Dynamic current equalization for light emitting diode (LED) and other applications

A system includes multiple dynamic current equalizers (DCEs). Each DCE includes a first control loop configured to regulate a current through a circuit branch associated with the dynamic current equalizer. The first control loop includes a first amplifier having two inputs. Each DCE also includes a second control loop configured to regulate a control signal. The second control loop includes a second amplifier having two inputs coupled to the inputs of the first amplifier. The first amplifier has an input offset compared to the second amplifier. The DCEs are configured such that one DCE regulates the control signal while one or more other DCEs regulate the currents through the associated circuit branches based on the control signal. The DCEs can be configured to achieve one or more ratios between multiple currents flowing through multiple circuit branches, where the one or more ratios are defined by resistances coupled to the DCEs.

TECHNICAL FIELD

This disclosure is generally directed to light emitting diode (LED) systems and other systems that can use current equalization. More specifically, this disclosure relates to dynamic current equalization for LED and other applications.

BACKGROUND

Many systems use light emitting diodes (LEDs) to generate light. For example, LEDs are often used in traffic control devices to generate light of different colors. As a particular example, a traffic lamp may use LED panels to generate red, yellow, and green light. Each LED panel could include multiple strings of LEDs, where each string includes multiple LEDs coupled in series. Each string generates light when a current flows through that string.

A problem with conventional LED devices is that individual LED strings can fail, which interrupts the current through the string. When this happens, the amount of light that is generated by the LED panel drops, which requires maintenance of the panel and the associated time, effort, and cost.

DETAILED DESCRIPTION

FIG. 1illustrates an example light emitting diode (LED) system100according to this disclosure. In this example, the system100includes an alternating current-to-direct current (AC/DC) converter102, an LED panel104, and a current control unit106. The AC/DC converter102receives an AC input signal and generates a DC output signal. For example, the AC/DC converter102could generate a DC input current IINfor the LED panel104. The AC/DC converter102includes any suitable structure for converting an AC signal into a DC signal. As a particular example, the AC/DC converter102could represent a converter operating in a constant current (CC) mode, such as a converter that generates a 3 A current.

The LED panel104here includes multiple strings108a-108n. Each string108a-108nincludes multiple LEDs110coupled in series, and the strings108a-108nare coupled in parallel with each other. Each string108a-108ncan include any number of LEDs110, any suitable number of strings could be coupled in parallel, and any other suitable configuration of LEDs110can be used. Each LED110includes any suitable semiconductor device for generating light. In this example, the LED panel104receives the input current IIN, which causes the LEDs110in the strings108a-108nto generate light. The amount of current flowing through an LED string controls the amount of illumination provided by that string. Higher currents typically result in more illumination, while lower currents typically result in less illumination.

During operation, one or more of the LED strings108a-108ncan fail. This could be due to any number of reasons, such as damage caused by an external object or degradation caused by normal use. When an LED string fails, this can disturb the distribution of currents in the remaining LED strings, so the total light output of the LED panel104can vary significantly over time.

To help compensate for this problem, the current control unit106controls the currents ILED1-ILEDnflowing through the LED strings108a-108n. As described in more detail below, the current control unit106implements dynamic current equalization in order to control the currents ILED1-ILEDn. If one or more LED strings108a-108nfail, the current control unit106dynamically adjusts the currents in the remaining strings to compensate. This may allow the system100to maintain the light output of the LED panel104even when one or more LED strings108a-108nfail (or at least provide more illumination than in conventional systems when one or more LED strings fail). The current control unit106includes any suitable structure for dynamically controlling currents in multiple LED strings. Details of example dynamic current equalizers and their arrangements in the current control unit106are provided below.

The current control unit106can equalize the currents in functioning or active LED strings108a-108n, allowing the active strings to receive currents according to specified ratios. For example, in some embodiments, the LED strings108a-108ncan receive substantially equal currents. In other embodiments, the current control unit106can apply a scaling factor to one or more currents and equalize the scaled currents. For instance, the current control unit106could make first currents in some strings substantially equal, while making a second current in another string substantially equal to twice the first current. This can provide great flexibility in the generation of light, such as by allowing different LEDs (like different colored LEDs) to receive different currents.

Among other things, the use of dynamic current equalization may increase system robustness. Light output could be maintained even when one or several LED strings fail, which reduces the need to replace the LED panel104each time an LED string fails. This can significantly reduce maintenance costs associated with the LED panel104. Moreover, embodiments of the dynamic current equalizers in the current control unit106work with standard off-the-shelf AC/DC converters102or any other current supply, which can reduce the overall system costs. Further, the dynamic current equalizers could be implemented without requiring the use of switching elements, which can reduce or eliminate concerns regarding electro-magnetic interference (EMI). In addition, the dynamic current equalizers can be easily set up (such as by simply tying a single resistor to each equalizer), reducing installation costs.

AlthoughFIG. 1illustrates one example of an LED system100, various changes may be made toFIG. 1. For example, the system100could include any number of AC/DC converters, LED panels, and current control units. Also, the use of an AC/DC converter is for illustration only. An input current for an LED panel could be generated or provided by any suitable structure, such as a DC/DC converter or a linear current regulator. Further, the relative positions of the components inFIG. 1are for illustration only. The illustrated components could be rearranged and additional components could be added according to particular needs. In addition, current equalization can be used in other systems unrelated to LEDs. In these embodiments, the current control unit106can be used to control the current through multiple branches of a circuit.

FIG. 2illustrates a more specific configuration of an example LED system200according to this disclosure. The system200is similar to the system100ofFIG. 1, butFIG. 2illustrates details of particular implementations of various components. In this example, the system200includes a current supply202, an LED panel204, and a current control unit206. The LED panel204includes multiple strings208a-208nof LEDs210.

As shown inFIG. 2, the current supply202includes a current source212, a diode214, a voltage source216, and a capacitor218. The diode214and the voltage source216are coupled in series between an output of the current source212and ground. The capacitor218is also coupled between an output of the current source212and ground. Note that the current supply202could represent an AC/DC converter, a DC/DC converter, a linear current regulator, or any other suitable structure for providing an input current IIN.

The current supply202generates the input current IINfor the LED panel204, which is associated with an LED voltage VLED. Assuming an LED string208a-208nis functioning properly, the LEDs210in that string cause a voltage drop across the string. This results in various voltages VD1-VDnat outputs of the LED strings208a-208n. Each LED string208a-208nalso has an associated current ILED1-ILEDnflowing through that string.

In this example, the current control unit206includes dynamic current equalizers (DCEs)222a-222ncoupled to the LED strings208a-208n, respectively. The DCEs222a-222nregulate the amount of current flowing through active LED strings208a-208n. In this particular example, when all LED strings208a-208noperate normally, the DCEs222a-222noperate such that the currents ILED1-ILEDnare substantially equal. If one or more LED strings208a-208nfail, the DCEs222a-222nadjust the currents such that the currents through remaining (non-failed) LED strings are substantially equal.

In this example embodiment, each DCE222a-222nincludes an ILEDinput, which is configured to receive the current ILED1-ILEDnflowing through the associated LED string or the voltage VD1-VDnat an output of the string. Each DCE222a-222nalso receives an equalization voltage VEQ. As described below, the equalization voltage VEQcan be set by one of the DCEs222a-222nfor use by the other DCEs222a-222nduring current equalization. This allows the DCEs222a-222nto operate together to control the currents ILED1-ILEDneven as conditions in the LED panel204dynamically change. The equalization voltage VEQmay therefore be referred to as a control voltage or control signal, since it is used to control the DCEs222a-222n. The equalization voltage VEQis coupled to a capacitor224, which represents any suitable capacitive structure having any suitable capacitance (such as a 1 μF or other bulk capacitor). Each DCE222a-222nfurther includes a ground pin. In this example, the DCEs222a-222noperate to make the currents ILED1-ILEDnthrough active LED strings substantially equal to IIN/N, where N is the number of active (non-failed) LED strings.

FIG. 3illustrates an example DCE300for LED systems according to this disclosure. The DCE300could, for example, be used in the current control unit206ofFIG. 2. As shown inFIG. 3, the DCE300includes an LED current pass element302and an LED current sense element304. The pass element302controls the amount of current that can pass through an LED string. The sense element304senses the amount of current that passes through the LED string and generates a sense voltage VSENbased on the amount of current. In this example, the pass element302includes an n-channel lateral diffused metal oxide semiconductor (NLDMOS) transistor, and the sense element304includes a resistor.

The DCE300also includes an open loop detector306that detects when little or no current passes through the pass element302. This could occur, for example, when an LED string fails and interrupts a current path through the string. In this embodiment, the open loop detector306includes a current source308and transistors310-312. The open loop detector306here detects when the sense voltage VSENfalls below some threshold (such as 36 mV), which is indicative of an open loop condition. When this condition is detected, the open loop detector306pulls an enable signal VENto a specified level (such as low). The current source308includes any suitable structure for generating a current, such as a 10 μA current source. The transistors310-312include any suitable transistor devices, such as NPN bipolar transistors.

The DCE300further includes a short circuit detector314, which detects a short circuit condition. A short circuit condition may occur when one or more LEDs of the string fail and form a short circuit. This condition causes the voltage at the output of the string with the short-circuit condition to increase rapidly. The short circuit condition can be detected, for example, when the voltage of any VD1-VDnincreases above some threshold. When this condition is detected, the short circuit detector314pulls the enable signal VENto a specified level (such as low) and causes a gate control signal VG1to go to a specified level (such as low) to shut off the pass element302. The short circuit detector314includes any suitable structure for detecting a short circuit condition in a circuit.

The DCE300also includes two resistors316-318. The resistor316is coupled to an upper supply voltage rail VDD. When the open loop detector306and the short circuit detector314detect no open or short circuit, the resistor316pulls up the enable signal VEN. The resistor318is also coupled to the voltage rail VDDand pulls up the equalization voltage VEQif necessary. Each resistor316-318includes any suitable resistive structure having any suitable resistance. For example, the resistor316could represent a 400 kΩ resistor, and the resistor318could represent a 100 kΩ resistor. In other embodiments, the resistors316-318could be replaced by current sources or other structures that pull up the enable signal VENand the equalization voltage VEQ, respectively.

In this example, the DCE300implements two different regulation loops, namely an ILEDregulation loop320and a VEQregulation loop322. The ILEDregulation loop320includes the pass element302, the sense element304, and a first operational amplifier324. This regulation loop320controls the current flowing through an LED string based on its own sense voltage VSENand the equalization voltage VEQreceived from an external source (such as another DCE). The amplifier324receives the equalization voltage VEQat its non-inverting input and the sense voltage VSENat its inverting input. The amplifier324generates and adjusts the gate control signal VG1for the pass element302. In this way, the ILEDregulation loop320regulates the sense voltage VSENto the equalization voltage VEQ(without attempting to alter the equalization voltage VEQ). The amplifier324can also drive the gate control signal VG1to a specified level when the short circuit detector314detects a short circuit condition. The amplifier324includes any suitable amplification structure. In this example, the amplifier324is arranged to operate as part of a differential amplifier or a differential gain stage.

The VEQregulation loop322regulates the equalization voltage VEQ. In this example, the regulation loop322includes a second operational amplifier326and transistors328-330. The operational amplifier326receives the current equalization voltage VEQat its non-inverting input and the sense voltage VSENat its inverting input. The equalization voltage VEQmay initially represent the voltage generated by the resistor318. The amplifier326generates and adjusts a gate control signal VG2for the transistor328, allowing the amplifier326to further adjust the equalization voltage VEQtowards the sense voltage VSENusing a feedback loop. The transistor330can also be cut off to prevent the regulation loop322from regulating the equalization voltage VEQwhen an open or short circuit condition is detected. The amplifier326includes any suitable amplifier structure. In this example, the amplifier326is arranged to operate as part of a differential amplifier or a differential gain stage. The transistors328-330include any suitable transistor devices. For instance, the transistor328could represent an n-channel MOS (NMOS) transistor, and the transistor330could represent an NLDMOS transistor.

In this example, the first amplifier324includes an input offset, namely an input voltage offset (VOS). This offset could be added to the sense voltage VSEN. The second amplifier326may lack an input offset or have a smaller input offset (meaning the offset of the amplifier324minus the offset of the amplifier326is positive). This difference in offsets helps to prevent both the regulation loop320and the regulation loop322from operating at the same time, thereby preventing the DCE300from regulating both the LED current ILEDand the equalization voltage VEQ.

DCEs300coupled to different LED strings operate differently depending on the situation. For example, during startup, the open circuit detector306can be triggered in each DCE300, cutting off the transistor330and the regulation loop322in each DCE300. The equalization voltage VEQin each DCE300is internally charged up gradually towards the supply voltage by the resistor318in that DCE. During this time, the regulation loop320in each DCE300is regulating its LED current ILEDto provide a soft startup.

After startup, the VEQregulation loop322in the DCE300associated with the “weakest” LED string begins regulating the equalization voltage VEQ. The weakest string represents the LED string with the smallest sense voltage VSEN, which would indicate that this LED string has the highest forward voltage and smallest current ILEDof any of the LED strings. The DCE300associated with the weakest LED string uses its VEQregulation loop322to regulate the equalization voltage VEQ, and the operational amplifier326in that DCE can regulate VEQto be substantially equal to the smallest sense voltage VSEN. The ILEDregulation loop320in this DCE300can fully turn on the pass element302to provide the minimum necessary voltage headroom (thereby providing inherent dynamic headroom control). Effectively, this DCE300is adjusting the equalization voltage VEQbased on the smallest current ILED1-ILEDnflowing through any of the LED strings. The DCEs300associated with the other LED strings cut off their VEQregulation loops322and use their ILEDregulation loops320to regulate their LED currents based on the equalization voltage VEQ.

If the input current IINincreases or decreases, this alters the charge on the capacitor218of the current supply202, which alters the voltage VLED. In the DCE300for the weakest LED string, the pass element302can be in a triode region of operation, so changes to the voltage VLEDcause changes to the current ILEDand changes in the sense voltage VSENof that DCE. This causes the DCE300to change the equalization voltage VEQ, which is then sent to the other DCEs. The other DCEs use the changed equalization voltage VEQin their ILEDregulation loops320to alter their currents ILED. Note that the capacitor224can slow changes in the equalization voltage VEQ, which helps to provide soft-start for the currents ILED1-ILEDnand to make the VEQregulation loop322a slower regulation loop compared to the ILEDregulation loop320so that they are not competing with each other.

If the weakest LED string breaks open, the open circuit condition is detected by its DCE300, and the transistor330in that DCE is cut off. This prevents the DCE300of the weakest string from regulating the equalization voltage VEQ. In each of the other DCEs300, its equalization voltage VEQis charged up by the associated resistor318, and its ILEDregulation loop320generates a sense voltage VSENthat equals the equalization voltage VEQplus the offset voltage VOS. The currents through those DCEs300continue to rise until their sum equals the input current IIN, at which point a new weakest LED string is identified (and its associated DCE300begins regulating the equalization voltage VEQ).

If a non-weakest LED string (a string that is not the weakest string) breaks open, the charge on the capacitor218in the current supply202increases, which increases the voltage VLED. This increases the current ILEDand the sense voltage VSENin the DCE300associated with the weakest string. The increase in the sense voltage VSENcauses the DCE300to increase the equalization voltage VEQ. The other DCEs300use the increased equalization voltage VEQto increase their own LED currents so that the currents through all of the functioning strings total the input current IIN.

As can be seen here, the DCEs222a-222ncan be used to force the currents ILED1-ILEDnthrough functioning LED strings208a-208nto be substantially equal. As a result, the failure of one or several LED strings may cause more current to flow through the remaining LED strings, increasing the light output of the remaining LED strings. Even if the light output decreases somewhat, the light output may still be adequate for the LED panel's intended use, meaning maintenance or repair of the LED panel or system may not be necessary.

InFIG. 2, a DCE is associated with each string208a-208nof LEDs. However, other configurations of LEDs and DCEs are also possible.FIGS. 4 through 8illustrate other configurations of example LED systems according to this disclosure. InFIG. 4, an LED system400includes a current supply402and multiple LED strings408a-408c. Each string408a-408cincludes multiple LEDs410, and each LED410is associated with its own DCE422. As a result, each string408a-408cis formed by multiple LEDs410with DCEs422embedded between the LEDs410. Also, each of multiple capacitors424(such as 1 μF capacitors) can be used with a subset of the DCEs422. Each capacitor424can store an equalization voltage VEQfor that subset of DCEs422.

FIG. 5illustrates an example LED system500that is similar in structure to the system400ofFIG. 4. InFIG. 5, a string of Zener diodes526a-526nis coupled between the upper and lower voltage rails. Each Zener diode526a-526nis coupled to the supply input VCCof a subset of DCEs522. The Zener diodes526a-526ncan be used for power up protection, and they can shunt current when all LEDs510coupled in parallel fail.

FIG. 6illustrates an example LED system600that is similar to the LED system200ofFIG. 2. The system600includes LED strings608a-608ncoupled to DCEs622a-622n, respectively. Resistors626a-626nare coupled to SRC pins of the DCEs622a-622n. These resistors626a-626ncan be used for various purposes. For example, if each of the resistors626a-626nhas an approximately equal resistance R, it is possible to identify the minimum necessary value of the voltage VLED. That is, the minimum value of VLEDcan be calculated as:
VLED=VFHIGHEST+ILED×(RDSON+R)
where VFHIGHESTdenotes the highest forward voltage of any LED string, ILEDdenotes the current in that LED string, and RDSONdenotes the specific on-resistance of the pass element302in the DCE for that LED string. With a known value of R, the minimum necessary VLEDvoltage can be identified, which can help to minimize voltage overhead. In these embodiments, the DCEs622a-622ncould operate to make the currents ILED1-ILEDnsubstantially equal.

However, the resistances of the resistors626a-626nneed not be equal. In fact, all of the resistors626a-626ncould have a different resistance value. In these embodiments, the specific resistances of the resistors626a-626ncould be selected to scale the currents ILED1-ILEDnin the different LED strings608a-608nto obtain different ratios between the currents. For instance, a lower resistance could allow more current to flow through the associated LED string. The current Ikin the kth LED string could be expressed as:

Ik=Iin×R1//R2//…//RNRk
where (R1//R2// . . . //RN) denotes the overall resistance of the parallel resistors626a-626nthat are associated with active (non-failed) LED strings, and Rkdenotes the resistance of the resistor associated with the kth LED string.

This could be useful, for example, when LEDs of different colors are used in the system600. Assume, for instance, that the strings608a-608dinclude white LEDs, while the string608nincludes amber LEDs. Also assume that there are five total strings. The resistors626a-626dcould each have a resistance of R, while the resistor626ncould have a resistance of 2.25×R. With this configuration, 90% of the current IINmay flow through the strings608a-608d, while 10% of the current IINmay flow through the string608n. This may be true regardless of changes to the input current IIN.

FIG. 7illustrates an LED system700with cascaded DCEs. InFIG. 7, DCEs722a-722dare coupled to LED strings708a-708d, respectively. Assuming resistances of resistors726a-726dare equal, the DCEs722a-722dcause the currents through the active LED strings708a-708dto be substantially equal. If at least some of the resistors726a-726dare unequal, the DCEs722a-722dcause the currents through the active LED strings708a-708dto achieve the ratios defined by those resistors726a-726d. These DCEs722a-722dform a first level of DCEs in the system700.

A DCE722eis coupled to the DCEs722a-722d, and a DCE722fis coupled to an LED string708e. The DCEs722e-722fform a second level of DCEs in the system700and perform another equalization. More specifically, assuming resistances of resistors726e-726fare equal, the DCEs722e-722foperate such that the total current flowing through the LED strings708a-708dsubstantially equals the current flowing through the LED string708e. In this example, the string708ereceives half of the input current IIN(assuming the resistors726e-726fare equal) as long as one or more of the strings708a-708dare active. The remaining half of the current flows through the active strings708a-708d.

In this way, hierarchical equalizations can be enforced using the DCEs. A DCE can control the current through a single string of LEDs, or a DCE can control the current through multiple strings of LEDs (possibly via other DCEs). Although not shown, the DCE722fcould be used to control the current through multiple strings of LEDs, and/or one or more additional layers of DCEs could be used in the system700. This provides great flexibility in how to manage the currents through a number of LED strings.

InFIG. 8, a DCE800for LED systems is similar in structure to the DCE300ofFIG. 3. Either DCE could be used in any of the LED systems shown in this patent document. The DCE800includes a pass element802and a sense element804. An ILEDregulation loop820includes a first amplifier824, and a VEQregulation loop822includes a second amplifier826.

In this example, the ILEDregulation loop820further includes a resistor832and a current source834. These components can be used in the DCE800to scale the current ILEDpassing through the pass element802. Moreover, these components in multiple DCEs800can be used to scale multiple currents ILED-ILEDnto obtain different ratios between those currents.

Assuming that currents coming out of an open circuit detector806and the inverting input terminals of the amplifiers824-826are minimal, the sense voltage VSENgenerated by the sense element804can be offset by a voltage generated by current from the current source834flowing through the resistor832. This offset alters the sense voltage VSEN, causing changes to the ILEDcurrent through that specific DCE800.

AlthoughFIGS. 2 through 8illustrate example arrangements of LED systems and example embodiments of DCEs and other components in those systems, various changes may be made toFIGS. 2 through 7. For example, an LED system could include any number of LEDs and LED strings in any suitable arrangement with any suitable number of DCEs. Also, while certain circuit elements are shown above (such as certain types of transistors or other components), other circuit elements could be used to perform the same or similar functions. In addition, the DCEs can be used in other systems to regulate the currents through multiple branches of a circuit, where those branches may or may not contain LEDs.

FIG. 9illustrates an example method900for dynamic current equalization in an LED system according to this disclosure. For ease of explanation, the method900is described with respect to the LED system200ofFIG. 2operating using the DCE300ofFIG. 3. The method900could be used with any other suitable LED system and DCE configuration.

A signal associated with one or more LEDs is received at a DCE at step902. This could include, for example, a DCE222a-222nreceiving a current or voltage associated with a string of LEDs208a-208n. The current could represent the current ILED1-ILEDnflowing through the string, and the voltage could represent the voltage VD1-VDnat an output of the string. The DCE generates a sense signal based on the received signal at step904. This could include, for example, the DCE222a-222ngenerating the sense voltage VSENusing the sense element304.

The DCE determines whether a short circuit condition is detected at step906. If so, the DCE disables its VEQregulation loop and blocks current from flowing through the one or more LEDs at step908. This could include, for example, the short circuit detector314causing the amplifier324to turn off or open the pass element302. This could also include the short circuit detector314disabling the VEQregulation loop322by cutting off the transistor330. The DCE determines whether an open circuit condition is detected at step910. If so, the DCE disables its VEQregulation loop at step912. This could include, for example, the open loop detector306disabling the VEQregulation loop322by cutting off the transistor330.

If no open or short circuit condition exists, the DCE is currently receiving a signal from one or more LED(s) that may or may not be the weakest LED(s), such as the weakest LED string. The detection of whether or not the DCE is associated with the weakest LED(s) occurs at step914, where the sense voltage VSENcan be provided to the amplifiers324-326, one of which includes an input offset (such as VOS).

If the DCE is associated with the weakest LED(s), the DCE enables its VEQregulation loop and disables its ILEDregulation loop at step916, and the DCE adjusts the equalization voltage VEQat step918. In this case, the amplifier324outputs a signal that causes the pass element302to pass the ILEDcurrent. Also, the amplifier326adjusts the operation of the transistor328to control the equalization voltage VEQso that it is substantially equal to the sense voltage VSEN, which can be output by the DCE to other DCEs for use.

If the DCE is not associated with the weakest LED(s), the DCE disables its VEQregulation loop and enables its ILEDregulation loop at step920, and the DCE adjusts the current through its LED(s) at step922. In this case, the amplifier326can turn off the transistor328to block adjustments to the equalization voltage VEQ. Also, the amplifier324adjusts the operation of the pass element302based on the equalization voltage VEQreceived from another DCE to control the current through its LED string.

In this way, the DCE can operate to either (i) regulate the equalization voltage VEQor (ii) regulate its LEDs' current based on the equalization voltage VEQ, but not both. Regulating the equalization voltage VEQallows the DCE to achieve some control over the currents flowing through other LEDs since the other DCEs regulate their currents based on the equalization voltage VEQ. Regulating the LED current based on the equalization voltage VEQallows the DCE to regulate its current in line with other DCEs.

AlthoughFIG. 9illustrates one example of a method900for dynamic current equalization in an LED system, various changes may be made toFIG. 9. For example, while shown as a series of steps, various steps inFIG. 9may overlap, occur in parallel, or occur in a different order. Also, the method900could be used to regulate the currents through multiple branches of a circuit, where those branches may or may not contain LEDs.

While this disclosure has described certain embodiments and generally associated methods, alterations and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this invention. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this invention as defined by the following claims.