Patent Grant
7164343
This invention relates to potentiometer circuits of the type where there is a wiper and first and second terminals at each end of a resistance between the terminals. In early potentiometers, the resistance may have been wire wound or a resistance pad. In these devices a wiper moved along the resistor and picked off the voltage from the resistor. These devices find application in high current applications. As a practical matter, impedance at the wiper would be high which would prevent current drain at the wiper. In these prior art devices, the wiper is physically moved by an operator.
Prior art devices generally utilize a resistor chain with taps between each pair of resistors. U.S. Pat. No. 4,849,903 to Fletcher and Ross is hereby incorporated by reference and teaches a digitally controlled variable resistor for effecting and presenting a selected electrical resistance. Resistance may be changed along a string of resistors by shorting out the particular resistor with a field effect device. However, such devices, since they are dependent upon field effect devices for shorting the resistor, are necessarily not useful in alternating current applications. In the U.S. Pat. No. 4,849,903, across each resistor (24a–24L) is a series of field effect device (54a–54L). When a device, such as (54a) is turned on, a resistor (24a) is effectively short circuited or removed from the series group of resistors or string of resistors. The resistors (24a–24L) are chosen in accordance with Table 2 and increase in value in accordance with an expression 2N-1(R) where R is equal to a selected electrical resistance. With this scheme, shorting out any combination of resistors (24a–24L) allows for selection of different values of resistance in increments of 1 ohm to 4095 ohms. However, the '903 patent includes a resistance (24a) of a value of 1 ohm and another resistance of 2048 ohms on resistor (24L). This means that very close tolerances are required for all resistors in order for changes in total resistance to be accurate. If the tolerance of a large resistor has a value greater than the smallest resister, value and resistance may decrease when an increase is intended.
U.S. Pat. No. 4,849,903 in FIG. 1 shows a control scheme for switching on and off optoisolators (36a) each of which include an LED (38) and anode (40). When any combination of lines to the optoisolators change state, the value of the resistance between terminal (57) and terminal (60) will change to a selected value. One or more lines from the computer may be selected in order to select a particular resistance value. In this disclosure, there is disclosed both a digital computer (17) for selecting the lines (19a–19L) to resistors (24a–24L) as well as an analog to digital converter (18). The control system of the U.S. Pat. No. 4,849,903 may be used with Applicant's invention to change the state of relays which control shorting of resistors. This control can be used to select a single relay, a double pole double throw or a four pole double throw relay in accordance with embodiments of applicant's invention.
Potentiometer circuits which digitally controlled taps or connections between pairs of resistors in a string of resistors are known in the art. U.S. Pat. Nos. 5,297,056 and 5,243,535 owned by Dallas Semiconductor are examples of this type of digital potentiometer, and are incorporated here in by reference. These devices use strings of resistors which are identical in value and have a “wiper” which is a plurality of switchable taps located between each pair of resistors. Each of these patents shows a digital potentiometer which has a plurality of resistors and at the low end of the potentiometer the “wiper” may be connected to a “LO” terminal directly.
In both the '056 and '535 patents, a wiper contact is provided where a contact point is selected between any two resistors in a string as shown in FIG. 8. These patents never connect more than one wiper contact in a passive resistor string. Therefore, there is no ability to short circuit any resistor in the string and therefore the resistance from the “LO” to the “HI” terminals never changes.
The potentiometer of both '056 and '535 maintains a constant value between the low terminal and the high terminal. There is no capability of changing the overall value of the string of resistors. The wiper is merely moved from string tap point to string tap point to select different resistances between the wiper and the low and high terminals. As disclosed, each resistor in a string of 256 resistors has the same value. This approach is analogous to a simple linear wire wound potentiometer, except that it selects taps with FET devices which are selected by a computer instead of a mechanical contact manipulated by hand.
This invention utilizes switches (preferably mechanical relays) which are controlled in any combination of on-off states by a computer. The switches are generally in the form of relays because the invention is not dependent upon field effect transistors. FET devices as taught by the prior art cannot respond to alternating currents or reverse currents.
In accordance with this invention, applicant uses a software controllable (digital computer) switching apparatus to short out or remove shorts from electrical connections between the wiper and first and second terminals of a potentiometer. The switches are relays, not semiconductor devices such as FET's. Programming as taught by U.S. Pat. No. 4,849,903 may be used to select relays in accordance with this invention. FET's will not work with this potentiometer because they conduct in only one direction and therefore cannot respond to alternating current measurements or alternating current control. Still further, FET based devices are necessarily connected so that the polarity is correct.
In this invention, a string of resistors having preferably different values are used with a programmable interface controller (PIC) chip having a customized code to implement the digitally controlled potentiometer. The switches are relays which may be individual relays, double pole double throw relays or double pole four throw relays. This potentiometer is configurable as a single potentiometer, or in a pair with master/slave capabilities for a Wheatstone bridge, stereo audio and other applications where a pair of potentiometers are required.
This potentiometer solves several problems associated with existing potentiometers when used in conjunction with an AC source. The first problem is resistance precision and drift associated with mechanical analog potentiometers. This potentiometer allows for precise setting of resistance especially in the embodiment which can connect the same resistor between the wiper and either terminal. A second problem with prior art devices is inability to handle zero-referenced AC signals as with semiconductor MOSFET potentiometers. This invention is resistor-based, producing no effects that interfere with AC wave forms applied to the device. A third problem is that high-current applications have only been controlled by mechanical analog potentiometers. Existing MOSFET potentiometers will fail when currents exceed 200 mA.
The present invention which provides a variable potentiometer which uses the different resistors between the wiper and the first and second terminals which comprises in combination: a wiper terminal; a first terminal electrically connected to said wiper terminal by a first group of resistors; a second terminal electrically connected to said wiper terminal by a second group of resistors; a plurality of switches for changing resistance between said wiper terminal and each of said first terminal and second terminal; wherein resistance is changed between said wiper and said first terminal by placing a short circuit across one or more of said first group of resistors or by removing a short circuit from across one or more of said second group of resistors; wherein resistance is changed between said wiper and said second terminal by placing a short circuit across one or more of said second group of resistors or by removing a short circuit from across one or more of said second group of resistors; and wherein resistors of said first group and resistors of said second group have values which correspond to each other.
The present invention which provides a variable potentiometer which uses the different resistors between the wiper and the first and second terminals wherein the switches comprise relays which replace resistors with short circuits and replace short circuits with resistors. The relays may comprise double pole double throw relays. A computer is used for controlling the plurality of relay switches. The computer may have a display which provides for a rotating pointer within a circle which is controlled by a mouse. The first and second resistor sets are made with resistors having the same standard value and tolerances.
The present invention which provides a variable potentiometer which uses the different resistors between the wiper and the first and second terminals wherein the first and second resistors are resistors having a tolerance value of which is determined so that where the smallest resistor value is R1 and the largest resistor is Rn, then:
Rn×(TOLERANCE)<R1
The present invention provides a variable potentiometer which uses the same resistors between the wiper and the first and second terminals which comprises in combination: a wiper terminal; a first terminal electrically connected to said wiper terminal; a second terminal electrically connected to said wiper terminal; a plurality of switches for changing resistance between said wiper terminal and each of said first and second terminals; wherein when resistance is reduced between said wiper and one of said first or second terminals a first resistor is replaced with a first short circuit; and wherein when resistance is increased between said wiper and another of said first and second terminals a second short circuit is replaced with the first resistor.
The present invention provides a variable potentiometer which uses the same resistors between the wiper and the first and second terminals which comprises in combination: a set of resistors; a wiper terminal; a first terminal electrically connected to said wiper terminal; a second terminal electrically connected to said wiper terminal; a plurality of switches for changing resistance between said wiper terminal and each of said first and second terminals; wherein when resistance is reduced between said wiper and one of said first or second terminals a subset of said resistors is replaced with first short circuits; and wherein when resistance is increased between said wiper and another of said first and second terminals second short circuits are replaced with the subset of resistors.
The present invention may use switches which comprise relays which replace the first resistor with a short circuit and replace a short circuit with the first resistor. The present invention may use single pole single throw, double pole double throw or four pole double pole relays.
The present invention may change resistance between the wiper and one of said first and second terminals in the case where the same resistor is used between the wiper and the first or second terminals wherein: a first subset of double pole double throw relays electrically remove the subset of resistors from the electrical connection between one of said first and second terminals; a second subset of double pole double throw relay insert a subset of shorting connections in the electrical connection between the one of said first and second terminals; the first subset of double pole double throw double throw relays electrically insert the subset of resistors between the other of said first and second terminals; and the second subset of double pole double throw relays remove a subset of short connections between the other of said first and second terminals.
The present invention may change resistance in a variable potentiometer which uses the same resistors between the wiper and the first and second terminals wherein when resistance is changed between said wiper and one of said first and second terminals: a subset of first double pole double throw relays which electrically remove a resistor subset from the electrical connection between one of said first and second terminals and replace the resistor subset with a subset of shorts; a subset of second double pole double throw relays which electrically insert the resistor subset and remove a subset of shorts in the electrical connection between the other one of said first and second terminals.
In the present invention when resistance is reduced between the wiper and a terminal and when resistance is increased between the wiper and another terminal, a subset of the plurality of resistors are replaced with first short circuits and a corresponding subset of second short circuits are replaced with the subset of resistors.
In the present invention, the resistors in the set of resistors have a tolerance value of which is determined so that where a smallest resistor value is R1 and a largest resistor value is Rn, then:
Rn×(TOLERANCE)<R1
In the present invention, the sum of the set of resistor values is constant and the total resistance between said first and second terminals always constant.
In
In
In
In
The invention may use a serial port to precisely set resistance values using a defined control sequence sent to the potentiometer device over a serial port. Actual switching of resistor shorts or resistor position from one side of a wiper to another can be easily implemented by relays controlled by the computerized sequence. This device when fully implemented may display the requested resistance on a computer screen or a display device incorporated into a test instrument. The potentiometer can be paired with a “slave” potentiometer to create a Wheatstone bridge circuit that works using either AC or DC signals.
The potentiometer of this invention which utilizes resistors and relays will have electrical characteristics exactly like a standard analog potentiometer in that there is no need to consider the polarity or absolute sign of the signal applied. In this invention, the speed at which the resistance can be changed is limited by the speed of the micro-mechanical relays. The frequency response is limited by stray capacitance and is generally above 1 Mhz. Ordinary relay response times exceed the response time for hydraulic systems.
This invention allows for easy software implemented minimum increment changes or software implemented major changes without passing through intermediate values (software-selectable resistance values), constant input impedance, current limits are set by resistor ratings, two board designs (master and slave), and maintenance of last resistance value in non-volatile memory (last relay state).
In the case where separate resistors are used between the wiper and a first terminal and the wiper and a second terminal, the digital potentiometer is constructed from a pair of resistor sets and a set of electromechanical relays. There may be two circuits (master and slave) which share a serial port input which provides control information (from the computer) regarding how the circuit(s) should behave electrically. Each circuit “listens” to the serial port for control information that determines which circuit is enabled, and which resistance value that circuit should select. If a circuit is enabled, the PIC chip on the circuit board determines the requested resistance from the control information. The software in the PIC chip then looks up the configuration of relays that should be opened or closed so that the resister series output resistance is the value requested by the computer which effectively creates a “wiper” as used in analog potentiometers. The “wiper” of the potentiometer is “moved” by changing the state of relays (pairs of single pole single throw) or the state of a double pole double throw relay. The circuit operates electrically like a conventional potentiometer in all important characteristics, including fixed resistance across the potentiometer (within tolerance limits). The number of resistors in the string, and the number of strings, is determined by the overall resolution required, i.e., 1 k, 10 k, 100 k.
Where the present invention provides a variable potentiometer which uses the same resistors between the wiper and the first and second terminals an electromechanical relay removes a resistor and replaces it with short circuit between the wiper and a first terminal and removes a short circuit and replaces it with the resistor between the wiper and a second terminal. Single pole single throw, double pole double throw or four pole double pole relays may be used. This configuration avoids tolerance problems which are unavoidable where different resistors are used between the wiper and different terminals. Otherwise, this embodiment is controlled like the case where separate resistors are used between the wiper and a first terminal and the wiper and a second terminal.
However, as a practical matter, resistors necessarily have tolerance limitations. For instance, all resistors should have close tolerances, otherwise if resistors having large values also have large tolerances, the shorting out of resistor Rn on the T1 terminal side of the potentiometer of
Resistors R1, R2, . . . Rn may vary in accordance with the following formula:
Rn=R1 (2n-1)
If resistors in the string on the T1 side of the wiper and the T2 side of the wiper of
If the resistor values and the resistors value selected are perfect (no tolerance variation), an increase in selected value (R) will produce an actual increase in resistance which is directly proportional to the selected value as shown in
In the embodiment of
In the embodiments shown in
Rn×(TOLERANCE)<R1
An illustration of the problem of maintaining monotonicity (ever increasing value of R) is illustrated by considering a string of resistors which follow the relationship
Rn=R1 (2n-1)
If the total resistance in the string (from W to T1 on W to T2 is 1023 ohms, then the highest resistance value will be a resistor of 512 ohms. The 512 ohm resistor and the sum of all the lower value resistors will total 1023 ohms. In selecting the resistance, if the previous sum of 511 ohms is correct, and a step up to 512 ohms is taken, the 511 ohm string of resistors will all be shorted and the 512 ohm resistor will be inserted. If the 512 ohm resistor has a tolerance of 1%, it is possible that the selection will result in a resistor of the value of 512−5.12 or 506.88 ohms. This would be a reduction in the resistance value of the string and violate the monotonicity requirement as illustrated in
The embodiments shown in
The relays of this invention may be wired to the short circuits and to the resistors in any manner so that when the relays are closed (normally closed, no power), there is any combination of resistances between W and T1 and W and T2. As shown in
Resistance values for resistors following the relationship R=R1 (2n-1) can be selected from standard resistance values which are commonly available. Table 1 shows a standard resistance value table which shows values which are readily obtainable.
An example of selection of resistors to yield a relationship R=R1 (2n-1) is shown in Table 2.
The resistance values for the embodiment of
| Number | Name | Date | Kind |
|---|---|---|---|
| 3808409 | Enns et al. | Apr 1974 | A |
| 3897685 | Beguin et al. | Aug 1975 | A |
| 4849903 | Fletcher et al. | Jul 1989 | A |
| 5218225 | Zanders | Jun 1993 | A |
| 5243535 | Bolan et al. | Sep 1993 | A |
| 5297056 | Lee et al. | Mar 1994 | A |
| 5519396 | Distinti | May 1996 | A |
| Number | Date | Country | |
|---|---|---|---|
| 20060103500 A1 | May 2006 | US |
