Method for monitoring the operation of a metal detection system and metal detection system

Abstract

A balanced coil system (2) of a metal detection system has a transmitter coil (21) connected to a first transmitter unit (1), to provide a transmitter signal (s1) comprising at least first and second operating frequencies (f.sub.TX1, f.sub.TX2). First and second receiver coils (22, 23), which compensate one another when the system is in balance, provide output signals (s22, s23) to a receiver unit (3). The respective operating frequencies are applied separately together each with a monitoring frequency (f.sub.MON) to inputs of first and second modulation units (52, 53), which provide first and a second modulated monitoring signals (s.sub.M1, s.sub.M2), each comprising a first or second modulated monitoring frequency (f.sub.MM1, f.sub.MM2) without a carrier. When applied to a summation unit (54), the modulated monitoring signals result in a combined output signal (s.sub.M12) that is applied to a monitoring coil (24), inductively coupled with at least one of the receiver coils.

Claims

1. A system for detecting metal, comprising: a balanced coil system, comprising: a first transmitter unit, generating a transmitter signal comprising at least a first and a second operating frequency, wherein each of the operating frequencies, the monitoring frequency and the multiplexing frequency are derived by division from a common reference frequency, such that the operating frequencies are lower than the reference frequency by a factor in the range from 30 to 600; a transmitter coil, connected to the first transmitter unit; a first and a second receiver coil, the receiver coils arranged to compensate each other when the metal detection system is in balance; and a receiver unit, receiving, as inputs, output signals from the respective receiver coils; a second transmitter unit, in which: a first modulation unit receives as inputs a first signal with the first operating frequency and a monitoring signal with a monitoring frequency and outputs a first modulated monitoring signal comprising a first modulated monitoring frequency without a carrier; and a second modulation unit receives as inputs a second signal with the second operating frequency and the monitoring signal with the monitoring frequency and outputs a second modulated monitoring signal comprising a second modulated monitoring frequency without a carrier; a summation unit that receives the first and the second modulated monitoring signals as inputs and that provides a combined output signal that comprises the two modulated monitoring frequencies, wherein the summation unit is a multiplexer that, in accordance with a multiplexing frequency, alternatingly switches the first modulated monitoring signal and the second modulated monitoring signal to its output; a monitoring coil that receives the combined output signal of the summation unit, the monitoring coil being inductively coupled with at least one of the receiver coils; a demodulation unit that receives the output signals of the receiver coils and provides a demodulated monitoring signal for each of the two operating frequencies; and a signal processor that compares the respective demodulated monitoring signals with the monitoring signal, the comparison being made in phase, in amplitude or in both amplitude and phase.

2. The system of claim 1, wherein: each of the modulation units is an XOR-gate.

3. A method for monitoring the operation of a metal detection system having a balanced coil system in which a first transmitter unit provides a transmitter signal to a transmitter coil, the transmitter signal comprising at least a first and a second operating frequency, and in which a first and a second receiver coil, which compensate one another when the metal detection system is in balance, each provide an output signal to a receiver unit, the method comprising the steps of: generating, in a second transmitter unit: a first modulated monitoring signal, as an output of a first modulation unit that receives as inputs a first signal with the first operating frequency and a monitoring signal with a monitoring frequency, the first modulated monitoring signal comprises a first modulated monitoring frequency without a carrier; and a second modulated monitoring signal, as an output of a second modulation unit that receives as inputs a second signal with the second operating frequency and the monitoring signal with the monitoring frequency, the first modulated monitoring signal comprises a first modulated monitoring frequency without a carrier; generating a combined output signal in a summation unit that receives the first and the second modulated monitoring signals as inputs, the combined output signal comprising the two modulated monitoring frequencies, wherein the summation unit is a multiplexer that, in accordance with a multiplexing frequency, alternatingly switches the first modulated monitoring signal and the second modulated monitoring signal to its output; applying the combined output signal to a monitoring coil that is inductively coupled with at least one of the receiver coils; generating a demodulated monitoring signal for each of the operating frequencies by demodulating output signals of the receiver coils in a demodulation unit; and comparing the demodulated monitoring signals in phase, amplitude or both with the monitoring signal, in order to obtain performance information that is used to control the measurement process; wherein each of the operating frequencies, the monitoring frequency and the multiplexing frequency are derived by division from a common reference frequency, such that the operating frequencies are lower than the reference frequency by a factor in the range from 30 to 600.

4. The method of claim 3, wherein: in generating the respective modulated monitoring signals, each of the operating frequencies is delivered with a selected phase shift from a divider unit to the respective modulation units.

5. The method of claim 4, wherein: each of the modulation units is an XOR-gate.

6. The method of claim 3, wherein; the monitoring frequency is selected to be in a frequency range that is higher that the frequencies induced by the presence of objects in the balanced coil system during the operation of the metal detection system.

7. The method of claim 6, wherein: the reference frequency signal, which comprises the reference frequency, is provided by a reference unit to a second divider unit, which in turn provides the monitoring signal with the monitoring frequency, in the range between 50 Hz and 1000 Hz.

8. The method of claim 7, wherein: the monitoring frequency is in the range of 500 Hz to 700 Hz.

9. The method of claim 6, wherein: a frequency source receives the reference frequency at an input thereof and provides selected multiples of the operating frequencies to the first and second transmitters.

10. The method of claim 3, comprising the further steps of: modifying, by at least one of filtering or amplifying, the first modulated monitoring signal to forward the first modulated monitoring frequency with a predetermined amplitude; and modifying, by at least one of filtering or amplifying, the second modulated monitoring signal to forward the second modulated monitoring frequency with a predetermined amplitude.

11. The method of claim 3, comprising the further step of: modifying the combined output signal, before being applied to the monitoring coil, by at least one of: filtering or amplifying.

12. The method of claim 3, wherein: the step of generating a combined output signal is achieved by: applying to the inputs of a first gate, having an AND or NAND function, the first modulated monitoring signal and, by way of an inverter, the reference frequency or a derivative thereof, applying to the inputs of a second gate, having an AND or NAND function, the second modulated monitoring signal and the reference frequency; and applying the output signals of the first and second gates to the inputs of a third gate, having an OR function or a NAND function, the output thereof defining the combined output signal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Some of the objects and advantages of the present invention have been stated, others will appear when the following description is considered together with the accompanying drawings, in which:

(2) FIG. 1 shows a block diagram of an inventive metal detection system that comprises a transmitter module 1 that provides a first transmitter signal s1 with two operating frequencies f.sub.TX1, f.sub.TX2 applied to a transmitter coil 21 and a second transmitter signal s.sub.M12 comprising two modulated monitoring frequencies f.sub.MM1, f.sub.MM2 applied to a monitoring coil 24;

(3) FIG. 2 shows a block diagram of the transmitter module 1 of the metal detection system of FIG. 1 in a preferred embodiment with a first transmitter unit 13 that delivers the first transmitter signal s1 and a second transmitter unit 5 that delivers the second transmitter signal s.sub.M12; and

(4) FIG. 3 shows the second transmitter unit 5 of FIG. 2 in a preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) FIG. 1 shows a block diagram of an inventive metal detection system, which comprises a transmitter module 1, a balanced coil system 2 with a transmitter coil 21, a first and a second receiver coil 22, 23, and a monitoring coil 24, a receiver unit 3, a signal processing unit 4, and a computer system 8 that comprises standard interfaces, input devices and output devices. FIG. 1 further shows a conveyor 6, on which products P are transferred through the transmitter coil 21 and through the receiver coils 22, 23.

(6) The inventive transmitter module 1, which is shown in a preferred embodiment in FIG. 2 comprises a first transmitter unit 13 that applies a first transmitter signal s1 with two operating frequencies f.sub.TX1, f.sub.TX2 to the transmitter coil 21 and a second transmitter unit 5 that applies a second transmitter signal or combined output signal s.sub.M12 with two modulated monitoring frequencies f.sub.MM1, f.sub.MM2 to the monitoring coil 24.

(7) The transmitter signal s1 induces signals s22, s23 in the identical receiver coils 22, 23 that are of the same amplitude but inverse polarity as long as the system is in balance, i.e. as long as the conveyed products P are not contaminated with metals.

(8) In the event that a product P.sub.C is contaminated with an electro-conductive object, then the signals s22, s23 in the identical receiver coils 22, 23 will change while that product P.sub.C passes through the balanced coil system 2.

(9) As a result the operating frequencies f.sub.TX1, f.sub.TX2 induced in the receiver coils 22, 23 get modulated with a base band signal, whose amplitude and frequency are dependent on the property, dimension and travelling speed of the electro-conductive object or contamination.

(10) Depending on the properties of the product P.sub.C and the contamination the signals s22, s23 induced in the receiver coils 22, 23 will change typically for both operating frequencies f.sub.TX1, f.sub.TX2. However, the impact on the signals s22, s23 will typically not be identical for each operating frequency f.sub.TX1, f.sub.TX2. Hence, for a first sort of contaminants the observation of the first operating frequency f.sub.TX1 may be preferable, while the observation of the second operating frequency f.sub.TX2 may be preferable for other contaminants.

(11) Since the electro-conductive object is not travelling through the monitoring coil 24 the magnetic field of the monitoring coil 24 is not disturbed. Interferences are further avoided by placing the monitoring coil 24 outside of the frame 20, in which the transmitter coil 21 and the receiver coils 22, 23 are arranged. As shown in FIG. 1 the monitoring coil 24 is wound around the leg of the second receiver coil 23 that is connected to the receiver unit 3. Hence, products P do not travel through the monitoring coil 24 and do therefore not influence the monitoring signals.

(12) The output signals s22, and s23 of the receiver coils 22, 23 and the combined modulated monitoring signal s.sub.M12, which has been induced into the receiver coils 22, 23, are applied to center-tapped primary windings of a balanced transformer 31 that mirror the receiver coils 22, 23. Further, the balanced transformer 31 comprises two identical center-tapped secondary windings whose opposite tails are connected to an amplifier 32. A receiver signal s.sub.R provided by the balanced transformer 31, that contains the operating frequencies f.sub.TX1, f.sub.TX2, which have been modulated by the products P or contaminations Pc, and the corresponding modulated monitoring frequencies f.sub.MM1, f.sub.MM2 is amplified in the amplifier 32 and subsequently filtered in a filter unit 33 which provides the amplified and filtered receiver signal s.sub.R to a demodulation unit 34.

(13) In the demodulation unit 34 the receiver signal s.sub.R is demodulated by applying demodulation signals sd1, sd2 with the demodulation frequencies, namely the operating frequencies f.sub.TX1, f.sub.TX2, which are supplied by the transmitter module 1.

(14) By demodulating the receiver signal s.sub.R a first product signal s.sub.P1 for the first operating frequency f.sub.TX1 and a second product signal s.sub.P2 for the second operating frequency f.sub.TX2 are obtained.

(15) Further for the first operating frequency f.sub.TX1 a first demodulated monitoring signal s.sub.M1 and for the second operating frequency f.sub.TX2 a second demodulated monitoring signal s.sub.M2 are obtained. The product signals s.sub.P1 and s.sub.P2 represent the influences of the product and contaminations Pc. The demodulated monitoring signals s.sub.M1 and s.sub.M2 contain information about the condition of the metal detection system and disturbing influences.

(16) The product signals s.sub.P1, s.sub.P2 and the demodulated monitoring signals s.sub.M1, s.sub.M2 provided at the output of the demodulation unit 34, preferably in-phase and quadrature signals, are forwarded to a filter unit 35, which allows the desired signals to pass to a gain unit 36 that allows setting the amplitudes of the processed signals to a desired value. Subsequently the filtered and calibrated signals are converted in an analogue to digital converter 37 from analogue form to digital form. The output signals of the analogue to digital converter 37 are forwarded to a signal processing unit 4, such as a digital signal processor, which compares the demodulated and processed monitoring signals s.sub.M1 and s.sub.M2 obtained for each operating frequency f.sub.TX1, f.sub.TX2 with reference values. The data resulting in the evaluation process are then forwarded to a data processing unit or to a computer terminal 8. In the event that the demodulated monitoring signals s.sub.M1 and s.sub.M2 differ from a given reference by more than a pre-set threshold then an alarm is raised. Alternatively information gained from the demodulated monitoring signals s.sub.M1 and s.sub.M2 can be used for adjusting parameters applied to the transmitter module 1 or the receiver stage 3.

(17) In order to control the measurement process the signal processor 4 is capable of controlling the functions of various modules provided in the transmitter module 1 and in the receiver unit 3. For this purpose, the signal processor 4 is forwarding a first control signal c32 to the amplifier unit 32, a second control signal c33 to the first filter unit 33, a third control signal c35 to the second filter unit 35, a fourth control signal c36 the gain unit 36 and a fifth control signal c37 to the analogue to digital converter 37. With these control signals c32, c33, c35, c36 and c37 the amplification and filter characteristics in the individual receiver units 32, 33, 35, 36 and 37 can be selected or adjusted. A sixth control signal c11 and a seventh control signal c11 are forwarded to the transmitter module 1 as described below. The mentioned control signals can be provided by the signal processor 4 as shown in FIG. 1 or by the computer system or control unit 8.

(18) FIG. 2 shows a block diagram of the transmitter module 1 of the metal detection system shown in FIG. 1, which comprises a first transmitter unit 13 and a second transmitter unit 5.

(19) The transmitter module 1 further comprises a reference unit 11 that provides a reference signal s0 with a reference frequency f.sub.REF to a frequency source 12, such as a frequency synthesiser that is controlled by the sixth control signal c11 received from the signal processor 4 or the control unit 8. The signal processor 4 or the control unit 8 can therefore select suitable operating frequencies f.sub.TX1, f.sub.TX2 or multiples 8f.sub.TX1, 8f.sub.TX2 thereof that are forwarded to the first transmitter unit 13 that contains a divider unit 131, a summation unit 132 and a power amplifier 133, which is providing the amplified transmitter signal s1 to the transmitter coil 21 of the balanced coil system 2. The divider unit 131 divides the multiples 8f.sub.TX1, 8f.sub.TX2 of the operating frequency f.sub.TX1, f.sub.TX2 by a corresponding factor in order to obtain the operating frequencies f.sub.TX1, f.sub.TX2, which are forwarded to the summation unit 132, which provides a single signal with both operating frequencies to the power amplifier 133. The summation unit 132 preferably operates in the same manner as the summation unit 54 that is described below.

(20) The reference signal s0 with the reference frequency f.sub.REF is further provided to a divider unit 14, which divides the reference frequency f.sub.REF preferably by an even number thus obtaining the monitoring frequency f.sub.MON that is forwarded with signal s.sub.M on the one hand to the signal processor unit 4 and on the other hand to the second transmitter unit 5 which is providing a second transmitter signal, i.e. the combined output signal s.sub.M12 comprising the two modulated monitoring frequencies f.sub.MM1, f.sub.MM2 to the monitoring coil 24.

(21) In the second transmitter unit 5 a divider unit 51 is provided, which receives and divides the multiples of the operating frequency 8f.sub.TX1, 8f.sub.TX2 selected in the frequency source 12 by a corresponding factor in order to obtain the operating frequencies f.sub.TX1, f.sub.TX2 preferably with a predefined phase shift, with the given divisor 8 by a multiple of 45°. A corresponding first signal s11 and a second signal s12 provided by the divider unit 51 are then modulated with the monitoring frequency f.sub.MON as follows.

(22) The first signal s11 with the first operating frequency f.sub.TX1 and the monitoring signal s.sub.M with the monitoring frequency f.sub.MON are applied to inputs of a first modulation unit 52 that outputs a first modulated monitoring signal s.sub.MM1 comprising a first modulated monitoring frequency s.sub.MM1 without a carrier.

(23) The second signal s12 with the second operating frequency f.sub.TX2 and the monitoring signal s.sub.M with the monitoring frequency f.sub.MON are applied to inputs of a second modulation unit 53 that outputs a second modulated monitoring signal s.sub.MM2 comprising a second modulated monitoring frequency s.sub.MM2 without a carrier.

(24) In this preferred embodiment the two modulation units 52, 53 are XOR-gates which provide first and the second modulated monitoring signals s.sub.MM1, s.sub.MM2 according to the double-sideband suppressed carrier principle (DSB-SC). Hence, the modulated monitoring signals s.sub.MM1, s.sub.MM2 comprise sidebands only, which lie outside the bandwidth of the frequency range around the modulated operating frequencies f.sub.TX1, f.sub.TX2, in which signals are induced by the measured and possibly contaminated products P, Pc.

(25) The modulated monitoring signals s.sub.MM1 and s.sub.MM2 are applied to inputs of a summation unit 54, which outputs a combined output signal s.sub.M12 that comprises the two modulated monitoring frequencies f.sub.MM1 and f.sub.MM2 and that is applied to a further processing unit 55, in which the combined output signal s.sub.M12 is filtered and/or amplified, before it is applied to the monitoring coil 24. The further processing unit 55 is controlled by the signal processing unit 4 or the control unit 8 by means of the control signal or control bus c12.

(26) FIG. 3 shows the second transmitter unit 5 of FIG. 2 with the summation unit 54 in a preferred embodiment. The summation unit 54 consists of two AND-gates 541, 542 whose outputs are connected to separate inputs of an OR-gate 543. The modulated monitoring signals s.sub.MM1, s.sub.MM2 provided by the modulation units 52, 53 or XOR-gates are applied to the corresponding first input of the AND-gates 541, 542. The reference frequency f.sub.REF is applied to the second input of the second AND-gate 542 and via an inverter 544 to the second input of the first AND-gate 541. Consequently only one of the AND-gates 541, 542 is enabled at a time and allows the related modulated monitoring signal s.sub.MM1 or s.sub.MM2 to pass through via the related input to the output of the OR-gate 543. Consequently corresponding to the duty cycle of the reference frequency f.sub.REF, which is preferably 50/50, the two modulated monitoring signals s.sub.MM1, s.sub.MM2 appear at the output of the OR-gate 543 and form the combined output signal s.sub.M12 that comprises the two modulated monitoring frequencies f.sub.MM1, f.sub.MM2.

(27) The combined output signal s.sub.M12 is then applied to the further processing unit 55, which is controlled by means of the control signal c12, which allows setting of parameters of a gain unit or preamplifier 551, setting of parameters of a filter unit 552 and setting of parameters of a power amplifier 553 whose output is connected to the monitoring coil 24.

(28) Hence, with control signal c12, the second transmitter unit 5 can be adapted to any operation mode or system configuration of the metal detection system. The filter stage 552 can be set to eliminate disturbing frequencies or sidebands for any selected operating frequency f.sub.TX or modulated monitoring frequency f.sub.MM.

(29) The inventive method has been described for the application of two operating frequencies f.sub.TX1, f.sub.TX2. However, as symbolically shown in FIG. 3 with gates 5X, 5Y, 5Z, a further advantage of the inventive solution is that the inventive metal detection system can easily be expanded for using three or more operating frequencies f.sub.TX1, f.sub.TX2, f.sub.TXn. XOR-gate 5X would serve as modulation unit that provides a modulated monitoring signal s.sub.MMn that is applied to the first input of AND-gate 5Y whose second input receives a multiplexing or time-sharing signal mux. The multiplexing signal mux, which would sequentially enable the AND-gates 541, 542, 5Y could be provided for example by a ring counter, such as an Overbeck counter, e.g., for four AND-gates a 4-register one-hot counter could be provided, which has an initial register value of 1000, and generates the repeating pattern: 1000, 0100, 0010, 0001, 1000, . . . . Individually controlled or addressed by this counter, the four AND-gates can sequentially be enabled so that four modulated monitoring frequencies can sequentially be switched through to the outputs of the AND-gates. The outputs of the four AND-gates could individually be connected to the inputs of two OR-gates, whose outputs are connected to a further OR-gate. Consequently, the four modulated monitoring frequencies s.sub.MM1, s.sub.MM2, s.sub.MM3, s.sub.MMn are sequentially present in timesharing mode at the output of this further OR-gate. Hence, modulated monitoring frequencies f.sub.MM1, f.sub.MM2, . . . , f.sub.MMn can be generated in the second transmitter unit 5 for any number of operating frequencies f.sub.TX1, f.sub.TX2, . . . , f.sub.TXn.