Abstract:
An escrow unit is an electromechanical device used to collect or return coins deposited in a pay telephone. An escrow management system 10 for use in controlling a pay telephone escrow unit 26 is provided which consists of an energy storage unit 20 which stores a voltage for use in activating escrow unit 26 upon control unit 40 closing discharge switch 30. The system also contains a detector 42 to determine the voltage requirement of escrow unit 26 by sensing characteristics of escrow unit 26 related to the impedance of the escrow unit. The system also contains a control unit 40 which controls energy storage unit 20 and detector 42 such that escrow management system 10 can both dynamically determine the voltage required to activate escrow unit 26 and supply the correct voltage for activation of escrow unit 26.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to field of electronic systems and, more particularly, to an improved dynamic escrow type detection system. 
     BACKGROUND OF THE INVENTION 
     An escrow is an electromechanical device used to collect or return coins deposited in a pay telephone. The escrow consists of an electrical coil wound around a core implementing an electromechanical solenoid. Several different types of escrow mechanisms exist which require different operating voltages, typically either 48 volts DC or 12 volts DC. 
     The current state of the art requires either (a) predetermination of escrow coil types or (b) driving all escrow types at a common drive level which suffices for the most power demanding of escrow coil types. 
     The disadvantage of approach (a) is a lack of flexibility in handling multiple types. In a maintenance environment, multiple escrow types may be used in servicing pay telephones. If a phone is serviced by replacing the escrow unit with a different type, the phone must be reprogrammed in some way to recognize the new escrow type. In a sales environment, when phone electronic kits are sold for retrofitting into existing phone housings, the installer must specify to the phone electronics what type of escrow unit is attached by reprogramming the telephone indicating the escrow type to expect or by use of a mechanical device such as a switch to indicate to the operating program the escrow type to expect which also adds cost. 
     The disadvantage of approach (b) is a life cycle impairment of low voltage escrow units being driven at high energy levels. This reduces the life of mechanical components by excessive wear caused by hard slapping of mechanical arms. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a dynamic escrow type detection system is provided that substantially eliminates or reduces problems associated with prior systems and methods of operation. 
     According to one embodiment of the present invention, a mechanism for determining escrow type dynamically is provided that adjusts needed drive energy at the run time of the operating program. This conserves power in low power applications such as telephones that receive their power solely from the telephone line. This capability also increases the life of escrow mechanisms by preventing over drive of escrow coils. An electrical energy storage device can be switched to discharge its energy into the escrow unit. When this is done, the escrow unit is activated. In addition, the characteristics of the discharge process can be monitored and used to determine the escrow type being driven. By empirically characterizing various escrow types driven with a known energy storage device and switch, characterization data can be stored by the operating program for dynamically determining the escrow type. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention and the advantages thereof may be acquired by referring to the accompanying FIGURE wherein: 
     The drawing is a block diagram of an escrow management system constructed according to the teachings of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a mechanism for dynamically, at the run time of an operating program, determining the type of escrow unit attached to a pay telephone and using that determination to adjust needed drive or activation energy for the attached escrow unit. This conserves power in low power applications such as line powered telephones and increases life of escrow mechanisms by preventing over drive of escrow coils. 
     The drawing is a schematic diagram of an escrow management system 10 constructed according to the teachings of the present invention. In part, the drawing represents a low to high voltage DC to DC converter unit 11. Converter unit 11 consists of an inductor 12, diode 14, and converter switch 16 arranged in a fly-back arrangement being driven at a fixed frequency and duty cycle. Inductor 12 may comprise, for example, on the order of 270 μH of inductance. The charge cycle consists of converter switch 16 connecting a low voltage DC source 18 to inductor 12. Low voltage DC source 18 may comprise, for example, a two-cell lead acid four volt battery. When this is done, the current in inductor 12 increases exponentially. The total energy stored in inductor 12 is indicated by the maximum electrical current reached and is a function of the time converter switch 16 is on. After inductor 12 is charged for a predetermined time cycle, converter switch 16 is turned off disconnecting low voltage DC source 18 from inductor 12. This changes the current in inductor 12 quickly. Inductor 12 has inductance L and current through inductor 12 is i. The voltage across an inductor is given by the expression: 
     
         v(t)=L*di(t)/dt 
    
     which indicates that the voltage across an inductor is proportional to how quickly the current changes. As the current changes quickly the voltage across the inductor changes polarity and increases significantly thus the fly-back term. The current in inductor 12 is discharged into an energy storage unit 20 comprising capacitor 22 and capacitor 24 for a fixed time interval, by way of diode 14, which acts as a switch. Capacitors 22 and 24 may each comprise, for example, on the order of 220μF of capacitance. Diode 14 may comprise, for example, a suitably rated Schottky diode. The above charge and discharge cycle of inductor 12 continues as an increasingly large voltage develops in capacitors 22 and 24. The energy stored in capacitors 22 and 24 is indicated by the current voltage across capacitors 22 and 24. By measuring the voltage on capacitors 22 and 24, the operating program can determine when there is sufficient energy stored for activating an escrow unit 26 of a given type and stop the above charge-discharge cycle. 
     An energy storage switch 28 is used to select whether both capacitors 22 and 24 are used to store voltage or only capacitor 22 is used. Both capacitors 22 and 24 are used when a lower voltage and higher current is needed to fire a particular escrow unit. Alternatively, energy storage switch 28 is left open and only capacitor 22 is used when a higher voltage and lower current is needed to fire a particular escrow unit. 
     After sufficient energy is stored in capacitors 22 and 24 to activate a given escrow unit type, a discharge switch 30, is turned on connecting capacitors 22 and 24 to an escrow unit solenoid resistor 32 and an escrow unit solenoid inductor 34 through a polarity switch 36. Polarity switch 36 controls the polarity of the signal sent to escrow unit 26 to either collect or return coins controlled by escrow unit 26 and may comprise, for example, a suitable double pole, double throw relay. Collected coins go into a coinbox within the pay telephone, and returned coins go into a coin return chute within the pay telephone. Discharge resistor 38 is the series resistance of discharge switch 30. Discharge resistor 38 may comprise, for example, on the order of 30Ω of resistance. As capacitors 22 and 24 discharge their energy into escrow unit 26, the voltage on capacitors 22 and 24 and the discharge curve of attached escrow unit 26 can be monitored and used for determining the current escrow unit type attached to escrow management system 10. 
     Converter switch 16, energy storage switch 28, discharge switch 30, and polarity switch 36 are controlled by a control unit 40 that runs the operating program of the unit. Control unit 40 may comprise, for example, a Motorola 68340 microcontroller with suitable operating memory. Control unit 40 is also used to control a detector unit 42 that is used to track the voltage on capacitors 22 and 24 in order to sense the amount of energy stored on the capacitors and to monitor and characterize the discharge profile for an attached escrow unit 26. This arrangement provides the system of the present invention the ability to dynamically detect an escrow unit type attached to escrow management system 10 and accordingly adjust the firing energy stored in capacitors 22 and 24 of energy storage unit 20. 
     Thus, the present invention provides a mechanism for dynamically, at the run time of an operating program, determining the escrow unit type attached to an escrow management system and using this determination to adjust the needed drive or activation energy for the attached escrow unit. This conserves power in low power applications such as line powered telephones and increases the life of escrow mechanisms by preventing over drive of escrow coils. This is accomplished by characterizing the discharge curves of known escrow units used with known phone electronics. 
     The discharge curve characteristics are dependent on the components in the discharge circuit consisting of capacitors 22 and 24 which store the escrow activation energy, discharge switch 30 having resistance R S , escrow unit solenoid resistor 32 having resistance R E , and escrow unit solenoid inductor 34 having inductance L E . The mechanical force of escrow unit 26 resisting activation along with the series resistance of the escrow coil causes the escrow coil impedance to look highly resistive. This effect causes the discharge curve of a given escrow unit to be largely RC in nature. The expression of an RC discharge curve is given by V 1  =V 0  *e -t/RC  where 1/RC is the time constant of the discharge curve represented by σ. 
     For a known discharge circuit, the time constant σ can be easily measured in conjunction with each known escrow unit type. These measurements produce a set of empirically derived time constants a for each escrow unit type. If sufficient spread exists in σ for each known escrow unit type, then a can be measured dynamically and used to indicate the escrow unit type attached. 
     To predetermine the time constant σ for each escrow unit type, the discharge curve is plotted on a graph with respect to time. Two voltage points V 0  and V 1  along the discharge curve are selected and the time between V 0  and V 1  is designated as T. The time constant for a given escrow unit type is then determined by σ=-1n (V 1  /V 0 )/T. Below is a list of time constants for three common escrow units used in conjunction with the Astiratel 2 phone electronics board RTC4-05 manufactured and sold by Intellicall. 
     
         ______________________________________Short Circuit         σ.sub.sh  = 32.112 V Escrow           σ.sub.12a  = 20.412 V Intellicall Escrow                 σ.sub.12  = 16.248 V Escrow           σ.sub.48  = 1.85Open Circuit          σ.sub.op  = 0.185______________________________________ 
    
     Given these values, only three thresholds need to be determined for judging dynamically if the escrow unit is shorted, if it is a 12 volt escrow or a 48 volt escrow, or if the escrow circuit is open. These thresholds σ 1 , σ 2 , and σ 3  are derived by taking the geometric mean of adjacent time constants. ##EQU1## 
     At run time of the operating program, when escrow unit 26 activation takes place or escrow unit 26 is fired, two voltage points V 0  followed by V 1  are measured on the discharge curve spaced 50 milliseconds apart. This measurement is accomplished by detector unit 42 under the control of control unit 40 and involves the sampling of the voltage on the top plates of capacitors 22 and 24. The ratio of V 1  to V 0  then indicates the time constant σ for the particular attached escrow unit and is calculated as 
     
         σ=-1n(V.sub.1 /V.sub.0)/50msecs. 
    
     To determine the attached escrow unit type, the calculated time constant is compared by control unit 40 against the three pre-calculated thresholds as shown below: 
     if (σ&gt;σ 1 ) then Escrow Unit is Short Circuited else 
     if (σ&gt;σ 2 ) then Escrow Unit is 12V else 
     if (σ&gt;σ 3 ) then Escrow Unit is 48V else 
     Escrow Unit is Open Circuited 
     Given the above determinations, the energy stored in capacitors 22 and 24 is limited to the needed activation energy for the firing of escrow unit 26 attached to escrow management system 10. 
     The return path from escrow unit 26 is connected through the discharge resistor 38 and discharge switch 30 to the positive pole of low voltage DC source 18 in order to return unused energy to the battery. 
     Accordingly, a system is provided that can dynamically (at run time of an operating program) determine the escrow unit type attached to a phone electronics board. In addition, the system of the present invention can dynamically determine the failure of an escrow unit exhibiting a short circuit condition, an open circuit condition or non-attachment to the phone electronics board. The system of the present invention also allows for the determination and control of the minimum needed activation energy of an attached escrow unit and allows for the provision of high voltage activation on an as needed basis for escrow unit drive from a low voltage source. 
     Although the present invention has been described in detail, it should be understood that various changes substitutions and alterations may be made to the systems disclosed herein without departing from the spirit and scope of the invention.