Vehicle suspension controller

Abstract: A vehicle suspension controller (15) includes a differentiator (33) for deriving an analogue relative suspension velocity signal from a relative suspension position sensor (13); and the analogue relative suspension velocity signal is digitised by digital signal processing apparatus (30) at a predetermined sampling frequency. The differentiator (33) comprises a bandpass filter having a linearly increasing gain and a 90 degree phase lead for differentiator action in a low frequency range of vehicle suspension resonances (0.5-20 Hz) in which signal differentiation is desired, a peak in gain and group delay at an intermediate frequency above the low frequency range of vehicle suspension resonances, at which vehicle suspension inputs are small, and a decreasing gain from the intermediate frequency to a higher aliasing frequency (500 Hz) related to the sampling frequency of the digital signal processing apparatus. The circuit is used in combination with a two-pole low-pass filter (32) to decrease further gain above the intermediate frequency; and, even with the low-pass filter (32), it has low group delay in the low frequency range.

Description: This invention relates to a vehicle suspension controller which includes a differentiator circuit for converting a suspension position signal from a relative suspension position sensor between the sprung and unsprung masses to a relative suspension velocity signal useful in deriving a control signal for a suspension actuator to produce a desired suspension behaviour. Ideally, such a differentiator should have several properties. The first is that it operates as a differentiator within a certain frequency range of interest. Passenger motor vehicles typically have suspension resonances in the region of 1 Hz for the vehicle body or sprung mass and about 8-10 Hz for the wheel apparatus or unsprung mass. In addition, the frequency of greatest sensitivity of a seated occupant to vertical vibration is about 5-6 Hz. Therefore, the circuit should preferably have the characteristics of a differentiator - that is, substantially linearly increasing gain and a substantially constant 90 degree phase lead - through a frequency range including these frequencies: for example, 0.5 to 20 Hz. A typical differentiator circuit of the type capable of producing such a response is shown in US-A-4,579,366, which circuit comprises an operational amplifier with a series input capacitor and a negative feedback resistor. However, the linearly increasing gain of this circuit continues past the upper limit of the frequency range of interest until a high frequency roll-off begins at a significantly higher frequency due to the inherent capacitances of the circuit. The result is an extended high gain above the frequency range of interest which tends to amplify high frequency noise. A preferred form of suspension control for motor vehicles not shown in US-A-4,579,366 comprises a digital microcomputer or other programmed digital signal processing apparatus. An analogue relative suspension velocity signal is digitised for use in such apparatus; and the digital signal processing apparatus thus samples the analogue signal at a predetermined sampling frequency, for example 1 KHz. In order to prevent aliasing distortion, the sampling frequency is at least twice the highest frequency in the analogue input signal. Thus, any component of the analogue relative suspension velocity signal above the aliasing frequency of 500 Hz, half the sampling frequency, is suppressed. This means that the gain of the differentiator circuit, in combination with any additional filters, decreases to a predetermined low value at the aliasing frequency of 500 Hz. Typically, the natural high frequency roll-off of the inherent circuit capacitance is insufficient to accomplish this task and further circuit elements are added to produce additional low-pass filter poles and thus accelerate the roll-off with frequency. However such low-pass filter poles also cause accelerated phase change with frequency, which introduces group delay. This group delay can degrade the performance of a real time suspension control system. Increasing the number of low-pass filter poles increases the roll-off in gain but also increases the group delay. In fact, the main problem becomes a tradeoff between high frequency gain roll-off and group delay so that it is difficult to provide simultaneously (1) the required differentiator characteristic in the range of vehicle suspension resonant frequencies, (2) sufficient roll-off in gain at the aliasing frequency of analogue to digital conversion, and (3) sufficiently low group delay in a differentiator circuit. The present invention seeks to provide an improved suspension controller. According to the present invention, there is provided a suspension controller for a vehicle suspension as specified in claim 1. In a preferred embodiment the differentiator comprises a bandpass filter having a linearly increasing gain and a 90 degree phase lead in a low frequency range of vehicle suspension resonances in which signal differentiation is desired, a peak in gain and group delay at an intermediate frequency above the low frequency range of vehicle suspension resonances in which vehicle suspension inputs are small, and a decreasing gain from the intermediate frequency to a higher aliasing frequency related to the sampling frequency of the digital signal processing apparatus. Preferably the differentiator comprises an operational amplifier with a first resistor connected in negative feedback around the operational amplifier. A differentiator input is provided through a first capacitor connected to an inverting input of the operational amplifier. The first capacitor and first resistor provide a linearly increasing gain and 90 degree phase lead for differentiating action over the low frequency range of vehicle suspension resonances. The differentiator may also comprise a second resistor connecting the differentiator input in series with the first capacitor and a second capacitor connected from an output of the operational amplifier to the junction of the second resistor and first capacitor. The second resistor and second capacitor, together with the first resistor and first capacitor, preferably form a high quality factor (Q) bandpass filter with a peak at an intermediate frequency above the low frequency range of vehicle suspension resonances. The second resistor and second capacitor together can provide a falling gain from the intermediate frequency to provide a predetermined low gain at a higher aliasing frequency related to the sampling frequency of the digital signal processing apparatus. The differentiator may also include a third capacitor connected in parallel with the first resistor to control the quality factor (Q) of the circuit so as to balance the amplitude of the peak in gain at the resonant frequency with the group delay in the lower frequency range of vehicle suspension resonances. Although group delay may be substantial above the low frequency range of suspension resonances, due to the fast changing phase near the intermediate frequency, the suspension inputs in this frequency range are minimal. Minimal group delay is maintained in the low frequency range of suspension resonances by the substantially constant 90 degree phase lead through this frequency range. The group delay of the differentiator in the low frequency range is sufficiently low that a two-pole low-pass filter may precede the differentiator for additional rejection at frequencies above the intermediate frequency. The resulting combination of low-pass filter and differentiator circuits can have high frequency noise rejection and low frequency group delay providing satisfactory service in a digital vehicle suspension control system at comparatively low cost. An embodiment of the present invention is described below, by way of illustration only, with reference to the accompanying drawings, in which: Figure 1 shows a schematic diagram of a motor vehicle incorporating an embodiment of suspension controller; Figure 2 is a block diagram of part of the controller of Figure 1; Figure 3 is a circuit diagram of a differentiator for use in the suspension controller of Figures 1 and 2; Figure 4 is a circuit diagram of a low-pass filter for use in the suspension controller of Figures 1 and 2; and Figure 5 is a series of graphs of gain, phase and group delay for the differentiator and low-pass filter of Figures 1-4. Referring to Figure 1, a motor vehicle has a body 10 comprising a sprung mass. Body 10 is generally rectangular in shape and is supported on a wheel 11 at suspension points at each corner by suspension apparatus 12 comprising a weight bearing suspension spring in parallel with a suspension actuator connected to exert a controllable force in parallel with the spring between the body 10 and wheel 11 at that suspension point. The suspension actuator may be a hydraulic or electric actuator capable of providing power to the suspension, as used in a fully active suspension controller. Alternatively, the actuator may be a variable damper, which is capable only of dissipating power, as used in a semi-active suspension controller. As a variable damper, the actuator may provide damping control in either a continuous or a discrete manner. Suspension apparatus 12 further includes an axle for rotatably supporting wheel 11 and such other suspension components, such as control arms, as are required and comprise the unsprung mass of a standard vehicle suspension. At each corner of body 10, a suspension position sensor 13 is connected between the body and unsprung mass to measure the relative vertical position thereof and generate an output suspension position signal for input to a controller 15. The relative suspension position signal is differentiated to produce a relative suspension velocity signal. An acceleration sensor 16 may be positioned at each corner of body 10 to generate an absolute vertical acceleration signal of that corner of body 10 for input to controller 15. The absolute body corner velocities at the four corners of the body may be derived from these acceleration signals. From the difference between the body corner velocity and relative suspension velocity at each corner, controller 15 is able to compute the vertical wheel velocity at that corner; and from the vertical wheel velocity and various body corner velocities, controller 15 is able to compute the desired force between the body and wheel at that corner of the vehicle. Additional signals which may optionally be generated are a vehicle deceleration or braking signal by brake sensor 17, a vehicle speed signal from vehicle speed sensor 18 and a vehicle steering signal from vehicle steering sensor 19. The hardware configuration of suspension apparatus 12 and controller 15 is described in more detail in US-A-5,071,157, the disclosure of which is incorporated herein by reference. For the purpose of this description, it is sufficient to state that controller 15 comprises at least a digital microprocessor 30 having a central processing unit (CPU), random access memory RAM, and read only memory ROM; and the relative suspension position signals from position sensors 13 are provided to microprocessor through input/output (I/O) apparatus. Input/output apparatus comprises a low-pass filter for receiving and filtering the relative suspension position signals from position sensors and a differentiator circuit for differentiating the filtered signals and providing relative suspension velocity signals to microprocessor. Either the input/output apparatus or the microprocessor includes an analogue to digital converter to convert the relative suspension velocity signals to digital form at a predetermined sampling frequency of 1 KHz. A representative circuit for use as low-pass filter is shown in Figure 4 for providing two-pole low-pass filtering with a half power frequency sufficiently above 10 Hz to affect minimally signals having frequencies of 10 Hz or lower. Referring to Figure 4, an operational amplifier 40 has its non-inverting input grounded and its inverting input connected to its output through a negative feedback resistor 41 (of 94kΩ resistance) and capacitor 42 (of 0.01µF capacitance) connected in parallel. An input terminal 46 in use receives the relative suspension position signal from a sensor 13 and is connected through two resistors 43, 44 (each of 47kΩ resistance) in series with the inverting input of operational amplifier 40. The junction of resistors 43 and 44 is connected through a capacitor 45 (of 0.022µF capacitance) to ground. The circuit is a standard two-pole active low-pass filter, with a half power frequency of approximately 100 Hz and a stop-band slope of 12 dB/octave and provides at its output a filtered relative suspension position signal. The differentiator circuit is shown in Figure 3. An operational amplifier 50 has a grounded non-inverting input and an inverting input connected to an output through a negative feedback resistor 51 (of 820kΩ resistance) and capacitor 52 (of 0.001µF capacitance) connected in parallel. An input terminal 56, which receives the filtered relative suspension position signal from the output of low-pass filter 32, is connected through a resistor 53 (of 8kΩ resistance) connected in series with a capacitor 54 (of 0.1µF capacitance) to the inverting input of operational amplifier 50. Finally, a capacitor 55 (of 0.01µF capacitance) is connected from the output of operational amplifier 50 to junction 57 of resistor 53 and capacitor 54. In the circuit of differentiator 33, capacitor 54 and feedback resistor 41 provide a differentiating characteristic of a linearly increasing gain through a low frequency range of 0.5 to 20 Hz and a 90 degree phase lead substantially through the same frequency range. This range includes, at its low end, the 1 Hz typical resonance of vertical vibration of a passenger vehicle sprung mass. It further includes, at its high end, the 8-10 Hz typical resonance of vertical vibration of a passenger vehicle unsprung mass. It also brackets the 5-6 Hz typical frequency of vertical vibrations to which the human body is most susceptible in a passenger vehicle. Thus, these components provide a differentiating action in the frequency range of greatest interest to a passenger vehicle suspension control system. This can be seen with reference to the graphs of Figure 5, the curves 60, 70 and 80 of which depict gain, phase and group delay, respectively, as functions of frequency, for a system including low-pass filter 32 and differentiator 33. It can be seen that in the frequency range 1-10 Hz the gain is increasing linearly, the phase is constant (90 degrees leading) and the group delay is constant (3 msec). This low group delay is achieved even with the contribution to group delay from low-pass filter 32. The curves emphasize the differentiating action with low group delay in the frequency range in which suspension related signals will appear, due to vehicle resonances. In an intermediate frequency range above the low frequency range of suspension resonances but below 500 Hz, the gain 60 continues to increase to a peak at a frequency of approximately 60 Hz and thereafter falls at the rate of 18 dB/octave, of which 12 dB is due to low-pass filter 32, as previously described, and another 6 dB is due to differentiator circuit 33. The phase 70 starts changing significantly in the lagging direction as it passes 10 Hz and reaches its greatest rate of change at approximately 60-100 Hz, after which the change gradually slows but continues within the range. The group delay 80 increases rapidly in the range 20-60 Hz to a peak of approximately 10 msec, after which it falls rapidly to an insignificant level within the range. Clearly, both gain and group delay reach maximum values within this range which would be unacceptable in the lower frequency range of suspension resonances. However, since suspension inputs in this intermediate frequency range are much smaller and excite no significant resonances in the vehicle, the larger gain and group delay have no significant effect on the output signal of the differentiator. At the aliasing frequency of 500 Hz, the gain of the circuits has fallen significantly: in this embodiment, to unity or below. It continues to fall with frequency in the high frequency range above 500 Hz. The phase, which eventually approaches 270 degrees lag, and the group delay are substantially irrelevant, since the gain is so low that signals at these frequencies are substantially suppressed. Thus, aliasing distortion is avoided. In this manner, the differentiator derives a relative suspension velocity signal in the suspension control system. The resulting circuit is basically a relatively high quality factor (Q) bandpass filter having differentiating characteristics on the low frequency side of an intermediate frequency of peak gain which are controlled by the RC time constant (0.082 in this embodiment) of resistor 51 and capacitor 54. The differentiating gain is predetermined for the vehicle sprung and unsprung masses, which limit the maximum relative suspension velocity, the range of the displacement sensor and the voltage range of the differentiator circuit power supply. The peak frequency and roll-off characteristics on the high side thereof are controlled by the RC time constant of resistor 53 and capacitor 55, as well as the coordinated values of the components of low-pass filter 32. However, this circuit, without capacitor 52, can have a very high quality factor (Q) with high noise amplification at the intermediate frequency and a tendency to ring. Therefore, capacitor 52 provides an adjustment of the quality factor (Q) of the circuit to control its resonance. In the embodiment shown, an increase in the capacitance of capacitor 52 will provide a lower and flatter peaked gain 60 for lower high frequency noise. In addition, phase change 70 will begin at a lower frequency and proceed somewhat slower with increased frequency. Therefore, group delay 80 will be lowered at its peak but will be greater in the low frequency range of suspension resonances. A decrease in the capacitance of capacitor 52, on the other hand, produces a higher quality factor (Q) for a higher gain peak and greater high frequency noise. It also provides a sharper and later change in phase for a higher group delay at the intermediate frequency but a lower group delay in the lower frequency range of suspension resonances. Unfortunately, it also provides a greater tendency for instability and ringing. Capacitor 52 allows the circuit designer to achieve the optimum balance between the amplitude of peak gain, which affects stability and high frequency noise, and group delay in the low frequency range.

Claims: A suspension controller for a vehicle suspension provided between a sprung mass and an unsprung mass and which includes a suspension actuator and a relative suspension position sensor; the suspension controller comprising a digital signal processor (30) for receiving and digitising at a predetermined sampling rate an analogue relative suspension velocity signal, and a differentiator (33) for deriving the analogue relative suspension velocity signal from an analogue output of a relative suspension position sensor, characterized in that the differentiator comprises a filter including a substantially linearly increasing gain and a substantially 90 degree phase lead in a low frequency range of vehicle suspension resonances, a peak in gain and group delay at an intermediate frequency above the low frequency range, at which intermediate frequency vehicle suspension inputs are relatively small, and a decreasing gain from the intermediate frequency to a higher aliasing frequency related to the sampling frequency of the digital signal processor (30). A suspension controller according to claim 1, wherein the filter of the differentiator (33) comprises an operational amplifier (50); a first resistor (51) connected to the operational amplifier so as to provide negative feedback; a first capacitor (54) connected to an inverting input of the operational amplifier so as to provide with the first resistor a substantially linearly increasing gain and a substantially 90 degree phase lead over the low frequency range; a second resistor (53) connected between an input of the differentiator and the first capacitor; a second capacitor (55) connected between an output of the operational amplifier to the junction between the second resistor and the first capacitor; the first and second resistors and the first and second capacitors forming a high quality factor (Q) bandpass filter with a peak in gain at the intermediate frequency; the second resistor and second capacitor together providing a decreasing gain from the intermediate frequency to the aliasing frequency; and a third capacitor (52) connected in parallel with the first resistor to control the quality factor (Q) of the filter substantially to balance the amplitude of the peak in gain at the resonant frequency with the group delay in the lower frequency range. A suspension controller according to claim 1 or 2, comprising a low-pass filter (32) in use connected between a relative suspension position sensor and the differentiator (33) for reducing the gain above the intermediate frequency. A suspension controller according to claim 3, wherein the low-pass filter (32) is a two-pole low-pass filter. A suspension controller according to any preceding claim, wherein the low frequency range of vehicle suspension resonances includes the frequencies of 1 Hz and 10 Hz. A suspension controller according to claim 5, wherein the low frequency range comprises the frequencies from substantially 0.5 Hz to substantially 20 Hz. A suspension controller according to any preceding claim, wherein the aliasing frequency is substantially 500 Hz.

Inventors: FULKS GARY CHRIS, FULKS, GARY CHRIS
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1993
Main classification: B60G17
IPC codes: B60G17, B60G23, G06G7