The present invention relates to a process for improving the computation of the advance speed of a railway vehicle, when all the axles of the vehicle are in the slipping phase due to degraded conditions of adhesion on the rails.
The most precise knowledge of the advance speed of a railway vehicle is of particular importance for example for driving control systems, such as anti-skid systems, and for odometrical references installed on board.
A known method for accurately determining the speed of a railway vehicle is to maintain a “dead” axle, not subjected to traction or braking torques, so that the measurement of its speed is the best reproduction of the actual speed of said vehicle. This solution is particularly effective in the case of particularly low adhesion between the wheels and the rail, when, during traction or braking, all the wheels may enter into a slipping condition and therefore not be in a position to provide correct information regarding the actual speed of the vehicle. In this case, a “dead” axle not subjected to traction or braking torques could continue to be a reliable indicator of the vehicle speed.
The modern architectures of railway vehicles, especially in the case of subway vehicles, tend to have very limited compositions, e.g. they are made up of only two carriages. In such a case, maintaining a “dead” axle could lead to a significant loss of the train's traction and braking capacity.
An object of the present invention is therefore to propose a new method that allows one to fully recover the use of the “dead” axle for the purposes of traction and braking, even in the case of particularly reduced adhesion, thereby increasing the traction and braking capacity of the train, while permitting said axle to accurately track the speed of the train for a precise assessment of the advance speed.
The description of the present invention refers to the specific case of braking implemented by means of an anti-slip system. Those skilled in the art may however easily deduce a way to implement the present invention through an independent system. Also, those skilled in the art may deduce the dual application, relating to the case of traction, to which the subsequent claims of the present application refer.
The object defined above is achieved according to the invention with a method the salient features whereof are defined in the appended claim 1.
Further features and advantages of the invention will become apparent from the following detailed description, provided purely by way of non-limiting example with reference to the accompanying drawings, in which:
Electronic systems are installed on board most modern rail vehicles, which typically include wheel slide control subsystems, intended to intervene both when the vehicle is in the traction phase and when it is in the braking phase. These subsystems are known as anti-skid or anti-slide systems, or also WSP (Wheel Slide Protection) systems.
A system for controlling the adhesion of the wheels, as an anti-skid function, according to the prior art, is schematically represented in
In the drawings, only one wheel of each axle is generally illustrated.
The WSP system of
The electronic control unit ECU is provided to operate a modulation of the torque applied to each axle according to a predetermined algorithm if, in the case of applying torque during traction or braking in a degraded adhesion condition, the wheels of one or more axles end up in a possible incipient skidding condition. Torque modulation is implemented in such a way as to prevent a total locking of the axles, possibly so as to bring each axle into a condition of controlled slipping with the intention of recovering adhesion and in any case for the entire duration of the degraded adhesion condition.
The reference speed VR(t) is obtained as a fraction of the instantaneous speed of the vehicle, for example, according to the expression:
V
s(t)=Vs(t)˜(1δ) (1)
where VV(t) is the instantaneous (computed) speed of the vehicle, and δ represents the related slip of the axle A to be obtained during the skidding phase.
It is evident how the knowledge of the vehicle's instantaneous speed VV(t) is essential for properly controlling skidding.
In the event of braking, the algorithm most used for the estimation of the vehicle's actual speed VV(t) normally uses a function of the type:
V
v(Tj)=max [S1(Tj), . . . , Sn(Tj), (Vv(Tj−1)+amax·T)] (2)
while in the event of traction, the following function is used:
V
v(Tj)=min[S1(Tj), . . . , Sn(Tj), (Vv(Tj−1)+amax·T) (3)
where amax is the maximum acceleration permitted for the vehicle in operation, this acceleration having a positive sign in the case of a traction condition and a negative sign in the case of a braking condition.
The contribution (Vv(Tj−1)+amax·T) in the relationships (2), (3) serves to contain the variation of the Vv(t) within physical limits allowed by the train, when excessive instantaneous and concurrent variations of the speeds of the axles due to particularly degraded adhesion conditions, in traction or braking conditions, could lead to a loss of significance of the speed Vv(t) computed with these relationships (2), (3).
More accurate variants of the relationships (2), (3) are known but still based on the instantaneous measurement of the individual speed of the axles. It becomes evident here how the availability of a “dead” axle would make (2), (3) extremely accurate if all the axles were subjected to torque during skidding phase.
By way of non-limiting example, one possible embodiment of a torque control apparatus TC of the torque applied to an axle is described and illustrated in the previous Italian patent application No. 102015000086465 filed Dec. 22, 2015.
A torque control apparatus may be made according to many variants known to persons skilled in the art.
As is well known, the adhesion coefficient μ(δ) between wheels and rails varies according to the slip δ substantially in the way illustrated in
In
It has been found experimentally that the values of δ at the adhesion peaks a1, a2, a3 vary with the change in the adhesion conditions, which move along a curve as indicated at A in
Experimental measurements demonstrate how the curve A lies in an area corresponding to values 0≤δ≤0.02 even in very degraded adhesion conditions.
If one or more axles, for example the one previously defined as the “dead” axle, can be maintained on the curve A during traction or braking, it is achieved the dual effect of using, for said axles, the maximum available adhesion and at the same time tracking the actual speed of the train, corresponding to δ=0, with a maximum error of 2%.
F
m
·R=F
A
·R−J·{dot over (ω)} (5)
where:
F
A
=μ·m·g (6)
for which:
F
m
=μ·m·g−J/R·{dot over (ω)} (7)
where Fm is the tangential force applied to a wheel by the traction and/or braking system, R is the radius of the wheel, J is the moment of inertia of the axle, m is the mass applied to the wheel-rail contact area, and {dot over (ω)} is the instantaneous angular acceleration of the axle.
It is clear that at the same instantaneous angular acceleration, the maximum applicable force Fm is obtained in correspondence with the maximum value of adhesion μ, i.e. at the points lying on the curve A of
The method according to the present invention uses an adhesion observer to evaluate in real time the adhesion value μ at the contact area between the wheels and rails for one or more axles during a skidding phase and, by processing these μ values in real time, identifies continuously over time the δ value to be assigned to a slip control system.
An adhesion observer adapted to dynamically identify the instantaneous value μ(Tj) of the adhesion in a generic sampling period Tj of a predetermined duration T at the wheel-rail contact area during skidding is definable using the equation (7) from which with some simple steps the following relationship is obtained:
where
Downstream of the adhesion observer, a low-pass type filter may appropriately be provided, to remove or at least mitigate instantaneous and noise variations present outside of the frequency band useful for a correct observation of the adhesion values.
An embodiment of a system for implementing a method according to the present invention is illustrated in
The method provides for identifying and tracking the slip value δ of at least one axle, such that the curve μ(δ) illustrated in
For this purpose, a system implementing an LMS algorithm (Least Mean Square) may be used. For an accurate description of the general features of the convergence criteria and the implementation variants of LMS algorithms, please refer to the available literature and in particular to the text: B. Widrow, S. D. Stearns, “Adaptive Signal Processing”, New Jersey, Prentice-Hall, Inc., 1985.
With reference to
The output of the adhesion observer 1201 is connected to the input of a module 1202 which computes the value of the derivative
e.g. according to the equation:
where the value of δ is obtained in real time in accordance with the equation (4).
An adder 1203 outputs an error signal e(Tj) as the difference between the desired value of said derivative (i.e. the value 0) and its instantaneous value computed by the module 1202, and such error is used to adapt the LMS algorithm implemented in a block 1204.
The latter provides in output a torque request C(Tj+1) for said axle, which is transmitted to a torque control module 1205 of a per se known type, having for example the architecture described in the previous aforementioned Italian patent application with reference to
In a manner known per se, the module 1204 continuously corrects the output C(Tj+1) in order to minimize or nullify the error e(T), i.e. in order to obtain a nullification of the aforementioned derivative, i.e. in order to bring and maintain said controlled axle to the adhesion peak value.
By applying, therefore, the solution according to
A simplified implementation of the group of modules included in the dashed line block 1206 of
the integrator 805 increases the torque value C(Tj+1), when
the integrator 805 decreases the torque value C(Tj+1), and when
the integrator 805 keeps the torque value C(Tj+1) stable.
In this way, the system brings and maintains said controlled axle to the peak adhesion value.
The gain K regulates the identification speed of the average adhesion peak value μ and simultaneously ensures the stability of the closed loop system.
A further simplified variant of embodiment of the dashed block 1206 of
The output of the block 903 being equal to +1 or −1 (positive and, respectively, negative sign), a subsequent integrator 805 performs simple unitary sums.
The integrator 805 may be replaced with an up/down type counter updated with period T=Tj+1−Tj.
The diagrams according to
but requires me use or a certain number of computations in real time.
The diagram according to
The diagram according to
Therefore, the two expressions (2), (3) provided above always allow a very reliable value of the vehicle's speed Vv to be provided, even in very degraded adhesion conditions.
If it is desired to further increase the accuracy of the tracking of the train's speed, it is sufficient to compute the error with respect to values of
i.e. on the left side of the curve illustrated in
Naturally, without altering the principle of the invention, the embodiments and the details of implementation may vary widely with respect to those described and illustrated purely by way of non-limiting example, without thereby departing from the scope of the invention as defined in the appended claims.
Number | Date | Country | Kind |
---|---|---|---|
102016000034579 | Apr 2016 | IT | national |
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/IB2017/051902 | 4/4/2017 | WO | 00 |