cow can be illustrated using Ohm's Law: ##EQU1## Assuming the cow has a body resistance R.sub.COW, the resulting current I.sub.COW is related to the given voltage V.sub.STRAY, as shown in FIG. 1.
In addition to R.sub.COW, the amount of actual current that flows through the cow is also related to the electrical resistance of the contact points that have stray voltage. As shown in FIG. 1, the current I.sub.COW is determined by dividing the stray voltage value, V.sub.STRAY, with the total series resistance, R.sub.TOTAL : ##EQU2## where: EQU R.sub.TOTAL =R.sub.CONTACT1 +R.sub.COW +R.sub.CONTACT2 Equation ( 3)
R.sub.CONTACT1 and R.sub.CONTACT2 represent series contact resistances of the entry and exit points of the electric current I.sub.COW.
Unpredictable environmental conditions on dairy farms may cause the series resistance contact points to be good conductors at some times, but poor conductors at other times. For example, if a cow touches a contact point that is covered with dry dirt, the electrical connection would be not as good as a contact point that is wet or damp from manure or urine. The wet or damp contact point would have less resistance than the dry contact point. As a result of the unpredictable R.sub.CONTACT1 and R.sub.CONTACT2 values, there is a high degree of variance in R.sub.TOTAL and a corresponding variance in I.sub.COW, even though the voltage reading, V.sub.STRAY, may be constant.
Modern dairy farms typically feature an assortment of electrical equipment which may have unknown faults. These faults create stray voltages on dairy farms which may effect cows. Such electrical faults may arise as a result of poor electrical connections, corrosion of switches, frayed insulation, faulty equipment, or heavily loaded power lines. Since many of these faults do not result in equipment failure, the existence of these electrical faults is not always readily apparent. Accordingly, dairy farmers are often unaware of conditions that may cause cows to suffer the effects of stray currents.
The effects of electric currents in cows vary depending on the sensitivity of the individual cow. Studies have linked the effects of stray currents in cows to lower yields in milk production and increased milking times. Other observed responses include: 1) increased incidence of mastitis, 2) elevated somatic cell counts, 3) incomplete milk letdown, 4) extreme nervousness while in the milking parlor, 5) reluctance to enter the milking parlor, 6) rapid exit from the parlor, 7) reluctance to use water bowls or metallic feeds, and 8) altered consumatory behavior ("lapping" of water from the watering device). Since these results are generally undesirable, dairy farmers are interested in methods to identify the times that cows suffer from these effects to prevent the negative consequences.
In order to detect the existence of stray currents in cows, researchers have used generally two methods: 1) the measurement of voltage between point-to-point, and 2) the measurement of voltage from point-to-reference ground.
Referring to FIG. 2, the measurement of voltage point-to-point involves measuring the voltage between two points which the cow may contact at the same time. Commonly used points include metallic structures, such as metal stall pipes, and the floor. Thus, similar to FIG. 1, a resulting current can be determined by dividing the measured voltage with the series resistances of the body of the cow and the respective contact points that touch the cow.
Referring to FIG. 3, the measurement of voltage from point-to-reference ground involves measuring the voltage between various metallic equipment the cow may touch, such as a stanchion, metallic feeder, or waterer, and a reference ground. When using the point-to-reference ground method, a ground rod must be driven into the earth to a depth of at least four feet at a distance of at least twenty-five feet from any electrical system grounding electrode. The point-to-reference ground method generally returns higher voltage readings than the point-to-point method and is more useful in identifying specific sources of stray voltages. Similar to the previously described point-to-point method, a resulting current can be determined by dividing the measured voltage with the series resistances of the body of the cow and the corresponding external contact resistance points.
Each of the above referenced detection methods has drawbacks. First, neither of the detection methods continuously monitor for the existence of stray current flowing through a cow at any given time. Each method relates only to the times a cow touches the preconfigured contact points. In addition, the measured voltage readings do not accurately convey the actual current flowing through a cow because of the high degree of variance in contact point resistances.
Other shortcomings to consider are the requirements of having to drive a ground rod into the earth for point-to-reference measurements or the expense of having to install cables in the milking facilities to accommodate the measurement techniques. These requirements may be impractical for some dairy farmers to implement. In addition, since cows must physically touch the contact points, the techniques may be more suited for simply identifying specific sources of stray current instead of detecting the actual presence of stray current flowing through a cow.
While conducting studies, researchers have employed a number methods to measure resistance values between points on a cow. These techniques have included having the cow stand on top of a metallic plate, and/or physically attaching electrodes to the body of the cow, as shown in FIG. 4. In some instances, efforts were made to reduce the effects of the contact resistances in measurements by attaching the electrodes to shaven areas of the cow with conducting paste. The metallic plates and electrodes are correspondingly coupled to the current measuring equipment with cables to complete a "cow circuit." To determine a resistance value, researchers applied known voltages to the cows and measured the corresponding current flow. Using Ohms Law, resistance values could be determined.
Although this technique may be useful to measure the electrical resistance in cows, its use is impractical for measuring the current in cows that results from stray voltages found on ordinary dairy farms. Furthermore, such measurements are still effected by variable contact resistances, even though the effects of the resistances are reduced with the use of metallic plates and the conducting paste. Moreover, the technique is limited in use to the times that a cow is physically interfaced with the measuring apparatus. Thus, in order to monitor continuously the current flowing through a cow, prior art methods confine the cows to limited areas since the cow must remain physically connected to stationary measuring equipment. Depending on the technique used, the cow cannot walk off of the metallic plate or the cow cannot move a distance greater than the length of the test cable from the current measuring hardware. Accordingly, it is impractical for dairy farmers to monitor the current flowing through cows continuously and continue daily milking activity using prior art techniques. In order to be effective, such techniques would impose severe limitations on dairy farm environments.
2. Four Terminal Resistor Measurements
A technique researchers use when making measurements of low resistances is to use a four-terminal resistor. This technique is used because one of the difficulties researchers encounter when making such precision measurements is caused by the contacts between the resistor and its connecting wires. This configuration has been used for devices such as ammeter shunts and other low resistance precision standards.
FIG. 5 illustrates a four-terminal resistor. When making precision measurements, contact resistances can have a high degree of variance. Conditions which may effect the precision measurements include the mechanical pressure of the contact as well as the physical condition of the contact surfaces. The outer pair of terminals provide a current connection and the inner pair of terminals provide a voltage connection. The resistance between the inner pair of terminals does not include the contact resistances of the outer pair of terminals and is therefore independent of varying resistances that may occur as a result of unpredictable physical conditions at the outer terminals.
An additional benefit of the four-terminal resistor configuration is that the contact resistances of measurement apparatus at the inner terminals is of no importance. Assuming a measurement apparatus with a sufficiently high input impedance, the voltage drop across the two inner terminals can be measured without drawing current from them. Since there is no current drawn, there can be no voltage drop across the contact resistances at the inner terminals. Hence, the contact resistances at the inner voltage terminals are immaterial. cl II. SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an unintrusive method for continuously, accurately, and reliably measuring the electric current flowing in a cow. Such a method could be used by dairy farmers to identify, locate, and detect stray voltage conditions on the farm that could result in cow discomfort and corresponding losses in milk production. It is an additional object of the present invention to provide a method for recording the measured electric current values to make the measured readings available for subsequent analysis and study.
The method is realized by utilizing the four-terminal resistor precision measurement technique with a cow. This technique provides for precision measurements of electric current flowing through a cow and is independent of the unpredictable environmental conditions found on farms that may effect contact resistances between the cow and stray voltage sources.
The present invention is realized by attaching to a cow two sensing electrodes that measure the voltage drop between two locations on the cow. These two locations correspond to the inner voltage terminals of a four-terminal resistor. It is assumed that the stray electric current enters and exits the cow at two locations that correspond with the outer current terminals of a four-terminal resistor.
The two sensing electrodes are connected to a voltage sensing device with a high input impedance. The voltage sensing device is also attached to the cow. Once the cow's body resistance between the two locations is determined, the current flowing through the cow between the two locations can be determined at any time by dividing the measured voltage between the two locations by the body resistance of the cow. In the currently preferred embodiment, the voltage sensing device also includes a transmitting device which allows data to be transmitted to a remote receiver. The voltage sensing device may also include a recording device which provides a means for storing data for subsequent analysis and study. Thus, the described method provides for continuous, accurate, and reliable monitoring of electric current flow through a cow without unnecessarily interfering with ordinary dairy farm activity.