WO2006084520A1

WO2006084520A1 - Method and circuit arrangement for recording a current of a pulse-width modulation-controlled electric motor

Info

Publication number: WO2006084520A1
Application number: PCT/EP2005/056770
Authority: WO
Inventors: Andreas Mueller
Original assignee: Robert Bosch Gmbh
Priority date: 2005-02-14
Filing date: 2005-12-14
Publication date: 2006-08-17
Also published as: EP1851849A1; DE102005006503A1

Abstract

The invention relates to a method and a circuit arrangement for recording a current of a pulse-width modulation-controlled electric motor (10) which is powered by a constant voltage supply network and connectable thereto by means of a semiconductor switching device (18) and a shunt device (20), wherein the controll of the semiconductor switching device is modifiable by a voltage drop. According to said invention, the recording and adjustment of the motor current (I1) are carried out by detecting the voltage drop (USH) of the shunt device (20) with the aid of a scanning and holding element (T2, 26) and by deactivating the semiconductor switching device with respect to the scanning and holding element at least in a time-delay manner in such a way that a measurement value proportional to the actual current quantity (I1) of the motor is recorded and the adjustment is based thereon

Description

Method and circuit arrangement for detecting the current of a pulse width modulation controlled electric motor

State of the art

The invention is based on a method and a circuit arrangement for detecting the current of a fed from a DC power supply, controlled by pulse width modulation electric motor according to the preamble of claims 1 and 6, as they have become known for example from DE 197 56 461 A. This document discloses a method for influencing the electrical power of a load with a pulse width modulated signal, the detection of a

Current change by an inductive load by means of an inductive element through which the current flows. In this case, the current detection evaluates the inductive voltage drop at an inductive element using an integrator, which adds up the current changes to the inductive element and from this provides a signal which is proportional to the current through the inductive load. Upon the occurrence of an overcurrent, the current detection arrangement generates a signal which modifies the pulse width modulated signal for the control of the load current.

From DE 29 30 863 A, it is known in the power control of an electric motor to evaluate the voltage drop across a shunt through which the load current flows to control the motor current. Because of the short duration of the motor current pulses at a with

Pulse width modulation operated electric motor and because of the constantly alternating current pulses and current pauses, however, it is difficult to determine the actual height of the motor current without complex detection and evaluation measures and to realize an accurate current control. The invention has for its object to realize using a low-cost analog control arrangement, a direct and accurate measurement of the current absorbed by the electric motor. This is achieved by the characterizing features of the preceding claims 1 and 6.

A particularly simple and inexpensive circuit construction is obtained by the fact that the semiconductor switching device used as well as the sample and hold element of the current detection of the same control unit, preferably directly controlled by the PWM control unit and in this case at least at the shutdown of the motor current, the signal for driving the semiconductor switching device is delayed. In this way, it is ensured that at the time of detection of the motor current in the form of the voltage drop at the arranged in the motor circuit shunt of the detection device still flows the full motor current. The semiconductor switching device expediently switches on and off in a delayed manner relative to the sample and hold element. This is technically easy to implement; Measurement errors with small pulse widths are tolerable in practice.

Furthermore, it is advantageous if both the semiconductor switching device and the sample and hold member are delayed on and off, wherein the delay is selected to be larger in the semiconductor switching device than in the sample and hold member. In this way, the turn-on of the semiconductor switching device can be attenuated more than would be necessary for the pure measurement process, so that the electromagnetic compatibility of the switching arrangement is improved.

A particularly expedient circuit arrangement results if both the semiconductor switching device and the shunt arrangement between the motor and the ground line of the DC voltage network and in this case the shunt arrangement, a sample and hold element (sample and hold) is connected in parallel. In this way, on the one hand requires the semiconductor switching device no control voltage, which is above the supply voltage and therefore causes additional costs for their provision, on the other hand, the

Shunt voltage detected directly against ground and thereby the circuit even further simplified and the measurement result can be improved.

The delay of the signals are selected to be greater for both the driving of the semiconductor switching device and for driving the switch of the sample and hold element is carried out in a particularly simple manner by a respective low-pass, wherein the time constant τi of the low-pass filter for the semiconductor switching device needs than the time constant τ ₂ for the actuation of the switch of the sample and hold member, wherein expediently both switches are designed as MOSFETs - with large motor currents optionally with a parallel connection of several MOSFETs in the load circuit.

Further details and advantageous embodiments of the invention will become apparent from the description of the embodiment. The pictures show in

1 shows a circuit arrangement for detecting the current of a fed from a DC power supply, controlled by pulse width modulation electric motor and in

Figure 2 diagrams of the various control voltages, the resulting current through the semiconductor switching device and the Ausgangsleitwertes κ (T ₂ ) of the switching element of the sample and hold member.

In FIG. 1, denoted by 10 is a DC motor, which serves as a drive for the fan of a motor vehicle. The engine will - A -

fed with a battery voltage U _B from a DC power supply, the positive pole 12 and the negative pole 14 are designated. The motor current Il flows from the positive pole 12 of the DC voltage network via a supply line 16 to the electric motor 10 and from this first via a semiconductor switching device 18 in the form of a single or optionally also a plurality of parallel-connected MOSFETs Tl and a shunt 20 and a ground line 22 to the lying on ground Negative pole 14 of the DC voltage network. To avoid overvoltages, a freewheeling diode 24 is connected in parallel to the electric motor 10.

The shunt device 20 is designed as a low-impedance precision resistor and supplies a measurement voltage XJ _SH , which can be fed to a capacitor 26 via a switch T2. The switch T2 is also a MOSFET, which together with the

Capacitor 26 forms a sample and hold circuit for the shunt voltage U _SH . The gate electrodes of both MOSFETs Tl and T2 are driven by the same PWM control unit, wherein, however, a low pass of a resistor 28 and a capacitor 30 between the PWM control unit 32 and the gate electrode of the MOSFET Tl has a larger time constant τi> τ ₂ than a parallel-arranged low pass having a resistor 34 and a capacitor 36 between the PWM control unit 32 and the gate electrode of the MOSFET T2 of the sample and hold member.

The switching arrangement according to the invention is completed by a current regulator 38 and a voltage regulator 40. The current regulator 38 receives at its inputs on the one hand, the charging voltage of the capacitor 26 and on the other hand, a voltage value corresponding to a predetermined desired value of the motor current I _so u. At the inputs of the voltage regulator 40 are the terminal voltage of the motor 10 and a target voltage U _so u corresponding to the predetermined

Operating conditions of the engine 10. The output of the voltage regulator 40 is on the one hand to the input of the PWM control unit 32 and on the other hand connected via a rectifier element 42 to the output of the current regulator 38.

The circuit operates as follows:

According to the required speed of the motor and the connected load, in the present case the fan blower of a motor vehicle air conditioning system shown schematically, are at the input of the voltage regulator 40, a target voltage U _so u and the hereby to be compared terminal voltage of the motor 10th According to the output voltage of the voltage regulator 40th is the PWM control unit 32 is driven and supplies at its output pulse width modulated control signals U _PWM for the gate of the semiconductor switching device 18 in the form of the MOSFET switch Tl. However, the drive signals U _PWM are not directly but via a low pass from the resistor 28 and the capacitor 30 as the control voltage U _T i to the gate of the MOSFET Tl.

Parallel to the low-pass with the resistor 28 and the capacitor 30 is located at the output of the PWM control unit 32, a second low pass of a resistor 34 and a capacitor 36 which delays the gate of the further MOSFET switch T2 with the output of the PWM control unit 32nd connects. The dimensioning of the resistors and capacitors of the two low-pass circuits is chosen such that for the low-pass of the resistor 28 and the capacitor 30, a time constant τi results, which is greater than the time constant τ2 for the low-pass of the resistor 34 and the capacitor 36th In the embodiment, the resistors 28 and 34 are set equal in size, each having a resistance of 10 kΩ, the capacitance of the capacitor 30 is twice that of the capacitor 36 having values of 100 pF and 50 pF, so that the time constant τi is twice as large is like the time constant τ ₂ . It follows that the MOSFET T2 is switched on and off significantly before the MOSFET T1. The MOSFET switch Tl is in the exemplary embodiment for a load current of the motor leading semiconductor power switching device 18, which can be constructed at high motor currents and several parallel-connected MOSFET switches. in the

Motor circuit is still the shunt 20 at which a motor current Il corresponding voltage U _SH drops. Basically, the shunt 20 can be installed at any point of the load circuit. For the detection of the measurement voltage U _SH , however, it is advantageous if the shunt 20 is connected directly to ground with one connection and is connected with its other connection to the source electrode of the MOSFET T1, so that at the shunt 20 only the measurement voltage in mV - Range and not the supply voltage UB is applied, whereby the measurement error is significantly reduced.

The MOSFET switch T2 lies with its drain electrode via a capacitor 26 likewise at the ground line 22 and with its source electrode at the terminal remote from the earth of the shunt 20, so that the switch T2 and the capacitor 26 is a sample and hold circuit (Sample and Hold) for the measuring voltage U _SH at the shunt 20. Here, the facing away from the ground electrode of the capacitor 26 is connected to an input of the current controller 38, whose second input receives the setpoint value I _as appropriate and the load current Ii reference value, while the output of the current regulator 38 is connected to the cathode of a diode 42 whose Anode at a connection 44 of the connecting line between the voltage regulator 40 and the PWM control unit 32 is located. Thus, the output value of the current controller 38 and the input value of the PWM control unit 32 is lowered when the load current Ii exceeds the set value I _so u and a compensating current flows from the terminal 44 via the diode 42 to the output of the current controller 38th

FIG. 2 shows, in a schematic form, the course of the various relevant voltages, the motor current Ii and the conductance K of FIG the charging current I _{2 of} the capacitor 26 leading switch T2 of the circuit arrangement according to the invention. The uppermost diagram shows the circuit sequence determining output voltage U _{PWM of} the PWM control unit 32. A pulse of the control voltage U _PWM lasts from the time t ₀ to the time ti and determined

Start and end of the control voltages U _T i at the MOSFET switch Tl and U _τ2 at the MOSFET switch T2 with the same start times t ₀ and the same cut-off points ti. Because of the larger time constant τi of the low pass with the resistor 28 and the capacitor 30 against the time constant τ _{2 of} the low-pass filter with the resistor 34 and the capacitor 36, the control voltage U _T i increases more slowly at the control electrode of the switch Tl and drops more slowly than the control voltage U _τ2 at the switch T2. If one assumes that the switch-on points of the switches Tl and T2 are in each case at half the control voltage, the switch-on point t ₂ for the current Il and the earlier switch-on point t ₄ for the current I ₂ at the switch Tl. In accordance with the switch-off points t ₃ and t _{5 in} each case on half of the falling edges of the control voltages U _T i and U _τ2 , the switch-off point t _{3 of} the current Ii is later than the switch-off point t _{5 of} the current I ₂ . In this way, it is ensured that the switch T2 in each case switches on earlier and earlier than the switch T1, so that the correct start and end value of the measuring voltage U _{SH are} applied to the capacitor 26 acting as a holding element. In contrast, a delayed drop in the MOSFET switch T2 on the capacitor 26 would already be a reduced voltage drop across the shunt 20 due to a decaying motor current Ii via the switch Tl and lead to a false measurement result.

Previously, it has already been stated that it is expedient to arrange the shunt 20 on the side of the electric motor 10 facing the ground connection 14, because in this case the measuring voltage applied to the shunt 20 in the mV range is measured only with respect to ground and is not set in relation must be to the full voltage U _B the DC voltage network, which is in applications in the motor vehicle at about 14 V and thus orders of magnitude above the measurement voltage U _SH , to which they would have to be set in proportion. Corresponding considerations also apply to the arrangement of the semiconductor switching device 18 on the ground-facing side of the electric motor 10. In this case, no gate voltage is required for the control of the MOSFET switch, which is above the mains DC voltage and would cause additional costs for their provision.

The inventive method and the associated circuit arrangement for measuring the current consumption of a fed from a DC power supply, controlled by pulse width modulation electric motor thus results in advantages that allow for mass production applications such as a Klimage blower motor for a motor vehicle significant cost savings. The inexpensive analog components used thus replace an integrated circuit and in particular a microcontroller. Compared with other known operating modes there are technical advantages, for example a reduction in power loss compared to a speed setting by series resistors or the possibility of a speed control by a clocked operation compared to a linear operation of the engine. In addition, in the current detection according to the invention a motor current limit allows without additional effort. The otherwise in a PWM energization of the electric motor 10 by the high frequency of, for example, 25 kHz existing difficulty of accurately detecting the current level is effectively eliminated by the inventive use of the sample and hold member in the form of the switch T2 and the capacitor 26 in a particularly simple manner , The two switching elements Tl and T2 of the same, already existing control unit 32 can be supplied synchronously with control pulses, if at least in the supply line for driving the circuit breaker Tl a delay element in the form of a Low pass is inserted. The voltage drop produced by the motor current Ii at the shunt 20 can be precisely determined by detecting the peak value of the current Ii instead of an average because the sample and hold circuit is activated earlier than the power switch Tl and thus at the grounded current regulator terminal 46 a DC voltage is present, which is proportional to the motor current Ii. There is a synchronous rectification of the square wave voltage at the shunt 20, because the power switch Tl and the sample and hold member T2, 26 are controlled synchronously by the PWM control unit 32. By inserting separate Tiefpassgliedern in the leads to the control electrodes of the switches Tl and T2 different time constants τi and τ ₂ can be realized for both leads, the time constant τi must be on the one hand greater than the time constant τ ₂ , but on the other hand can be selected in that the electromagnetic compatibility of the circuit arrangement is improved by the delayed connection of the circuit breaker Tl. A small error in the measurement result with very small pulse width ratios is negligible in practice. It is important, in particular, that in the region of the falling edge of the pulse width signal U _PWM the sampling switch T2 first switches off before the current flow Ii through the shunt 20 decreases because otherwise too low a shunt voltage U _{SH is} transmitted and during the entire pause times of the PWM signal held at the current regulator connection 46 by the capacitor 26 and thereby a considerable measurement error would be caused.

Claims

claims

1. A method for detecting the current of a fed from a DC power supply, controlled by pulse width modulation electric motor, which via a

Semiconductor switching device and a shunt arrangement can be connected to the DC voltage network, wherein the driving of the semiconductor switching device is variable by the voltage drop across the shunt arrangement, characterized in that the voltage drop (U _SH ) at the shunt arrangement (20) by a sample and hold member (T2, 26 ) and the semiconductor switching device (Tl, 18) with respect to the sample and hold member is at least delayed off.

2. The method according to claim 1, characterized in that the semiconductor switching device (Tl, 18) and the sample and hold member (T2, 26) are driven by the same control unit (32) and thereby delays the signal (U _PWM ) for the semiconductor switching device becomes .

3. The method according to claim 1 or 2, characterized in that the semiconductor switching device (Tl, 18) with respect to the sample and hold member (T2, 26) delayed on and off.

4. The method according to any one of the preceding claims, characterized in that both the semiconductor switching device (Tl, 18) and the sample and hold member (T2, 26) are at least delayed off, wherein the delay in the semiconductor switching device is greater than in the sample and holding member.

5. The method according to any one of the preceding claims, characterized in that the semiconductor switching device (Tl, 18) and the sample and hold member (T2, 26) together from the PWM Control unit (32) are controlled according to a by the size of the motor current (Ii) modified control of the PWM control unit (32).

6. Circuitry for detecting the current of a fed from a DC power supply, controlled by pulse width modulation electric motor which is connectable via a semiconductor switching device and a shunt arrangement to the DC network, wherein the driving of the semiconductor switching device by the voltage drop at the

Shunt arrangement is variable, characterized in that the series connection of the semiconductor switching device (Tl, 18) and the shunt arrangement (26) between the motor (10) and the ground line (22) of the DC voltage network (12, 14) and the shunt arrangement (26) a Sample and hold member (T2, 26) is connected in parallel.

7. Circuit arrangement according to claim 6, characterized in that the control electrode of the semiconductor switching device (Tl, 18) via a Tiefpassglied (28, 30) to the PWM control unit (32) is connected.

8. Circuit arrangement according to claim 6 or 7, characterized in that the control electrode of the semiconductor switching device (Tl, 18) and the semiconductor switch (T2) of the sample and hold member (T2, 26) via separate

Low-pass elements (28, 30, 34, 36) are connected to the PWM control unit (32), wherein the low-pass element (28, 30) assigned to the semiconductor switching device has a greater time constant (ii> X ₂ ) than that of the semiconductor switch (T2) of FIG Scanning and holding member associated low-pass filter (34, 36).

9. Circuit arrangement according to one of claims 6 to 8, characterized in that the semiconductor switches (Tl, T2) for the Motor current (II) and for the charging current (12) of a capacitor (26) of the sample and hold member (T2, 26) are designed as MOSFETs.

10. Circuit arrangement according to one of claims 6 to 9, characterized in that on the voltage at the shunt arrangement (U _SH ) chargeable capacitor (26) of the sample and hold member (T2, 26) via a current regulator (38) to the input the PWM control unit (32) is connected.