Patent Description:
<CIT> discloses a resistive torsional mode damping system for a shaft of a machine which includes: a sensor configured for sensing a signal representative of torque on the shaft; a controller configured for using the sensed signal for detecting a presence of a torsional vibration on the shaft corresponding to a natural frequency of the shaft and for generating control signals for damping the torsional vibration; and a damper including a damping converter and resistor coupled to a DC output of the damping converter, the damping converter being coupled to the machine through a power bus and having a power rating on the order of less than or equal to about five percent of a nominal power of the machine.

According to the present disclosure, there is provided a damping system according to claim <NUM>.

As may be appreciated, based on the disclosure, there exists a need in the art for an electrical damper to absorb ripple in a mechanical torque or speed of a drive shaft of an electrical generator. Further, there exists a need in the art for an electrical damper integrated with a constant power load of the generator. Additionally, there exists a need in the art for an electrical damper that absorbs or reflects unwanted ripple power in the presence of a filtering capacitor at the output of a rectifier of the electrical generator.

Referring to <FIG>, in one aspect of the present disclosure, a damping system <NUM> can be combined with a generator <NUM> having a drive shaft <NUM> and providing a direct current (DC) link voltage <NUM> across a load resistance (R) <NUM>. A DC link capacitor <NUM> can have a load capacitance (C) <NUM> smoothing the DC link voltage <NUM> of the generator <NUM>. For example, DC link voltage <NUM> can be a rectifier output having alternating current (AC) ripple needing low-pass filtering to provide a steady regulated DC link voltage <NUM> for load resistance <NUM>. For instance, the electrical load represented by load resistance <NUM> might tolerate a +/- <NUM>%, <NUM>% or <NUM>% variation in DC link voltage <NUM> and can require DC link capacitor <NUM> to filter out noise or high-frequency components. Prime mover <NUM>, such as a gas turbine engine, can provide a nominal rate of rotation <NUM> to the drive shaft <NUM> with a shaft input <NUM> to the generator <NUM>. Additionally, the drive shaft <NUM> will have a torsional oscillation (or speed ripple). Load resistance <NUM> can reflect a substantially constant mechanical load to shaft input <NUM> which can, without a damping action, sustain or exacerbate the speed ripple.

Turning now also to <FIG>, the speed ripple can be steadily periodic, such as represented by a sine wave, with a speed ripple amplitude <NUM> and a speed ripple phase <NUM>, or it can persist for a limited number of cycles at an angular frequency <NUM> before changing in speed ripple amplitude <NUM> or speed ripple phase <NUM>, or it can be impulsive or intermittent, depending on the stability of components associated with the prime mover <NUM>, the drive shaft <NUM>, and the generator <NUM>. For example, an undamped generator may present a constant horsepower load to the drive shaft which can cause the speed ripple to react positively or negatively. The generator <NUM> can be a highly efficient energy converter, and the electrical power delivered by the generator <NUM> can result in approximately the same mechanical power presented at shaft input <NUM>. For example, over <NUM>% of the mechanical power transmitted to shaft input <NUM> can appear as electrical power across load resistance <NUM> and load capacitance <NUM>. Generator efficiency can be lower than <NUM>% as well.

Continuing with <FIG>, in an aspect, damping system <NUM> can include a ripple sensor <NUM> disposed to measure the speed ripple amplitude <NUM> of the drive shaft <NUM>. The ripple sensor <NUM> can connect to drive shaft <NUM> or to generator <NUM> by mechanical, electrical, or wireless means, and can also provide an indication of the speed, torque, horsepower, frequency, or acceleration of the drive shaft <NUM>. Damping system <NUM> can further include a feedforward circuit <NUM> connected to the ripple sensor <NUM> and to the generator <NUM>. A module <NUM> of the feedforward circuit <NUM> can be configured to determine a DC link phase angle <NUM> formed by the load resistance and the load capacitance <NUM> of the DC link capacitor <NUM>. The DC link phase angle <NUM> can define a power-to-voltage relationship for an RC load <NUM> of the generator <NUM>, the RC load <NUM> comprising the load resistance <NUM> in parallel with the load capacitance <NUM>. The module <NUM> can further be configured to calculate a voltage signal value <NUM> based on the speed ripple amplitude <NUM> and the DC link phase angle <NUM>, and to adjust the DC link voltage <NUM> according to the voltage signal value <NUM>. Beneficially, adjusting the DC link voltage <NUM> based on the speed ripple amplitude <NUM> and the DC link phase angle <NUM> can provide a damping power in the RC load <NUM> that substantially dampens or eliminates the torsional oscillation in drive shaft <NUM>.

In an aspect of the damping system <NUM>, feedforward circuit <NUM> can set a magnitude (not shown) of the voltage signal value <NUM> to be proportional to the speed ripple amplitude <NUM> of the speed ripple such that the DC link voltage <NUM> is increased as a speed or a torque of the drive shaft <NUM> increases, and decreased as a speed or a torque of the drive shaft <NUM> decreases, thereby accommodating a ripple power of the speed ripple (or torsional oscillation) transmitted to the generator <NUM>. Beneficially, the feedforward circuit <NUM> can also offset a phase of the adjusted DC link voltage <NUM> from the speed ripple by the DC link phase angle <NUM> (<FIG>) such that the damping power provided in the RC load <NUM> is in phase with the ripple power transmitted to the generator <NUM>. Adjustment of the DC link voltage <NUM> by the feedforward circuit <NUM> in the presence of a periodic speed ripple can be viewed as a wiggling of the DC link voltage <NUM> to create the damping power. Some of the damping power is dissipated in the load resistance <NUM> and some is temporarily stored in the load capacitance <NUM>.

Referring still to <FIG>, the ripple sensor <NUM> can also be configured to measure an angular frequency (ω) <NUM> of the speed ripple from which the module <NUM> can determine an impedance of the load capacitance <NUM> (<NUM>/jωC) at the angular frequency <NUM>. The DC link phase angle <NUM> can describe by how much the damping power leads a phase of the DC link voltage wiggling depending on the size of the load capacitance <NUM>, the load resistance <NUM>, and angular frequency <NUM>. In an aspect of the disclosure, the feedforward circuit <NUM> can retard the wiggling of the DC link voltage <NUM> with respect to the speed ripple by the DC link phase angle <NUM> being zero to ninety degrees so that the ripple power is in phase with the damping power, thereby substantially dampening or eliminating the torsional oscillation (speed ripple). In another aspect, the DC link phase angle (Φ )<NUM> can be calculated as an arctangent of one-half of a product of the angular frequency <NUM>, the load capacitance <NUM>, and the load resistance <NUM>, or Φ= arctan(ωRC/<NUM>).

Applying the arctangent relationship above, when the load capacitance <NUM> is essentially zero, the DC link phase angle <NUM> can be approximately <NUM> degrees because all of a current (not shown) flowing out of generator <NUM> is in phase with the DC link voltage <NUM>. For example, a load capacitance <NUM> whose impedance (<NUM>/jωC) is three times the load resistance <NUM> can be regarded as essentially zero because a phase shift of arctan(ωRC/<NUM>) is arctan(<NUM>/<NUM>) = <NUM> degrees, or approximately <NUM> degrees. Alternately, when the load resistance <NUM> is essentially infinite, the DC link phase angle <NUM> can be approximately <NUM> degrees because the current flowing out of generator <NUM> leads the DC link voltage <NUM> by ninety degrees or slightly less than <NUM> degrees. For example, the load resistance <NUM> being ten times greater than the impedance (<NUM>/jωC) of the load capacitance <NUM> at angular frequency ω can be regarded as essentially infinite because a phase shift of arctan (ωRC/<NUM>) is arctan(<NUM>) = <NUM> degrees, or approximately <NUM> degrees. Beneficially, the DC link capacitance <NUM> can alone provide torsional damping without the use of a load resistance. The present disclosure is an improvement in damping torsional oscillation by adapting the feedforward circuit <NUM> to various values of load resistance <NUM> and load capacitance <NUM>, which components perform other functions such as filtering. Additional advantages will be described below.

Continuing with <FIG>, in various aspects, a DC link reference (Vref) <NUM> can be provided at an output of the feedforward circuit <NUM> as a nominal operating set point for the DC link voltage <NUM> In addition, the DC link reference <NUM> can be combined in a summing node <NUM> with the voltage value signal <NUM> to superimpose the damping function described above. In one aspect, an output of the summing node <NUM> can connect to a control input <NUM> of the generator <NUM> to control the DC link voltage <NUM>. For example, the generator <NUM> can be a variable speed generator having a variable AC output frequency and an output voltage rectified by an internal rectifier (<FIG>), where the control input <NUM> can set the output voltage. In this case, the value of the DC link capacitance <NUM> can be chosen to perform filtering in accordance with a range of output frequencies. Alternatively, the generator <NUM> can be a fixed speed generator having a fixed output frequency where there is less constraint on the choice of the DC link capacitance. In other aspects, the voltage value signal <NUM> can connect directly to the control input <NUM> without the use of summing node <NUM> or the DC link reference <NUM>, and the nominal operating point for the generator output voltage may be set elsewhere, such as by a separate control input.

Referring now to <FIG>, in another aspect, a rectifier <NUM> can be connected between the generator <NUM> and a constant power load <NUM> to provide the DC link voltage <NUM> to the constant power load <NUM> and the load resistance <NUM>. The rectifier <NUM> can be a passive rectifier, such as a diode rectifier converting an AC voltage from the generator <NUM> to the DC link voltage <NUM>. The rectifier <NUM> can also be an active rectifier where a control input <NUM> of the rectifier can receive the DC link voltage adjustment from the feedforward circuit <NUM>. The constant power load <NUM> can accept a range of voltages supplied by DC link voltage <NUM> without a change in a power consumed by the constant power load <NUM>. Generator systems such as a turbine-driven generator aboard an aircraft can commonly drive constant power loads <NUM> such as a voltage inverter that produces an AC output or such as a voltage converter that produces a DC output. Advantageously, constant power loads <NUM> can tolerate deviations from a nominal DC link voltage <NUM> without malfunctioning, and operate at a high power conversion efficiency. However, adjusting the DC link voltage to generate a damping power can be independent of the size or operation of the constant power load <NUM>. Beneficially, the load resistance <NUM> is a constant resistance and an effective DC link phase angle <NUM> can be managed by simply knowing the values of the load resistance <NUM> and load capacitance <NUM>.

Since the load resistance <NUM> can change dynamically as various aircraft loads are switched on an off, the resistor current <NUM> can be measured by the feedforward circuit <NUM> to calculate an accurate load resistance using a knowledge of the DC link voltage <NUM>. The damping system <NUM> can then maintain an optimum DC link phase angle <NUM> while the load resistance <NUM> and the constant power load <NUM> draw varying amounts of power. Although the DC link capacitance <NUM> can be fixed in value, the constant power load <NUM> can also contain a capacitance which can be included in determining the overall load capacitance <NUM>. The load current <NUM> can be determined with a current shunt (not shown) in parallel with the load resistance <NUM>, or by receiving one or more data values indicating which load circuits are operating off the DC link voltage and their respective current drains, or by any other means known in the art.

The load resistance <NUM> can be a resistive load chosen to provide more or less damping for a given variation (wiggle) applied by the feedforward control circuit <NUM>, and can also include pre-existing load circuits (not shown) depending on the generator <NUM> for power within an operating environment. In one aspect, the load resistance <NUM> can be intentionally added across DC link voltage <NUM> to achieve a desired damping of mechanical oscillation in drive shaft <NUM>. Load resistance <NUM> can be set to be small enough so that the DC link 'wiggle' does not violate a maximum voltage tolerance of the constant power load <NUM>. The load resistance <NUM> can also be chosen to be large enough to avoid unnecessary power loss while providing a damping of the mechanical oscillation in drive shaft <NUM>. In another aspect, load resistance <NUM> is determined by the pre-existing load circuits in the operating environment and cannot be randomly adjusted for damping purposes. In yet another aspect, a combination of an intentionally added load resistance and pre-existing load circuits may determine load resistance <NUM>. The load resistance <NUM> can also be chosen to provide an RC time constant providing an optimized low-pass filter corner frequency for removing unwanted high-frequency signals from rectifier <NUM>.

<FIG> illustrate aspects of the phase relationships that can exist in the damping system <NUM> described by <FIG> and <FIG> above. There can be a periodic ripple in the speed or torque of the rotating drive shaft, as shown in <FIG>, where a period T can be equal to 2π/ω and ω is the angular frequency of the speed ripple. The ripple can also be non-sinusoidal, such as a tending toward a square wave, or can be impulsive, or have more than one frequency component. Referring to <FIG>, the ripple sensor (<FIG> and <FIG>) can detect the amplitude <NUM> of the speed ripple and thereby have an indication of a ripple power <NUM> of the torsional oscillation. For instance, the ripple power can be proportional to a square of the amplitude <NUM> of the speed ripple. A phase of the speed ripple can be determined by the ripple sensor detecting zero-crossings <NUM> of the amplitude <NUM>. Alternatively, a phase detector or peak detector could be used to determine a relative phase of the speed ripple. In <FIG>, the DC link phase angle <NUM> can be determined by the feedforward circuit <NUM> to describe by how much a damping power caused by the feedforward circuit <NUM> will lead a phase of the DC link voltage wiggling, depending on the size of the load capacitance, the load resistance, and the angular frequency.

The feedforward circuit <NUM> can adjust the DC link voltage <NUM> about its nominal operating point, proportional to the amplitude <NUM>, and delayed in phased by the DC link phase angle <NUM> with respect to the speed ripple in <FIG> illustrates that the damping power <NUM> can be caused to be in phase with the ripple power <NUM> such that the ripple power is absorbed substantially or completely by the load resistance and the load capacitance driven by the DC link voltage. Alternately, the DC link voltage <NUM> can be delayed or advanced in phase by an amount <NUM> degrees opposite that of the DC link phase angle <NUM>, or by an intermediate amount, in order to test for or accommodate various stability dynamics of the torsional oscillation. For example, it can be desirable to test a response of the drive shaft to a non-cancelling damping power in order to determine stability characteristics of the torsional vibration.

<FIG> illustrates one possible aspect of the feedforward circuit <NUM> where an integrator block <NUM> can calculate the DC link phase angle <NUM>, a sample of the speed ripple from the ripple sensor <NUM> can be applied to the integrator block <NUM>, and the results summed in the summing node <NUM>. A gain block <NUM> can set value K to calibrate the summed results of the summing node <NUM> to gain and efficiency factors within the feedforward circuit and the generator <NUM> and to provide a control input <NUM> which substantially dampens the torsional oscillation of the drive shaft <NUM>, shown in <FIG> and <FIG>. The dotted line illustrates the sensor <NUM> acquiring a sample of the speed ripple from the generator <NUM>, but the sensor <NUM> can also acquire a sample of the speed ripple from the drive shaft <NUM>, from a rectifier rectifying an output of the generator <NUM>, or from other components associated with the drive shaft <NUM> or generator <NUM>.

Claim 1:
A damping system (<NUM>) comprising:
a generator (<NUM>);
a drive shaft (<NUM>) coupled to the generator (<NUM>);
a DC link capacitor (<NUM>) across a load resistance (<NUM>);
a DC link voltage (<NUM>) across the load resistance (<NUM>);
a ripple sensor (<NUM>) connected to the generator (<NUM>) or the drive shaft (<NUM>) by mechanical, electrical or wireless means, and configured to:
measure a speed ripple amplitude (<NUM>) of the drive shaft (<NUM>); and
provide an indication of at least one of: a speed, a torque, a horsepower, a frequency, or an acceleration of the drive shaft (<NUM>);
characterized in that the damping system (<NUM>) comprises
a feedforward circuit (<NUM>) connected to the generator (<NUM>) and the ripple sensor (<NUM>) and comprising a module (<NUM>) configured to:
determine a DC link phase angle (<NUM>) formed by the load resistance (<NUM>) and a load capacitance (<NUM>) of the DC link capacitor (<NUM>), the DC link phase angle (<NUM>) defining a power-to-voltage relationship for an RC load (<NUM>) of the generator, the RC load (<NUM>) comprising the load resistance (<NUM>) in parallel with the load capacitance (<NUM>);
calculate a voltage signal value (<NUM>) based on the speed ripple amplitude (<NUM>) and the DC link phase angle (<NUM>); and
adjust the DC link voltage according to the voltage signal value (<NUM>), thereby to provide a damping power in the RC load (<NUM>) that dampens a torsional oscillation in the drive shaft (<NUM>).