FIELD OF THE INVENTION
The present invention relates in general to devices in which motion
is produced by a motor and in particular to the detection of a torque variation
of a reversible DC motor, that is the direction of rotation of which may be reversed.
BACKGROUND OF THE INVENTION
Sample applications of the present invention is for the detection
of an accidental block of an electric motor dedicated to lift and lower a glass
window of a car, a garage door and the like. In many applications of reversible
electric motors for moving parts that may accidentally be blocked by a number of
impredictable causes, it may be necessary to detect the presence of an obstacle
that stops the motion in order to protect either the motor and/or the transmission
organs from overloads that may damage them, or to prevent injuries when the obstacle
is a part of the human body (for example a finger, a neck etc.).
Figure 1 schematically depicts a motorized car window system. The
glass pane 2 of a door 1 of a car is moved by a DC motor 3 (M). The mechanical transmission
4 between the motor and the glass is sketched with a dashed line.
The motor 3 is driven by a full-bridge stage 5, composed of four switches
SW1, SW2, SW3 and SW4 that may be driven in pairs depending on the desired direction
of rotation of the motor, and thus of the desired direction (up or down) of motion
of the glass pane 2. The switches SW1 ... SW4 are connected in series along the
two parallel branches of the bridge, between the supply nodes 6 and 7. The switches
SW1, SW3 are connected in series to form a first branch between the nodes 6 and
7, and the switches SW1 and SW4 are connected in series to form a second branch
between the nodes 6 and 7. The motor 3 is connected to the intermediate nodes 8
and 9 of the two branches of the bridge Depending from the configuration of the
four switches, the current IM forced through the motor will circulate
either in a direction or in the opposite direction determining the direction of
rotation of the motor.
The switches SW1 ... SW4 are driven by a control circuit 10 CTRL in
function of a start command (H/L) bearing the information of the desired direction
of rotation. The control circuit 10 produces the four control phases for the respective
switches SW1 ... SW4. The bridge 5 and the circuit 10 are generally powered by a
regulator 11 (REG) of the car battery (not depicted) voltage (Vbat) providing a
regulated DC voltage Vcc. If required, the supply voltages of the bridge 5 and of
the control circuit 10 may even be different.
Generally, the current IM flowing in the motor 3 is monitored
and the instantaneous current information is input to the control circuit 10. In
Figure 1, the sensing of the current IM is indicated by an ampere meter
symbol 12 in series with the motor 3. In practice, this sensing may be done on a
sense resistor connected in series to one or to both branches of the bridge 5.
The instantaneous current information is used by the control circuit
10 in order to protect the motor against accidental overloads that may damage it.
The current sensing may be substituted or associated to the sensing of the temperature
of the motor in order to switch off electric supply when the temperature exceeds
a certain limit.
In any case, current and/or temperature sensing involve the fixing
of thresholds and are generally unsuitable for sensing torque variations of the
For instance, application of the type of a motorized window or door
with automatic detection of the end of the run it may often be required also a so-called
"anti pinch" function to prevent crushing with excessive force an accidental obstacle
such to cause injury to a limb or other part of a human body. Such an anti pinch
function must also reverse the motion such to open the window or door upon detecting
a pinch in order to free as quickly as possible the obstacle.
A problem is that comparing the instantaneous current or speed with
a threshold is not sufficient to have a reliable detection of an excessive torque.
In effect, the motor current IM may vary for other causes than that of
an obstacle blocking the movement of the part moved by the motor.
For a DC motor it is possible to write the following relations:
- VM = LM · dIM/dt+RM.IM
+ EM, where VM is the supply voltage of the motor,
IM is the current, LM is the inductance of the motor, RM
is the resistance of the motor and EM is the back electromotive force;
- EM = k · SM, where SM
is the rotation speed of the motor and k is a constant;
- KM = k'·IM, where KM is the motor
torque and k' is a constant; and
- KM - KR = JM ·
dΩM/dt, where KR is the resisting torque,
ΩM is the speed of the motor and JM is the moment of
inertia of the rotor.
The back electromotive force of the motor is thus directly proportional
to the rotation speed thereof while the motor torque is directly proportional to
the current. Except where expressly indicated, the word torque in the ensuing description
indicates the motor torque against the resisting torque representing the mechanical
load (e.g. the weight of the glass window) plus the friction (of the edges of the
glass pane sliding in the door guides).
Figure 2 is a diagram showing the range of variation and the general
characteristic of the current IM flowing in the motor when closing a
car window by lifting it up to its end of run. For sake of simplicity, oscillations
of the current due to noise (switchings of the motor brushes on the collector) are
The curve 21 (solid line) illustrates a mean functioning, that is
the closing of the glass pane under mean hygrometric and temperature conditions.
The user pushes a command button producing a close-window command
that is sent to the controller 10. The switches SW1 and SW4 or SW2 and SW3, depending
on the transmission, are turned on and the motor is powered. When the motor is switched
on (instant t0) the current IM flowing in its windings increases though
the speed is still null. The torque that is proportional to the current IM
increases. At a time t1, the current IM drops because the motor is starting
The start of rotation reduces the torque and thus the absorbed current,
until the time t2 when the current raises again. The instant t2 coincides with the
moment at which the glass pane (initially wholly retracted inside the door) is braked
by the horizontal gasket. Thereafter the torque and the current IM continue
to increase but at slower rate during the lifting of the glass pane.
Upon approaching the end of the lift run current and torque drop because
of the higher speed acquired by the glass pane and upon abutting against the upper
glass edge receiving channel, at t4, the speed becomes zero and the torque increases
End of run detection is generally carried out by means of a position
sensor that switch off the power to the motor with consequent drop to zero of the
The curve 22 traced with short dashes represents the current (and
thus the motor torque) in case of a wet glass subjected to a reduced braking action
by the gasket. The curve 23 traced with long dashes represents the case of an iced
glass. In this case, the torque necessary to lift the glass is definitely greater.
Should a relatively yielding obstacle (for example, a wrist) interfere
with the lift run of the glass pane, the run will be braked by the obstacle before
being definitely stopped. The motor will decelerate more or less abruptly, which
implies a drop of its back electromotive force and, as a consequence, a rise of
the current for increasing the torque.
In the absence of safeguard, an automatic lift of the glass could
be dangerous. Should a rigid (unyielding) obstacle be encountered similar consequences
are produced characterized by an instantaneous drop to zero of the rotation speed
and thus a more abrupt increase of the torque.
A difficulty of reliably detecting a blocked condition or an abrupt
deceleration is that for safety reasons the limit of tolerable compression of a
limb or finger may correspond to or be even weaker than the required lifting force
(accounting for the case of an iced glass window).
Another difficulty is that the required lifting force may increase
in time because of the aging of gaskets and linings and of progressive deformations
of parts of the mechanism and of the window structure.
Another difficulty is that abrupt variations of the motor torque stochastically
occur when lifting the window pane, for example when the car is travelling a road
with an irregular paving. Such in an occurrence, illustrated by the dot and dashed
peak 24 of the curve 21 of Figure 2, the resisting torque increases abruptly when
a wheel of the car rolls out of a pot hole. The torque and thus the current fall
abruptly. When the wheel gets out of the hole, the inverse phenomenon is produced,
that is the motor torque increases abruptly, and thus the current flowing in it.
For these reasons that render current monitoring approaches based
on the fixing of thresholds hardly discriminating, other techniques have been proposed
and are currently used.
Typically, mechanical or electromechanical devices are used as anti-pinch
sensor. Deformation of an elastic element between a mechanical detector and the
moving glass, when a body is being pinched, is a known approach. This technique
requires the use of an elastic conducting element of complex and costly installation.
Pinch detection may be performed by a switch fitted in a weather strip
of the car window. The switch that is normally off switches on when the weather
strip is subjected to a certain force.
This solution is rather simple but the particular weather strip assembly
implies a certain mounting complexity and maintenance may become necessary to maintain
effectiveness. Moreover, in some cases this approach does not meet safety specifications.
For instance, if the shape of the window determines a very slanted weather strip,
a pinching force may not be applied in a direction sufficiently orthogonal to the
strip, as shown in Figure 3. Therefore, the pressure necessary to activate the switch
may not be reached or may exceed the specified maximum value.
Alternative solutions are generally based on monitoring the motor
speed by the use of speed sensors (Hall effect sensors, encoders, and alike). When
a pinch condition occurs, the motor is blocked and its speed becomes zero. Therefore,
a pinch condition causes an abrupt speed variation, that may be detected by speed
Yet another known solution consists in processing a number of operating
parameter measurements such as temperature, supply voltage, motor speed and position
of the motorized part for comparing the actual displacement of the moved part in
respect to pre-established models. This approach requires a powerful calculator
and a large number of sensors for measuring the parameters necessary for the calculations,
besides the definition of particularly complex models in consideration of aging
OBJECT AND SUMMARY OF THE INVENTION
The present invention provides in general terms a novel and particularly
effective technique for detecting an increase of the torque of a DC motor without
the drawbacks and limitations of the known techniques.
More in particular the technique of this invention offers an excellent
solution to provide for a reliable and simple anti-pinch systems.
The technique of this invention overcomes the problems due to spurious
abrupt variations of the motor torque that may be caused by other events rather
than by an accidental pinching.
The invention provides a solution easy to realize that may be embodied
in a relatively simple integrated circuit, without requiring specific external sensors.
More in general, the invention provides a method of detection of an
increase of the motor torque particularly suited for detecting an accidentally block
of the motor.
The method of this invention permits to recognize a blocking condition
caused by a resisting force that may be weaker than the force necessary for activating
the motor during other functioning conditions.
The invention permits detection of a blocked condition (increase of
the torque) even when the load of the motor is greater than the limit blocking force
and even if the resisting force varies during the motion of the part driven by the
motor even in the long term as it is often the case with the aging of parts.
More precisely, object of the present invention is a method for detecting
variations of the torque of a DC motor, characterized in that it comprises the operations
- generating a first signal representing the current flowing in the motor;
- multiplying the first signal with a pre-established function producing a product
- generating a comparison signal to correspond to the slope of the product signal;
- signaling a torque variation if the comparison signal surpasses a certain threshold.
Preferably, the comparison signal is the difference between the product
signal and the moving average thereof over a certain time interval (t-Δt1;
Another object of the present invention is a control circuit for detecting
a torque variation of an electric DC motor, comprising
- sensing means of the current flowing in the motor, generating a first signal;
- first circuit means for generating a product signal of the first signal by a
- second circuit means for generating a comparison signal to correspond to the
slope of the product signal;
- a comparator of the comparison signal with a certain threshold, signaling a
torque variation when the comparison signal surpasses the threshold.
Preferably, the second circuit means comprise a low-pass filter of
the product signal and an adder that produces the comparison signal as the difference
between the product signal and the filtered replica thereof.
The invention is more precisely defined in the annexed claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The different aspects and advantages of the present invention will
become even more evident through the following detailed description of embodiments
thereof and by referring to the attached drawings, wherein:
DESCRIPTION OF SEVERAL EMBODIMENTS OF THE INVENTION
- Figure 1 is a sketch of a motorized car window;
- Figure 2 are time diagrams of the current through the motor;
- Figure 3 is a sketch of the motorized glass pane of the window of a car
door and the diagram of the force exerted on an obstacle trapped between the closing
glass and a window post;
- Figure 4 are diagrams illustrating the working principle according to
a preferred embodiment of the method of this invention;
- Figure 5 depicts an embodiment of the control circuit of this invention;
- Figure 6 is a diagram illustrating how the product signal and the comparison
signal change when a pinch occurs;
- Figure 7 is a diagram of the main signals of the control circuit of Figure
- Figures 8a and 8b illustrate the functioning of a control circuit of
this invention in a normal and in a blocked condition of a motorized car window.
For sake of clarity, only the steps of the method and the components
of the detection circuit of this invention that are necessary to fully understand
the invention are depicted and described hereinbelow. In particular, the end-run
detectors of a motorized window and the mechanical transmission organs that are
driven by the motor are not represented.
Substantially the method of the invention contemplates generating
a signal representing the current flowing in the motor IM, multiplying
this signal by a certain function WM_virtual(t) and generating a comparison
signal corresponding to the slope of this product. The comparison signal is compared
with a pre-established threshold, signaling a torque variation when the threshold
The function WM_virtual(t) is preferably a saturated linear
ramp function, which is null when the motor is switched on and saturates when the
start-up phase of the motor finishes. The advantages of choosing WM_virtual(t)
as a saturated linear ramp will be explained later on.
Such a comparison signal could be generated for instance by deriving
a substantially noise-free filtered replica of the product signal or, according
to a preferred embodiment of the method of the invention, by subtracting from the
product signal an integrated replica thereof. The time constant of integration may
be chosen such to let the filtered replica signal track any torque variation that
may occur during the normal functioning of the motorized implement but practically
results insensible to torque variations due to the presence of an obstacle that
blocks the motor. Preferably, the filtered replica is obtained by calculating the
moving average of the product signal with an integrator over a time interval (t-Δt1;
t) whose duration Δt1 is greater than the maximum time constant of a torque
variation to be ignored and smaller than the minimum time constant of a torque variation
to be detected.
The duration Δt1 is chosen in function of the yielding interval
of a part of the human body. It has in fact been found that these yielding times
typically last several tens of milliseconds, which makes possible even to discriminate
the squeezing of a limb from other events.
Preferably the signal representing the current in the motor is filtered
with a low-pass filter with a time constant of about a millisecond. By doing so
the switching noise of brushes and current variations that may be caused by running
over pot holes in the carriageway are effectively filtered out and do not influence
detection of torque variations due to the blocking of the motor.
According to a preferred embodiment, the function WM_virtual(t)
is a saturated linear ramp voltage. It could be possible to choose a constant function
WM_virtual(t), but in this case the system would recognize the inrush
current of the motor as due to a pinch. In this case, the detection of torque variations
should be disabled during the start-up phase of the motor, thus any pinching occurring
during the start-up phase would not be detected.
With reference to Figure 4, let us suppose that the motor be started-up
with a saturating ramp voltage VM(t). If there is not any obstacle that
blocks the motor, the current flowing in the motor is represented by the curve IM_nominal(t),
which is almost constant, otherwise in case of a blockage it is represented by the
curve IM_blocked(t). The product of the current IM flowing
in the motor by the saturated linear ramp voltage WM_virtual(t) is a
saturated linear ramp IM_nominal(t)*WM_virtual(t) if the motor
is not blocked, otherwise it is a saturated parabolic ramp IM_blocked(t)*WM_virtual(t)
if an obstacle stops the motor during its start-up phase.
A comparison signal corresponding to the slope of the product signal
is a linear ramp signal if the motor is blocked during the start-up phase, otherwise
it is equal to a certain constant value. It should be noted that the slope of the
product signal when the motor is not blocked is relatively small, because the current
in the motor is almost constant. On the contrary, when the motor is blocked the
slope of the product signal rises rapidly to large values. This is exploited for
discriminating a normal start-up condition from a blocked start-up condition by
comparing the comparison signal with a certain voltage threshold.
As it will be evident to the skilled person, the inrush of current
at the start-up does not produce any false blockage detection because the function
WM_virtual(t) is null when the motor is switched on, and thus it effectively
masks the initial abrupt variation of the current IM.
When the start-up phase finishes, the saturating ramp voltage WM_virtual(t)
becomes a constant (e.g. equal to 1) and thus the product signal corresponds to
the current in the motor IM. Therefore, after the start-up phase the
method of the invention substantially contemplates detecting torque variations from
variations of the current in the motor.
It is worth noticing that using the method of the invention it is
possible to detect a blocked condition even when glass pane is blocked at the turn
on of the motor. This cannot be done with known methods that contemplate the use
of Hall sensors because these methods are based on the detection of variations of
the motor speed.
A control circuit of the invention for detecting the abrupt speed
variation of the motor due to a blocked condition by monitoring the current circulating
in the motor, is depicted in Figure 5.
A current sensor, not depicted in figure and that may be of any type
as commonly employed for this purpose, generates a signal representative of the
current circulating in the motor. An optional Low-Pass filter generates a noise-free
replica 1 of the signal representative of the current in the motor, a multiplier
generates the product signal 2 and an integrator produces a moving average 3 of
the product signal. The integrator filters the variations of the filtered signal
the characteristics of which make them attributable to an obstacle, and generates
the signal 3. Finally, an adder generates the comparison signal 4 as the difference
between the product signal 2 and the moving average thereof 3. The logic signal
5 produced by the comparator is active when the comparison signal 4 exceeds the
threshold, signaling that a torque variation has been detected.
The comparator COMP compares the signal 4 with a pre-established threshold
PINCH_THRESHOLD that when is exceeded generates a comparison signal that signals
that an obstacle is blocking the run of the motor. Variations of current due to
strains of the glass pane guides or to particularly low temperature, humidity and
dirt typically cause a relatively slow variation of the motor current that is very
well discriminated from fast variations due to the presence of a block obstacle.
Figure 6 depicts in a schematic manner the evolution of the product
signal and of the comparison signal when a pinch occurs.
Before the pinch, the product signal is a linear ramp and the comparison
signal assumes relatively small values. When a pinch occurs, the current, and thus
the product signal, increase rapidly according to a typical parabolic curve, while
the comparison signal increases following a linear ramp.
Diagrams of the main signals of the circuit of Figure 5 during a normal
run of the motor, are depicted in Figure 7. The initial current peak due to clearances
in the transmission organs does not produce sensible variations of the comparison
After the start-up of the motor (from t0 to t2 in Figure 2) and neglecting
spurious disturbances and switching noise, the product signal is proportional to
the current IM flowing in the motor and the mean value thereof do not
change considerably for different functioning condition (wet, dry or iced glass).
Indeed, different operating conditions cause similar variations of the mean value
of the respective curve. Moreover, current (torque) variations during normal functioning
are significantly slower than a variation caused by the accidental blocking of the
More generally, for the same travel between two positions, slow variations
of the resisting torque (when an additional constant force is applied) for example
under certain conditions of humidity, temperature, and the like, modify negligibly
the motor torque in respect to the mean value of these rather slow variations of
the resisting torque. Therefore, from an assessed maximum variation during normal
functioning, it is possible to determine a proper duration Δt1 of integration
for calculating the moving average, for example, by considering the maximum variation
as the acceptable limit.
Figure 8a illustrates the waveform of different signals of a motorized
window in absence of accidental blocking of the motor.
Figure 8b illustrates the same waveforms in the case of an accidental
blocking of the motor while lifting the window closed.
In Figures 8a and 8b, the instantaneous value of the current in the
motor IM is represented by the curve traced with short dashes. The curve
of IC (which is a noise filtered replica of the current IM)
is traced with a solid line. The waveform of the moving average IS of
the current in the motor is traced with long dashes.
The curve IC of Figure 8a is similar to the already described
curve 21 of Figure 2. The curve IS raises slower than IC and
crosses it at an instant t11 successive to the inversion of the slope of the signal
IC after the start of the rotation of the motor.
After the instant t11, the curves IC and IS
coincide at the instant t12 during normal operating conditions of the motor and
remain identical as long as the window is eventually shut (peak at the instant t14).
The detection of torque variation is disabled upon reaching the end
of the run when the close shut condition is detected by commonly used end run detectors
(at the instant t14).
If the glass window is blocked while being lifted (instant t13 of
Figure 8b), the curve IC increases faster than the curve of the moving
When the difference between the two curves reaches the pre-established
threshold value PINCH_THRESHOLD, a torque variation is detected.
An instantaneous drop to zero of the current circulating in the motor
is depicted in Figure 8b. This drop to zero of the current is due to an inversion
of the direction of the current (not depicted) following a command for pulling down
the glass pane. In case of detection of a pinching it is not sufficient to stop
the motor but it is necessary to reverse the motor and to pull down immediately
the glass pane for freeing the trapped part of the body.
A feature of the method of the present invention is that the pinching
value, which is fixed (PINCH_THRESHOLD), is independent from the level of current
in the motor. Therefore, the functioning of an anti-pinch system or more generally
of a circuit for detecting a torque variation according to this invention, is not
affected by the current value of the torque. This means that, independently from
the lifting speed of the glass pane, it is possible to stop any pinching in a very
short time. Theoretically, it would be possible to fix a null pinch threshold, so
that even the smallest difference would be detected. In practice, the threshold
is fixed for a pinching force as weak as possible in consideration of tolerances
of the constituting elements of stabilization times of the quantities to be compared.
In particular the pinch threshold may be chosen such to correspond to a tolerable
Another feature of the present invention is that even abrupt variations
of the torque are ignored as long as they do not last. This phenomenon is represented
in Figure 8b by the peak 44 of the signal IM, representing the abrupt
current variation caused by the passage of the car on a pot hole while lifting the
glass pane. Because of the shortness of the phenomenon, the signal Is does not vary.
Because of the chosen time constant the peak 44 may cause a small variation of IC
as depicted in Figure 8b. This variation besides having a transient character has
also a rather small amplitude and thus the discriminating threshold PINCH_THRESHOLD
is not reached and thus there is no detection of a pinching.
Yet another feature of the method of the present invention is that
a pinching condition may be detected even if the blocked condition is not followed
by a yielding.
According to the described embodiment of a motorized window, this
represents the case in which the blockage of the glass pane is caused by a trapped
finger. Even in such an occurrence it will be necessary to free the trapped finger
by reversing the motor for pulling down the glass pane. Though the variation of
the instantaneous signal IM is not followed immediately by a variation
of the signal IC (the peak is not detected, because it is filtered by
the integrator 32), the blocked condition persists and therefore the signal IC
eventually increases, thus causing the detection of a blocked condition because
IC increases before the signal IS.
According to this invention, it is not necessary to use sensors of
operating parameters of the motor or other external transducers. The current sensing
that is normally performed for controlling the motor is perfectly sufficient also
for the aims of torque variation detection of this invention.
The present invention may be practiced in forms even different from
those described for illustrating purposes, as will appear to the skilled person.
In particular, selection of the duration Δt1 and of the time constant T2 of
the noise filter may be easily. done by the skilled person on the basis of the indications
given in this description, to suit the particular application. In case of a motorized
car window, the duration Δt1 may be comprised between 10 and 200 milliseconds
and the time constant T2 may be comprised between 0.5 and 10 milliseconds.
Though the invention has been illustrated in relation to an important
sample application of a motorized car window, it is applicable to any other kind
of implement motorized with a DC motor whenever it is desirable to detect a blocked
condition, or a more or less abrupt increase of the motor torque. For example, the
invention may be applied to automated doors, gates, as well as to motorized conveyors
and the like.