EP0377214B1 - Device for controlling an electric current - Google Patents

Description

The invention relates to an arrangement for controlling an electrical current according to the preamble of claim 1.

It is known to use pulse width modulation technology to control the coil current of the electromagnet of an electrohydraulic valve. Strictly speaking, this is a voltage control technique. However, since the force of the electromagnet is proportional to the coil current, this technique requires a method for compensating for changes in the supply voltage and the coil resistance, for example a sensitivity control that can be set by an operator.

Another known method for controlling the coil current uses a current feedback sensor connected in series with the coil. Here, the current is determined by a comparison device, which in turn moves the control unit back when the coil current exceeds a reference value and which drives the control unit back up when the coil current falls below the reference value. A switch-insensitivity range can be set for the comparison circuit by means of controllable switch-off and switch-on times. The valve actuation process provides precise control of the average coil current for both changing supply voltages and changing coil resistances. However, the cost and number of electronic components required for this method can be considerably higher than for the pulse width modulation method. Consequently, it becomes inexpensive and effective arrangement for controlling the electromagnet current.

From DE-A-3 530 966 an amplifier circuit for electromagnets of proportional or servo valves with the generic features is known, in which the input of a clocked pulse width modulator is connected to the output of a controller, the pulse width of which is determined by the output signal of the controller becomes. The output signal of the pulse width modulator is fed to a control logic which drives transistors arranged in a bridge circuit. The excitation winding of an electromagnet is located in a branch of the bridge circuit. In other branches of the bridge circuit there are two resistors whose voltage drop is measured and used to generate the actual value for the current in the field winding. The difference between the actual current value and a current setpoint is formed at a summing point and fed to the controller as a control deviation.

EP-A-0 220 559 specifies a method for driving a magnetic coil which is periodically fed by direct current pulses. After a certain period of time, the coil current is reduced in order to reduce the thermal load on the magnet coil after lifting work has been carried out, if the magnet coil is only intended to ensure a holding function.

The object to be achieved with the invention is seen in specifying an arrangement of the type mentioned at the outset, which is simple in construction and inexpensive. Furthermore, the current control arrangement should be able to be developed in such a way that it can determine whether the magnet coil is in an open circuit and that it enables the interruption frequency to be set easily.

The object is achieved in a generic arrangement by the characterizing features of claim 1. Expedient refinements emerge from subclaims 2 to 4.

A microprocessor periodically generates a peak current setpoint and energizes the solenoid. A voltage corresponding to the peak current set point is applied to an input of a comparator. The actual coil current is sensed through a resistor in series with the coil and a voltage corresponding to the sensed coil current is applied to the other input of the comparator. The comparison device generates an interrupt signal at its output as soon as the detected coil current reaches the peak current setpoint. The interrupt signal is fed to the microprocessor, which emits control signals for separating the magnetic coil from the earth potential. If the microprocessor does not receive an interrupt signal, it lowers the peak current setpoint after a predeterminable time interval, whereupon a new comparison is made. If an interrupt signal is still not generated, the peak current setpoint is lowered again and a comparison is carried out again. If there is also no interruption signal, a signal is generated which indicates that the circuit in which the magnet coil is located is interrupted.

This peak current detection method provides proximity control for the coil current that regularly requires fewer parts than other known ones Methods of current compensation control. Since the interruption time is controlled by the software of the microprocessor, the interruption frequency can also be conveniently set. The number of parts is reduced because the interrogation resistance can be inserted into the switchable earth line of the coil and because no switch insensitivity range of the comparison device is required. Another advantage of this modulation control method is that a coil short-circuit has a self-limiting effect, since the average current of a pure ohmic load is much smaller than the peak current. Furthermore, software that can be used allows a broken circuit to be sensed by the absence of a peak current break.

On the basis of the drawing, which shows an exemplary embodiment of the invention, the invention and further advantages and advantageous developments and refinements of the invention are to be described and explained in more detail.

Fig. 1a

ein vereinfachtes schematisches Blockdiagramm der vorliegenden Erfindung,

Fig. 1b

ein detailliertes Stromkreisschema des in Fig. 1 dargestellten Ventilansteuerkreises,

Fig. 2a, 2b und 3a bis 3c

Flußdiagramme von Algorithmen, die von dem in Fig. 1 dargestellten Mikroprozessor ausgeführt werden.

It shows:

Fig. 1a

a simplified schematic block diagram of the present invention,

Fig. 1b

1 shows a detailed circuit diagram of the valve drive circuit shown in FIG. 1,

2a, 2b and 3a to 3c

Flow diagrams of algorithms executed by the microprocessor shown in FIG. 1.

The valves 42a and 42b shown in FIGS. 1a and 1b are each driven by identical valve control circuits 512, each of which via a relay K501 with a +12 volt voltage source, via a D / A converter U203 with the microprocessor 508 and with one of the Microprocessor connections 6 and 7 and connected to a NAND gate U002. In principle, only one valve with a valve control circuit is required for the application of the present invention, it being possible to dispense with the NAND gate U002 shown. In the present circuit, only one of the two valves 42a and 42b is activated. The block diagram according to FIG. 1a and the valve control circuit according to FIG. 1b contain the following components:

Diodes

CR601

ultra-fast rectifier, MUR410

CR603

Zener diode, IN4745, 16 volts

CR605, CR606

Double diode SOT-23, BAV99

Integrated circuits

U203

8 bit digital / analog converter, AD558

U002

Quad NOR gate (non-OR gate), 74HC02

U004

Comparator, LM2901

Transistors

Q601

Power MOSFET, BUZ11

Q603

NPN Darlingon transistor, MPS A29

Resistances

R612

5.6 kOhm, 1/8 W

R613

330 ohms, 1/8 W.

R614

750 ohms, 1 W.

R607

120 ohms, 1/8 W

R601

Wire resistance 0.75 ohm, 7 W.

R602

1.0 kOhm, 1/8 W

R603

4.7 kOhm, 1/8 W

capacities

C602

150 pF, 50 V

C603

0.001 µF, 100 V

C604

0.001 µF, 100 V

C607

0.001 µF, 100 V

C608

0.001 µF, 100 V

C611

0.047 µF, 50 V

C612

0.047 µF, 50 V

C002

0.01 µF, 100 V

Inductors

L601

Axial ferrite bead

L602

Axial ferrite bead

In cooperation with the valve drive circuit shown in Figures 1a and 1b, the microprocessor 508 periodically executes valve drive program sequences (see Figures 2a and 2b) which respond as follows: A reference value for the peak valve current VCOM is sent to the minus input via the D / A converter U203 of the comparator U004. Microprocessor 508 also generates a signal that is present at terminals 6 or 7 and turns on transistor Q601 so that current flows through the solenoid of valve 42a or 42b. When the solenoid current sensed through resistor R601 reaches a value that corresponds to the peak value VCOM, the output of comparator U004 switches from one position to another, thereby providing an interrupt signal to terminal 12 of microprocessor 508. This interrupt signal triggers the program run (Steps 100 to 106) for the peak current detection, which is shown in FIGS. 2a and 2b and begins with the input step 100. This program then generates a signal in step 102 which opens the transistor Q601, whereby the current to the magnet coil is interrupted for a predeterminable period of time, which is predetermined by steps 104 to 116.

Microprocessor 508 also detects whether the solenoid is in an open circuit or not. This is done by lowering the reference value for the peak valve current twice, provided that no interruption signal is received within a certain time interval. If the queried valve current does not reach the twice reduced reference current value and no interruption has yet been received within an additional time interval, a signal is generated which indicates the open circuit. These steps are carried out with the time program (steps 120 to 156), which is shown in FIGS. 3a to 3c.

The time program is started with step 120 by a trigger signal every 80 microseconds. It is then determined in step 122 whether both valves are de-energized. If this is the case, the algorithm proceeds to step 130, otherwise it proceeds to step 124. In 124 it is determined whether an off time interval is currently in progress. If so, the algorithm proceeds to step 126. If not, the algorithm proceeds to step 132. If there is an off time interval, the value of the off timer is gradually reduced in step 126 and it is determined in each case whether the timeout has expired. If not, the algorithm proceeds to step 134, otherwise it proceeds to step 128. Step 128 emits a signal which indicates that no off-time interval is active and switches the corresponding valve on.

In step 130 it is indicated that the peak current interruption is no longer present, an open circuit timer is activated again and it is indicated that there is no current limitation. The open circuit timer has a duration of 0.6 seconds. If no interrupt signal is generated within this time, this means that the coil is in an interrupted circuit. Step 132, on the other hand, indicates that no peak current interruption has occurred. Step 134 counts down the timer of the main program loop and emits a signal when the time interval of the timer has expired. The main program timer has a duration of 10 seconds. Next, it is checked in step 136 whether the algorithm is waiting for a peak current interrupt. If not, the algorithm continues in step 152. Otherwise, the algorithm proceeds to step 138, in which the query is "is this an end of the circuit test?". If so, the algorithm proceeds to step 152 again. If not, go to step 140. Step 140 counts down an open circuit timer and determines if its time has expired. If not, the algorithm continues to step 152; if the time has expired, proceed to step 142.

In step 142 the current limit value is checked and, depending on this value, the program is continued with one of the steps 144, 146 or 148. If, for example, the current limit is zero, which means that there is no current limit, the program continues with step 144, which sets the current limit to the maximum value. If, according to step 142, the current limit value is equal to the maximum limit value, the program continues with step 146, where the current limit value is reduced to a lower value. If, according to step 142, the current limit value is equal to the lower limit value, this becomes The program proceeds to step 148, where an error signal is set which indicates that a faulty open circuit has occurred. In this case, the valve drive is switched off by opening the relay K501 and the current limit value is reset to zero.

After steps 144, 146 or 148, the timer for the open circuit is reactivated in step 150. In steps 152 and 154, other timers are reset or reactivated depending on the present case. Finally, the program flow is ended in step 156.

In a valve program (not shown), the peak current level can be set (using a digital / analog converter U203) and markers can be set which indicate to the timer program which valve is switched on. This valve program also controls the length of the time interval (delay) during which the valves are switched off. The delay (DELAY) is derived from the following relationship:

$\text{DELAY = G80 - VCOM x G81 / 256}$

G80 and G81 are predeterminable constants and VCOM is the peak current setpoint.

It can be seen that this time-out interval is reduced as the peak valve current reference increases.

Claims (4)

1. Device for controlling an electric current which is applied periodically to the coil of an electro-magnet, with means for detecting an actual current value, corresponding to the existing coil current, means for generating a reference current value, means for comparing the actual current value with the reference current value and for generating a first signal when the actual current value reaches the reference current value, and means which interrupt the coil current at intervals in dependence on the first signal, characterized in that the reference current value corresponds to a desired peak coil set-point current value and in that means are provided to reduce the reference current value should no first signal be generated within a predetermined time interval.
2. A device according to claim 1, characterized by means for generating a second signal, should the actual current value not reach the reduced reference current value within a presettable time interval.
3. A device according to claim 1 or 2, characterized in that means are provided to cause re-energisation of the coil within a presettable time interval after the coil current is interrupted, wherein the presettable time interval is reduced with increasing reference current value.
4. A device according to claim 3, characterized in that the length of the time interval DELAY during which the coil current is interrupted is determined from the following equation:
   
   $\text{DELAY = G80 - VCOM x G81/256,}$

   

   
   
   where G80 and G81 are presettable constants according to the device and VCOM is the peak coil set-point current.

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