Tow bar arrangement for control of a self-propelled trailer

Tow bar arrangement (100) for interconnection of a pulling vehicle (10) and a pulled trailer (20). The arrangement (100) comprises a control device (110) with a force sensor (111) arranged to sense an instantaneous relative force between vehicle (10) and trailer (20). The control device (110) is arranged to produce a control signal for an electric motor (21) arranged to propel the trailer (20) so that a motor force counter-acts said measured force. The invention is characterised in that the control device (110) is arranged to a first time series of force measurements, in that the control device (110) comprises a low-pass filter (112) producing a second low-pass filtered time series, and in that a regulator (113) is arranged to regulate said second time series to produce said control signal.

The present invention relates to a tow bar arrangement. In particular, the invention relates to a tow bar arrangement for use with an at least partly manually propelled vehicle, whereby the vehicle pulls a trailer using the tow bar.

It is well-known to arrange trailers of various types to be pulled by bicycles or other vehicles that are at least partly manually propelled by a user. For instance, trailers such as trailing carriages for children and various types of cargo are sold. Normally, such a trailer includes a tow bar arrangement for fastening the trailer to the vehicle so that the trailer can be pulled by the vehicle when the vehicle is propelled by the user.

More specifically, a parent can for instance connect a child trailer behind a bicycle using such a tow bar arrangement, such as connecting to the back hub of the bicycle and to a supporting structure of the trailer. The child can be placed in the trailer behind the bicycle so that the parent can bring the child along as the parent rides the bicycle.

Such trailers generally simplify logistical endeavours of users of bicycles and other manually or semi-manually propelled vehicles, by allowing larger amounts of cargo to be transported than what would otherwise be possible.

However, more cargo also means heavier load on the person propelling the vehicle. It would be desirable to be able to take more cargo without having to exert heavy pedalling force.

It is also known to provide bicycles with an electric help motor for helping the propelling user.

However, even with such a bicycle it may be difficult to tow a heavy trailer under various conditions, such as going downhill, uphill, at different speeds and/or with more or heavy load.

Another problem is to allow relatively powerful and/or heavy trailers to be towed by a manually or semi-manually propelled vehicle, such as a bicycle with or without a helper motor. Trailers having a powerful motor and/or being heavy may be difficult to control when being towed by such vehicles.

Hence, it would be desirable to provide a safe yet simple, inexpensive and robust way to allow a user of a vehicle to safely and comfortably tow a trailer, even a heavy and/or powerful trailer, without having to exert excessive force in doing so. Such a solution should preferably be compatible with a wide variety of vehicles without requiring complicated calibration or configuration.

The present invention solves the above described problems.

Hence, the invention relates to a tow bar arrangement for interconnection of a vehicle and a trailer for pulling the trailer by the vehicle, wherein the tow bar arrangement comprises a control device in turn comprising a force sensor arranged to sense an instantaneous pulling or pushing force with respect to the vehicle in relation to the trailer, wherein the control device is arranged to measure said instantaneous force using said force sensor and to produce a control signal for an electric motor arranged to propel the trailer so that a force developed by said motor counter-acts said measured force, and is characterised in that the control device is arranged to measure said measured force to produce a first time series, of force measurement values, in that the control device comprises a low-pass filter arranged to filter said first time series to produce a second time series, of low-pass filtered force measurement values, and in that the control device furthermore comprises a regulator, arranged to regulate said second time series to produce said control signal.

The Figures share the same reference numerals for same or corresponding parts.

Hence,FIG.1shows a tow bar arrangement100for interconnection of a vehicle10and a trailer20, and specifically for pulling the trailer20by the vehicle10when travelling on the ground G. The interconnection is a physical interconnection, achieved using a tow bar101of the tow bar arrangement100, resulting in that the trailer20substantially follows movements of the vehicle10(it being understood that small movements between the vehicle10and the trailer20can occur and be absorbed by a suspension of the tow bar100or similar, see below).

The vehicle10can be any type of vehicle used by a person for transport. However, it is preferred that the vehicle10is a completely or partly manually propelled vehicle. That a vehicle is “manually propelled” means that a user uses muscle power to set the vehicle10in motion. The vehicle10may also comprise an internal motor, such as an electric helper motor. A vehicle10both being arranged for manual and motor-assisted propulsion is herein denoted “semi-manually propelled”.

The vehicle10may hence be a bicycle with or without a helper motor. The vehicle10may furthermore be a moped or similar light-weight motorcycle. It is preferred that the vehicle10is a two-wheeled vehicle.

As will be discussed below, a tow bar arrangement100according to the present invention may also be useful for interconnecting several trailers20to form a trailer train300, in which case one trailer20may be the “vehicle” of another trailer, being towed by the first trailer20via the tow bar arrangement100.

The trailer20may be any suitable trailer, such as a bicycle trailer for transport of children and/or cargo. The trailer20may be a combined children's trailer and bicycle trailer, such as is well-known as such in the art.

The tow bar arrangement100may be a part of the trailer20, or be separately provided, for instance to replace (retrofit) an existing tow bar provided with the trailer20.

The tow bar arrangement100comprises a control device110. The control device110performs control as will be discussed hereinbelow. This control mechanism may be implemented in purpose-built electronic hardware or a combination of general-purpose electronic hardware and a suitable specifically designed software function arranged to execute on said hardware, as the case may be. The control device110may comprise a battery and/or be powered from an external source, such as from a battery comprised in said vehicle10and/or said trailer20.FIG.1illustrates a battery23of the trailer, for illustrative and exemplifying purposes.

InFIG.1, the control device110is arranged on (along) the tow bar101. However, it is realised that the control device110may be arranged in different locations, depending on the concrete application. For instance, the control device110may be arranged at or in a connection between the trailer20and the tow bar101; at or in a connection between the vehicle10and the tow bar101; or in any other suitable location. The control device110may also be distributed across more than one pieces of hardware.

The control device110is interconnected for communication with the trailer20, and in particular with a motor21of the trailer20, and with the below-described sensors. These communication interconnections may be wired and/or wireless, using suitable analogue and/or digital communication protocols as is well-known in the art as such.

Moreover, the control device110comprises a force sensor111(seeFIG.3), arranged to sense an instantaneous pulling or pushing force with respect to the vehicle10in relation to the trailer20. Hence, when the vehicle10and the trailer are pressed together, such as because the vehicle10is braked during transport and/or the trailer20is propelled in an accelerating manner by its motor21to speeds above that of the vehicle10, the sensed force is a “push” force; and when the vehicle10and the trailer are pulled apart, such as because the vehicle10is propelled by the user at speeds higher than that of the trailer20and/or the trailer20is braked during transport, the sensed force is a “pull” force.

That the force sensor111measures an “instantaneous” force means that the force measurement in question relates to a present force between the vehicle10and the trailer20. Such measurement may be performed continuously or intermittently, preferably at least 5 times per second or even at least 10 times per second. Measurement values may be immediately pushed to the control device110upon reading, or be queried from the control device110, such as at regular intervals.

The force sensor111may, by way of example, be a strain gauge sensor, an impedance force sensor, a HALL sensor or a magnetoelastic sensor.

Using this force sensor111, the control device110is arranged to measure said instantaneous force and to produce a control signal U[t] for the electric motor21of the trailer. In other words, the control device110produces said control signal U[t] to control an electric power applied by the electric motor21to propel the trailer20. This means that the control signal U[t] produced by the control device110indirectly controls a speed, or at least the above described force between the vehicle10and the trailer20. It is preferred that the power applied by the motor21to the trailer20is arranged to be entirely controlled by the control device110, via said control signal U[t], at least intermittently or in at least one control program (in particular the below-described second control program) implemented by the control device110.

In generally, the control device110may be arranged to produce said control signal U[t] as a braking or electricity generation signal to the motor21in case the measured force is a pushing force.

In particular, the motor21is arranged to propel the trailer20so that a force developed by said motor21counteracts the above discussed measured force between the vehicle10and the trailer20.

It may be so that the trailer20comprises its own control device for controlling the motor21. When applying the present invention, the control device110at least partly, and preferably fully, replaces or supplements such an existing control device of the trailer20, so as to implement the control according to the present invention. This may imply entirely bypassing such an existing control device, or providing a communication between the control device110and the existing control device so that the control device110at least partly controls the existing control device.

The tow bar arrangement100may also comprise the electric motor21. For instance, the tow bar arrangement100may comprise, in addition to the tow bar101and the control device110, also the motor21and possibly even one or several wheels22of the trailer20. Then, one or several existing wheels of the trailer may be replaced (retrofitted) by corresponding electrically driven wheels, the motors of which are directly controlled by the control device110in a tow bar arrangement according to the present invention.

According to the invention, the control device110is arranged to measure said measured force between the vehicle10and the trailer20so as to produce a first time series, of force measurement values. Preferably, the first time series is a continuously updated series of force measurement values, with the last measurement value in said series being a last measured value. Hence, the first time series may be continuously updated by a most recently measured force value being added at an end of the series, possibly deleting one or several measurement values from the opposite end of the series, in other words a moving window type time series.

Further according to the invention, the control device110comprises a low-pass filter112(seeFIG.4a), arranged to filter the above mentioned first time series to produce a second time series, of low-pass filtered force measurement values. The second time series may be of the same vector size as the first time series and may be of a similar moving window constitution.

In addition to the low-pass filter112, the control device110also comprises a regulator113, arranged to regulate said second time series to produce said control signal U[t]. Expressed differently, the regulation performed by the regulator113produces the control signal U[t], which is then fed, directly or indirectly via circuitry in the trailer20, to the motor21. The motor21is power regulated using the control signal U[t], which is hence output from the regulator113.

It is understood that the low-pass filter112and the regulator113may be implemented using any suitable technology, such as suitable purpose-designed hardware, purpose-designed software executing on general-purpose hardware; or a combination of these. Preferably, all functionality of the control device110is implemented in the digital domain, using digital signal processing so as to achieve said control signal U[t].

The low-pass filter112may receive, as a feedback signal based upon which the regulation in the regulator113takes place, a currently/recently measured value of the above described force F[t−1] between the vehicle10and the trailer20, and said control signal U[t] may be produced so as to regulate this force F[t−1] to a desired force value. Such a desired force value may be predetermined or in turn depend on one or several variable parameters, such as a current trailer20inclination, a current velocity and so forth. Examples with be provided below.

Hence,FIG.4aillustrates an exemplary control loop useful in a tow bar arrangement100according to the present invention, in which t denotes time and t−1 denotes the time one sample period prior to the time t. F[t] denotes force at time t; Fd[t] denotes, at time t, the desired force to which the control loop is arranged to control the relative force between the vehicle10and the trailer20. This desired force may be static or dependent on circumstances, such as current velocity and/or inclination of the vehicle10and/or trailer20. T[t] denotes motor torque at time t. U[t] is the control signal, at time t, implying a corresponding torque applied by the motor21onto a wheel axis of a trailer20wheel22.

As can be seen fromFIG.4a, the low-pass filter112performs a filtering of a force signal being calculated (such as by a sum function) based on said desired value Fd[t] and further based on said measured relative force F[t−1] between the vehicle10and the trailer20. The output of the low-pass filter112is used to calculate a desired motor torque value, which in turn is used, in combination with a measured torque value T[t−1] output from a suitable sensor in the motor21, such as using a sum function, in turn being fed into the regulator113. The torque required may be represented by a gain value that is set to the motor21. In practise, the regulation of the desired motor torque value can be achieved based on a priori knowledge of a known relationship (such as a linear relationship) between input current and produced torque, which relationship is a characteristic of the motor used. The current may be supplied by setting a corresponding voltage. Hence, for the regulation in terms of a desired torque value is equivalent to a regulation in terms of a supplied current or even an applied voltage, via known motor-specific relationships. In typical motors, the current input may be 0-20 A, and the achieved torque may be 0-15 Nm peak; 0-8 Nm average.

Hence, the regulator113may be arranged as a part of an inner control loop, while the low-pass filter112may be arranged as a part of an outer control loop. The final control signal value U[t] is regulated to be a control signal value controlling the motor21to apply a dynamically changing power with the aim of achieving the desired relative force Fd.

The present inventors have discovered that such a setup, with a low-pass filter112in combination with a regulator113for controlling the motor21, results in greatly improved user comfort. In particular, such a basic setup in combination with one or several of the various specific implementation aspects described herein, depending on application, makes it possible for a user to handle even very powerful and/or heavy trailers20without increasing the risk of accidents.

In a particularly preferred embodiment, the control device110is arranged to produce said control signal U[t] so that the trailer20does not push the vehicle10, preferably so that the trailer20will never push the vehicle10irrespective of operating prerequisites as long as the trailer20has the capacity of braking enough (see below).

Put differently, the control signal U[t] may be produced to always achieve a “pulling” force between the vehicle10and the trailer20. This pulling force, which is effectively controlled using the control signal U[t] produced by the control device110, is preferably small, such as maximally 20%, such as maximally 10% or even maximally 5%, of a force currently provided onto the trailer20by the motor21via a torque applied by the motor21to the wheel22of the trailer20.

In particular, the control device110may be arranged to produce said control signal U[t] so that the motor21counteracts the measured force, between the vehicle10and the trailer20, to at least 80%, or even at least 90% or even 95%, and at the most 100%, or preferably at the most 98%.

This counteraction is then provided as long as the motor21is capable of producing sufficient instantaneous power, since it is possible that the user on the vehicle10is able to produce an instantaneous power which is significantly higher than the maximum power output of the motor21. In the latter case, the motor21will typically run at full power to counteract the measured force between the vehicle10and the trailer20to as large a degree as possible.

As mentioned above, in case the vehicle10is braked the control device110may be arranged to activate a braking function of the trailer20with the aim of maintaining a non-pushing function of the trailer20. Such braking function may be a part of a control loop such as the one shown inFIG.4a, or be implemented using a separate control loop in the control device110. For braking, an internal electricity generating function of the motor21may be used (in which case the generated electrical power may be fed to the trolley20battery23), and/or the trolley20may be arranged with a separate brake activatable directly or indirectly (such as via the motor21) from the control device110.

It is realized that such regulation will regulate the force between the vehicle10and the trailer20to a value being close to zero but being a small pulling force, as long as the motor21(and or brake) is capable of producing sufficient power. The desired pulling force that the regulator113is arranged to use as its target value may be at least 5 N, such as at least 10 N. The desired pulling force may also be at the most 50 N, such as at the most 30 N.

This will provide a user experience with a small but not considerable effort increase when towing the trailer20, as compared to when not towing the trailer20, in a way which is substantially independent of a weight of the trolley20and also of if the vehicle10travels uphill or downhill. However, since the trailer20at no time pushes the vehicle10, even a heavy and/or powerful trailer will not jeopardise traffic security.

In order to offer adequate propulsion help when travelling on non-horizontal ground G surfaces (seeFIG.1), in particular when the force is regulated to be a small pulling force as described above, the control device110may further comprise a velocity sensor115and a trailer inclination sensor114.

Namely,FIG.3illustrates an exemplifying tow bar arrangement100arranged between two parts101a,101bof the tow bar101. Hence, the two bar101comprises two parts101a,101bbeing movable in relation to each other in a main movement direction of the vehicle10and the trailer20, where a first such part101ais arranged to be connected to the vehicle10while a second such part101bis arranged to be connected to the trailer20.

The tow bar arrangement100may comprise a resilient suspension130, arranged to absorb small horizontal movements of the trailer20in relation to the interconnected vehicle10. The resilient suspension130, in turn, may be spring-loaded both in a forward and a backward direction of the trailer20in relation to the vehicle10. In particular as illustrated inFIG.3, the resilient suspension130may comprise spring means116, such as comprising a pair of compression springs, such as a pair of metal coil springs, said compression springs being arranged on and supported by said first tow bar part101a, and said compression springs further being arranged on either side of a part117rigidly moving with said second tow bar part101b.

The above described force sensor111may then be arranged as a tension or compression sensor at said suspension130, and in particular at the suspension of said part117in relation to said second tow bar part101b. inFIG.3, the force sensor111is provided as a pair of pressure-sensitive pieces of material, one on either side of the part117and arranged to sense a relative instantaneous force, in a main travel direction of the vehicle10and the trailer20, applied between the first101aand second101btow bar parts.

FIG.3also discloses that the tow bar arrangement100comprises the velocity sensor115. In general, the velocity sensor115may be arranged to measure both an absolute velocity of the tow bar arrangement100and a relative velocity between the vehicle10and the trailer20. This may be achieved by the use of two sensors, one rigidly connected to the vehicle10and one rigidly connected to the trailer20. There may also be a separate sensor measuring a relative velocity difference between the vehicle10and the trailer20, in which case only one additional sensor is needed in order to measure an absolute velocity. InFIG.3, only one velocity sensor115is displayed, for reasons of simplicity.

FIG.3also shows said trailer inclination sensor114, arranged to measure an instantaneous inclination A (FIG.1) of the trailer20in relation to the horizontal, and hence substantially a ground G inclination A in relation to the horizontal.

It is realized that the control device110is in communication with all sensors described herein for reading of read measurement values, as described above.

The control device110may hence be arranged to measure, using said velocity sensor115, an instantaneous absolute velocity of the trailer20and/or of the vehicle10. Furthermore, the control device110may be arranged to measure, using said trailer inclination sensor114, an instantaneous inclination A of the trailer20in relation to the horizontal.

In this and other cases, the control device110may be arranged with at least two different control programs. As the term is used herein, “control program” means a set of control parameters or functions defining a distinct type of control of the motor21provided input from various connected sensors.

In particular, the control device110may be arranged to produce said control signal U[t] according to a first such control program in case said measured instantaneous velocity is non-zero and below a predetermined velocity threshold value, and said measured instantaneous inclination A in relation to the horizontal is simultaneously above a predetermined inclination threshold value.

The predetermined velocity threshold value may be at least 3 km/h and at the most 10 km/h, preferably 5-7 km/h, such as around 6 km/h. The predetermined inclination threshold value may be at least 1% and at the most 10%, such as between 2-5%, such as around 3%.

In the opposite case, that is in case said measured instantaneous velocity is not below said predetermined velocity threshold value or said measured instantaneous inclination A in relation to the horizontal is not above said predetermined inclination threshold value, the control device110may instead be arranged to produce said control signal U[t] according to a second or further control program. Whether the second control program is selected, or a further available control program, such as a third or fourth available control program, when the first control program is not selected, may be determined based on certain parameters that can be read or determined by the control device110. Such parameters may include one or several of the measured inclination A; the measured velocity; a measured acceleration of the vehicle10and/or the trailer20; a current weight of the trailer20; and user-set or predetermined threshold values for one or several of these parameters.

Said first control program may be defined so that, when the control device110, when being operated in the first control program, is arranged to produce said control signal U[t] so that the motor21is operated either at a predetermined power or at a power which is dependent on said measured instantaneous inclination A in relation to the horizontal. One option is to use a control loop designed so that, the larger the inclination A, the larger the power of the motor21. Alternatively, a predetermined fixed power, such as a maximum available motor21power, may be used irrespectively of the inclination A as long as the first control program is used.

Furthermore, the first control program may be defined so that, when the control device110is operated in said first control program, it is arranged not to apply the above mentioned low-pass filter112. However, the second control program, and possibly also any further control program mentioned above, is then defined so that, when the control device110is operated in that control program, it is arranged to apply said low-pass filter112.

Preferably, the first control program uses both the low-pass filter112and the regulator113in some or all the ways illustrated inFIG.4aand as described in connection to thatFIG.4a.

In contrast,FIG.4billustrates an example of a control loop which the control device110can use to produce the control signal U[t] when being operated in said first control program. An inclination value A currently measured by the inclination sensor112is used to calculate the control signal U[t] which is fed to the motor21so as to achieve a desired torque tot eh trailer20wheel22.

Using such first and second (and possibly further) control programs, a user of the vehicle can get adequate help to start under uphill conditions without jeopardising security once having started.

In order to avoid false positive readings, the control device110, while in said second or further control program, may be arranged not to activate said first control program until said measured instantaneous velocity has been measured to be non-zero and below said predetermined velocity and said measured instantaneous inclination A in relation to the horizontal has simultaneously been measured to be above said predetermined inclination across the entire completion of a predetermined trailer20rolling distance and/or a predetermined trailer20wheel22rolling angle. In other words, even if the criteria for activating the first control program are met, the first control program is nevertheless not implemented before the criteria in question have been met for a certain time period. This time period can be measured in terms of trailer20rolling distance and/or trailer20wheel22rolling angle. This distance or angle may be measured using any suitable sensor, such as the velocity sensor115or a wheel22motor21angle sensor (such as the one described below).

In addition, the control device110, while in said second or further control program, may also be arranged not to activate said first control program if said predetermined trailer20rolling distance or predetermined trailer20wheel rolling angle takes more than a predetermined amount of time to be achieved. This predetermined maximum time may be measured using a clock of the control device110from a point in time when the first control program criteria were first met.

In addition, the control device110, while in said second or further control program, may also be arranged not to activate said first control program in case a currently measured velocity change of the vehicle10and/or the trailer20indicates that the two are currently decelerating from a velocity above said predetermined velocity to a velocity below said predetermined velocity, or a currently measured pushing relative force, since this would imply that the user is trying to come to a stop, which could happen in an uphill slope.

At any rate, in order to allow the user to brake efficiently and securely, the control device110, while in said first control program, may also be arranged to immediately activate said second or further control program (and hence inactivate the first control program) in case a decrease of said measured relative force between the vehicle10and the trailer20is detected, which decrease exceeds a predetermined threshold value or which decrease is to at least a predetermined pushing relative force between the vehicle10and the trailer20.

As explained above in connection toFIG.4a, the control signal U[t] may be arranged to, directly or indirectly, control a torque applied by said electric motor21, as opposed to a speed of the motor21or of the trailer20. It is understood that such a torque indirectly affects such a speed, but what is meant here is that the control loop of the control device110does not calculate or directly regulates such a speed but instead such a torque.

In addition to the force F[t−1] and torque T[t−1] feedback loops shown inFIG.4a, the control loop used in said first control program may use additional feedback loops or control mechanisms.

For instance, the control device110may be arranged to receive a measured instantaneous speed of the motor21and/or of the trailer20, and to feed this value into the regulator113as an input value affecting the determined control signal U[t]. For instance, the regulator113may implement a maximum speed of the vehicle10and/or of the trailer20; or the regulator113may use a relative velocity difference between the vehicle10and the trailer20to calculate a vibration pattern between the two and use that information when calculating the control signal U[t] so as to counter such vibration pattern (active dampening).

In another example, a measured electric current value, representing an electric current being fed to the motor21, may be monitored and the regulator113may cap the electric current, via the control signal U[t] at a predetermined electric current value so as not to damage the motor21, such as by overheating.

In general, the regulator113may a PI regulator or a PID regulator, the general properties of which regulator types are well-known per se in the art.

Turning to the low-pass filter112, this may have properties that filter out self-oscillating frequencies of the towing system30and non-wanted high frequencies. In particular, the low-pass filter112may be designed with a cut-off frequency ωcclose to a horizontal resonance frequency ωoscof a system30comprising the towed trailer or trailers20, the tow bar arrangement100itself and an interconnected towing vehicle10, or a main such horizontal resonance frequency ωosc. This resonance frequency ωoscis easily measured for different such systems for which the tow bar arrangement100is intended to be used.

That the cut-off frequency ωcis “close to” such a horizontal resonance frequency ωoscmeans, herein, that the cut-off frequency ωcis selected so that the low-pass filter112is arranged to substantially filter out said resonance frequency ωoscand higher frequencies, but substantially not to filter out lower frequencies. For instance, the cut-off frequency ωcmay be selected to be between 25% and 75% of said resonance frequency ωosc, or between 40% and 60% of said resonance frequency ωosc. Using such a low-pass filter112in combination with a regulator113as described above will generally provide for a smooth towing operation.

Ideally, the frequency response of the low-pass filter112is:

Implementing such an ideal filter, however, is not practical given the restricted hardware and software resources possible to build into a tow bar system100such as the present one. Therefore, the present inventors have identified a number of implementation details providing well-balanced compromises between computing resources, signal stability and ultimately towing performance under varying conditions. In particular, one object is to minimize ripple both in the passband and the stop band of the low-pass filter112.

A number of such advantageous implementation details will be described in the following.

Hence, the low-pass filter112may be designed as a FIR (Finite Impulse Response) filter. Such an (ideal, continuous) filter may be represented in the time domain as

FIG.5shows the impulse response of a corresponding discrete filter. It is noted that in practical applications, implemented using digitally operating hardware/software, the low-pass filter112will be implemented as a discrete defined filter.

As we want to suppress the resonance frequency ωoscand all higher frequencies, a sample rate for the above-mentioned relative force F[t−1] is selected as more than 2 times ωosc(to satisfy the Nyquist sampling criterion). Preferably, the relative force is furthermore oversampled to a certain extent. This oversampling should at the same time not be too large, since this has been found to result in that the filter characteristics of the low-pass filter112deteriorate.

If ωN=ωs/2 is the Nyquist frequency (ωsbeing the sampling frequency used), the present inventors have found the following relations to provide a well-balanced low-pass filter112:
ωN>=ωosc*3(oversample)
ωc<ωosc<ωN
ωc/ωN<0.5,preferably <0.1

In general, said relative force may be measured at a sampling frequency ωsof at least five times said horizontal resonance frequency ωoscof said system30.

In practical tests with systems30comprising a bicycle as vehicle10and a bicycle trailer20, it has been found that a cut-off frequency ωcof at least 0.2 Hz, such as at least 0.4 Hz; and at the most 1.5 Hz, such as at the most 1.0 Hz yields good results.

In practical applications, in which the low-pass filter112is implemented in digital electronics hardware/software, computation resources are limited and the impulse response of the low-pass filter112may be truncated to meet these limitations. In general, the low-pass filter112may truncated to use at least 10 and at the most 200, such as at the most 100 or even 60, samples and/or between 1-10 s, such as between 2-6 s of historic force measurement data.

FIG.6illustrates the impulse response for such a FIR filter which has been truncated to 21 consecutive samples.

The filter characterised inFIG.6is, however, not causal. In order to make the low-pass filter112causal, it may be time shifted at least 50% of the total (time) length of a used truncation. For instance, if the truncation is 21 samples, the time shift may be at least 11 samples. The trade-off here is that a truncation in combination with a time-shift of such a discretely defined low-pass filter will introduce a filter delay. The present inventors have discovered that using a time shift of at least 50% of the total truncation length provides an adequate compromise for a low-pass filter112used in the present application, together with a PI or PID regulator for controlling a motor on a towed trailer20. In particular, this compromise provides for a stable control loop and also low-pass filter112characteristics being sufficiently close to a corresponding ideal filter.

A comparison between such an ideal filter (broken lines) and a discrete FIR low-pass filter112designed in accordance with the above principles (full lines) is illustrated inFIG.7.

Hence, the control device110operated in said second control program, which may its default control program, is arranged to control the trailer20to, at all times whenever possible due to any motor21constraints, propel the trailer20so that it barely pulls the vehicle10in a direction opposite to a current movement direction of the vehicle10. In other words, the relative force between vehicle10and trailer20is controlled, by the control device110, to be the desired force—a force being close to zero but slightly negative (a slight pulling force).

As is illustrated inFIG.1, the present tow bar arrangement100may comprise a hub connection140arranged to be releasably fastened to the back hub of a bicycle. Such hub connections are known as such, and may for instance be of a quick-release type, or comprise a hub axis arranged to replace an existing hub axis of a bicycle to be used as the towing vehicle10according to the present invention.

Similarly, the tow bar arrangement100may comprise a trailer connection150, arranged to be releasably fastened to a supporting structure of the trailer20. Such a trailer connection150may be designed in any suitable manner, the important thing being that the tow bar arrangement100may in itself constitute a standalone device attachable to any compatible vehicle10and/or any compatible trailer20for interconnection of a vehicle10to a trailer20, or the tow bar arrangement100may be a fixedly integrated part of a trailer20arranged to be interconnected to any compatible vehicle10.

In fact, the present invention also relates to a trailer arrangement200comprising a tow bar arrangement100according to the present invention and also to a trailer20. Then, the tow bar arrangement100, and in particular to tow bar101, may be an integrated part of the trailer20, and even permanently attached to the trailer20.

Moreover in this case, the trailer20may in turn comprise the electric motor21arranged to propel the trailer20, and also the battery23arranged to power the electric motor21.

In particular in this case, the trailer arrangement200may be arranged to be connected, by said tow bar arrangement100, to a second trailer arrangement201of the same type (seeFIG.2), forming a train300of trailer arrangements200,201interconnected by said tow bar arrangement100.

In such a trailer arrangement200,201, the trailer arrangement200,201in question may be arranged to control its respective electric motor21based on said control signal as described above, so that the electric motor21in question exerts a force which instantaneously counteracts the measured relative force between the trailer arrangement200,201and its towing vehicle. For the trailer arrangement200, the relative force is measured between the vehicle10and the trailer arrangement's200trailer20; while for the trailer arrangement201in the example shown inFIG.2, the relative force is measured between the trailer20of the trailer arrangement200and the trailer20of the trailer arrangement201. This way, smooth and safe towing can be performed by one vehicle10of two or more trailer arrangements200,201that are connected one after the other in a train300of such trailer arrangements200,201.

It is understood that the trailer20and the tow bar arrangement100may be arranged as described above.

In a particularly preferred embodiment, the electric motor21is of a type in which the stator2comprises a number of stator2poles7which is not an integer multiple of a corresponding number of rotor3poles7and in which the stator2poles7are subdivided into at least three magnetically and electrically identical subsets8that are mounted one after the other around the angular direction of the electric motor21. This is detailed in the following.

Hence, the present invention may make use of a particular type of electrical motor illustrated inFIG.8, namely a brushless electric motor with a stator comprising a number of stator poles and a rotor comprising a number of rotor poles. In this type of motor, the stator comprises a number of stator poles which is not an integer multiple of a corresponding number of rotor poles, and the stator poles are subdivided into at least three magnetically and electrically identical subsets that are mounted one after the other around the angular direction of the electric motor. Preferably, the stator poles are equidistantly arranged along a stator periphery. Correspondingly, the rotor poles are preferably equidistantly arranged along a rotor periphery.

Preferably, a motor21of this type may be arranged to directly drive a respective wheel22of the trailer20. For instance, the motor21can be mounted on a wheel axis and drive a wheel22in relation to said axis, in which case two, preferably substantially identical, motors can be mounted on either side of the axis, driving one respective wheel22each. In this case, it is desired that the above-described control mechanism is performed in unison for each such individually driven wheel22. Alternatively, a motor21of said type can be mounted to drive a wheel axis in turn propelling two wheels22at the same time. Wheels22on more than one wheel axis may also be driven, in the corresponding manner. Hence, it is foreseen that between 1 and at least 4 motors21can be used with one trailer20. Preferably, all motors21are controlled by, and communicate with, the same control device110.

The use of such an electric motor21for propelling a trailer20assembly of the present type achieves a number of advantages.

Firstly, such a motor provides very low cogging of the motor, which provides for a smooth rotation of the wheel.

Secondly, this type of motor21can be precisely controlled, based upon a feedback algorithm taking into consideration rotation positional feedback from the motor itself, across a broad spectrum of rotation velocities and patterns. This includes the above-described control loops, but also a variety of other types of control loops that may also be implemented by the control device110.

For instance, such a motor21can be used to implement a free-wheeling control program, in which neither pushing nor braking force is added to the vehicle10but the trailer20simply rolls along as would a non-motorised trailer. In many situations, it may be desirable to be able to simply activate such a free-wheeling control program, for instance when the battery23is depleted or when a user of the trailer20wishes to manually manoeuvre a vehicle10with an attached trailer arrangement200. Hence, the control device110may be arranged to activate such a free-wheeling program when certain prerequisites are detected, including a manual activation by a user.

Thirdly, a motor21of this type allows being used as an efficient generator for motor braking the trailer20while recharging the battery23using the generated current.

At the same time, there are many types of electrical motors that are as such in general suitable for use in a trailer arrangement200of the present type, such as a conventional stepping motor or a conventional brushless motor of any other type. The present inventors have discovered that, in contrast thereto, a motor21of said specific type is particularly well-suited for the various trailer functionality described herein, and provides a very simple yet robust construction with a minimum of movable parts and wear details.

A motor of the above described type is, as such, known from EP 0996217 B1, which is specifically referred to herein for information pertaining to the motor as such.FIG.2of EP 0996217 B1, which is also shown, in an edited version, asFIG.8of the present application, schematically illustrates such a motor1, having a stator2and a rotor3. InFIG.8, the rotor3surrounds the stator2, it is however realized that the situation can also be the reversed.

The stator2is arranged with stator poles7, and the rotor3is arranged with rotor poles6. Between the poles6,7, there is a gap, such as an air gap, so that the poles6,7do not physically come into contact with each other. This allows the motor1to turn substantially without friction.

The rotor poles7are divided into pairs of alternate south pole4and north pole5rotor poles, preferably in the form of permanent magnets.

The motor is preferably arranged to be fed with an alternating current, either using a motor-internal AC/DC converter or using such a converter arranged as a part of, or an external part controlled by, the control device (see below). In the exemplifying case shown inFIG.8, the motor is a three-phase AC motor.

More precisely, the stator poles7are fed, by said AC/DC converter (which may comprise phase-shifting circuitry), with DC voltage according to the following:

In general, in the document EP 0291219 A1, to which specific reference is made herein regarding the details of the pole arrangement and power feeding to the poles of the motor1, it is described how to select such phase values, the number of stator poles7and the number of rotor poles6.

All poles with the same reference (A1, A3, A2, etc.) can be connected in series or, preferably, in parallel.

What is important for the present purposes is that the stator2comprises a number of stator poles7(in the present case45poles7) which is not an integer multiple of a corresponding number of rotor poles6(in the present case40poles6), and that the stator poles7are subdivided into at least three magnetically and electrically identical subsets8, or coherent zones, that are mounted one after the other around the angular direction of the motor1. This can also be expressed so that the stator2is arranged with a five-fold rotation symmetry with respect to its poles7. InFIG.8, the subsets8are five of number, but it is realized that they can be more or fewer, as long as they are at least three.

The combination of using such different numbers of poles6,7, with arranging the stator poles7in such subsets8, provides for low cogging in the motor1while supporting all the desired features of such a motor for the present purposes in a way which has proven very advantageous in practical tests by the inventors, especially in terms of efficiency, smoothness, noise levels and exactness.

Furthermore, the motor1supports using a hall position sensor9, schematically illustrated inFIG.8. The position sensor9may be arranged to sense the rotation position of the motor, or comprise logic so as to provide a value for the current rotation velocity directly.

According to a preferred embodiment, the position or velocity sensor9is a hall effect sensor, arranged to provide the control device110with rotary motor position or velocity information.

Above, preferred embodiments have been described. However, it is apparent to the skilled person that many modifications can be made to the disclosed embodiments without departing from the basic idea of the invention.

For instance, the tow bar arrangement100described herein may be arranged to control the motor21of the trailer20in even more complex ways than described herein, as long as the described principles are employed. For instance, the control programs implemented by the control device110may take into consideration parameters set as a part of a user profile, or may be dynamically updated based on parametric data characterising a currently employed vehicle10driving style or pattern.

Similarly, the trailer20itself may comprise many additional details and parts, in addition to the ones described herein.

Hence, the invention is not limited to the described embodiments, but can be varied within the scope of the enclosed claims.