Abstract:
A circuit for sensing the presence of an inductive load that is particularly applicable to sensing when a solenoid is being driven by a pulse width modulation (PWM) signal. The circuit includes a high side connected transistor having an output driving a load, with the transistor driven by a PWM signal. A circulating diode is coupled to the driving output of the transistor. The circuit further comprises an operational amplifier (op amp) circuit that is coupled to the circulating diode and operates as an inverting operational amplifier (op amp). The op amp circuit charges a first capacitor when the transistor releases driving an inductive load.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to a sensing apparatus and more particularly to an apparatus for sensing the presence of an inductive load driven by a pulse width modulated (PWM) signal. 
   2. Description of the Related Art 
   Media storage systems are well known in the art and are commonly used to store data cartridges at known locations and to retrieve desired data cartridges so that data may be written to or read from the data cartridges. Such media storage systems are often referred to as an autochanger media storage system or, simply, autochanger (“media storage system”). 
   A typical media storage system can include a slot for holding a single data cartridge, or cartridge storage racks or “magazines” that hold several data cartridges. Another type of data cartridge holding device is a cartridge read/write device for reading data from or writing data to the data cartridges within the media storage system. The cartridge storage racks can be accessed by a system operator by opening drawers in the front of the media storage system housing to reveal the cartridge storage racks. 
   A typical media storage system is also provided with a cartridge handling system for transporting the data cartridges between the cartridge racks and the cartridge read/write device. A typical cartridge handling system includes a cartridge engaging assembly or “picker” for engaging the data cartridges and a positioning device for moving the cartridge engaging assembly between the racks and the read/write device. 
   Media storage systems of the type described above are usually connected to a host computer system, which can access or store data on the data cartridges. A control system associated with the autochanger actuates the positioning system to move the picker along the cartridge storage locations until the picker is positioned adjacent the desired data cartridge. The picker can then remove the data cartridge from the cartridge rack and carry it to the cartridge read/write device. Once properly positioned adjacent the cartridge read/write device, the picker can insert the selected data cartridge into the cartridge read/write device so that the host computer can read data from or write data to the data cartridge. After the read/write operation is complete, the picker can remove the data cartridge from the cartridge read/write device and return it to the appropriate cartridge rack. 
   One concern with the type of media storage system is that after a drawer is opened by the operator to access the data cartridges, it can be left fully or partially open or not properly closed. If the picker attempts to remove a data cartridge from a rack in an open or improperly closed drawer, the picker can be damaged. To help prevent this damage, the media storage system is equipped with a solenoid adjacent to each of its drawers to lock the drawers in the closed position prior to operation of the picker. Each drawer has a funnel receiver arranged adjacent to its solenoid so that when the solenoid is activated the plunger engages the funnel receiver to lock the drawer in the closed position. When the solenoid is deactivated it disengages the funnel receiver and the drawer is free to open. 
   One disadvantage of this type of system is that one or more of the solenoids can be disconnected from its power connection during assembly of the media storage system or through use. If the solenoid is not connected to its power connection it will not lock its drawer. One way to test for this disconnect condition is for the operator to manually pull each one of the drawers to be sure they are locked after the solenoid activate signal is given. This method of testing is inconvenient and time-consuming. Another way to verify that the solenoids are present and connected properly is to provide an apparatus that applies an AC waveform to drive the solenoid and then monitors the resulting waveform. While effective, this approach is complex and expensive. 
   SUMMARY OF THE INVENTION 
   In accordance with one embodiment of the invention, a circuit for sensing the presence of an inductive load comprises a high side connected transistor having an output driving a load, the transistor driven by a pulse width modulated (PWM) signal. A circulating diode is coupled to the driving output of the transistor. An operational amplifier (op amp) circuit is coupled to the circulating diode operates as an inverting amplifier. It charges a first capacitor when the transistor is driving a load. 
   In accordance with another embodiment, a system for verifying that a signal is driving an inductive load comprises a solenoid that is driven by a transistor. The transistor is driven by a pulse width modulation (PWM) signal. The system further comprises a first capacitor and an op amp circuit, with the op amp circuit charging a first capacitor when the transistor is driving the solenoid. 
   These and other further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view of a media storage system that utilizes an embodiment of a sensing apparatus according to the present invention; 
       FIG. 2  is a perspective view of the media storage in  FIG. 1 ; 
       FIG. 3  is a diagram of a media storage system drawer utilizing an embodiment of a sensing apparatus according to the present invention; 
       FIG. 4  is a diagram of an embodiment of a sensing apparatus according to the present invention; 
       FIG. 5  is a timing diagram of a signal applied to the input of the sensing apparatus in  FIG. 4 ; 
       FIG. 6  is a timing diagram of a signal at a point in the sensing apparatus of  FIG. 4 ; 
       FIG. 7  is a timing diagram of a signal at a second point in the sensing apparatus of  FIG. 4 ; 
       FIG. 8  is a timing diagram of a signal at a third point in the sensing apparatus of  FIG. 4 ; 
       FIG. 9  is a timing diagram of the output of the sensing apparatus of claim  4 ; and 
       FIG. 10  is diagram of an embodiment of a resistor-capacitor (RC) filtering circuit according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2  show a media storage system  10  that utilizes an embodiment of a sensing circuit according to the present invention. The media storage system  10  includes data cartridges  12  that can be arranged in different locations including at least one data cartridge storage rack(s)  14 , a read/write device  16 , and/or a mail slot (not shown). A control system (not shown) associated with the media storage system  10  moves a cartridge “picker”  18  along a positioning rail  20  that is adjacent to storage racks  14  and read/write device  16 . 
   In operation, a host computer (not shown) is linked to the media storage system  10  (direct connection, remote connection, network connection, etc.) and the host computer can issue a request to access a data cartridge  12  stored in one of the storage racks  14  to read and/or write data. In response, the control system moves the picker  18  along the positioning rail  20  and positions the picker  18  adjacent the requested data cartridge  12 . Once positioned, the control system signals the picker  18  to withdraw the data cartridge  12  from the storage rack  14  and carry it to the read/write device  16 . The linked computer can then read and/or write computer readable data to the cartridge  12 . 
     FIG. 2  is a perspective view of a media storage system  10 , which includes a case or housing  22 . The housing  22  is included primarily to protect the internal components of the system  12  from dust and/or other foreign objects, but is also included for aesthetic and safety reasons. In a preferred embodiment, the housing  22  includes an interface panel  24  and two drawers  26 . The drawers  26  slide within the housing  22  and allow access to the cartridge storage racks  14  shown in FIG.  1 . The interface panel  24  includes a display unit  30  and a keypad  32 . The display unit  30  can be provided for menu-driven information retrieval, diagnostics, etc. 
     FIG. 3  shows one of the media storage system drawers  26  that can be locked in a closed position using a solenoid  36  to avoid damage to the picker during picker operation. 
   The drawer  26  has a funnel receiver  38  aligned so that the solenoid plunger  40  is aligned with the opening  42  in the receiver  38 . When the solenoid is activated as shown in  FIG. 3 , the solenoid plunger  40  is extended into the funnel opening  42  to lock the drawer  26  in the closed position. The funnel shape of the receiver  38  helps to lock the drawer  26  in the closed position even if the drawer is not completely closed or fully aligned. When the plunger  40  closes, it strikes the angled funnel wall  44  and draws the drawer  26  to the appropriate closed position by the action of the extending plunger  40  against the funnel wall  44 . If the solenoid  36  is not connected properly, the plunger  40  would not engage the funnel receiver  38  to lock the drawer. The drawer  26  could then be left improperly closed or could be opened during picker operation, both of which could result in damage to the picker  18  (shown in FIG.  1 ). 
     FIG. 4  shows a circuit  60  according to the present invention for sensing whether an inductive load is being driven by a PWM signal. The circuit can be used in many applications, with the circuit shown being particularly applicable to testing whether a drawer solenoid  36  (shown in  FIG. 3 ) is being driven by a PWM signal. 
   When initially driving the solenoid  36 , a direct current (DC) signal is applied to give maximum initial throwing force to extend the solenoid plunger. However, a continuous DC signal applied to the solenoid after the initial signal can cause the solenoid to overheat and a DC signal is not needed to keep the plunger in its extended position. Instead, the plunger can be kept extended with a periodic signal after the initial DC signal is applied to the solenoid. A PWM signal (Vg)  62  is the typical periodic signal applied, and in the circuit  60 , the signal  62  drives a solenoid though a FET  68 . The signal  62  can be generated under microprocessor control and can be an operational or test signal. Many different signal frequencies and duty cycles can be used, with a suitable signal  62  having a 30 KHz frequency and a 33 percent (%) duty cycle. 
   The circuit  60  generally uses the PWM signal  62  to charge capacitor  64  through an operational amplifier (“op amp”) op amp circuit  66 , with the charge on the capacitor  64  reflecting whether the solenoid is present and is being driven by PWM signal  62 . The circuit elements are arranged so that the first capacitor  64  remains charged as long as the solenoid is being properly driven. The charged capacitor provides voltage Vc, which can be read by a processor as the pass/fail condition for the solenoid connection. 
   The op amp circuit  66  acts as an inverting amplifier with a charge storage/integrate feature in the first capacitor  64 . The first capacitor  64  is connected in series with a second diode  65 , with one of the capacitor&#39;s two connection points coupled to the cathode of the second diode  65 . The other of the capacitor&#39;s two connection points is coupled to the negative input of an op amp  72  and the anode of the second diode is coupled to the output of the op amp  72 . V− is the signal at the negative input of the op amp  72  and the op amp&#39;s positive input is coupled to ground. As more fully described below, when Vi is a negative voltage, V− is either a negative or zero voltage. With a negative or zero voltage from V− at the op amp&#39;s negative input, the output of the op amp will increase positively charging capacitor  64  through the forward biased diode. Capacitor charging only occurs during the highs of the PWM signal  62 . One high from a cycle may not be enough to fully charge the first capacitor  64  and it can take several signal cycles to initially fully charge the first capacitor  64 . Vc represents the voltage charge stored in capacitor. 
     FIGS. 5 through 9  show various waveforms at different points in the circuit  60  and are discussed in combination with the circuit  60  to describe its operation. The waveforms show the operation of the circuit  60  after the capacitor  64  has been fully charged and the circuit  60  is in its “steady state” condition. 
     FIG. 5  shows the input PWM signal (Vg)  62  in more detail, which is applied to a high side connected field effect transistor (FET)  68  of the circuit  60 . The frequency of the PWM signal  62  is 30 kilohertz (kHz) and with a 33% duty cycle PWM signal as shown, the high  82  of each cycle lasts 11 microseconds (μs) and the low  84  of each cycle lasts 22 μs. The high  82  of each cycle is 12 volts and the low  84  is 0 volts. 
     FIG. 6  shows the waveform  90  at point Vi in the circuit  60 . During the low  84  of the PWM signal  62  in  FIG. 5 , the PET  68  is on, which causes Vi to have a corresponding high  92  of approximately 12 volts. The first diode  70  acts as circulating diode for the PET  68 , and during the high  82  of the PWM signal  62 , the PET  68  is off and effectively forward biased. The solenoid (inductive load) coupled to the FET  68  cannot change its current instantaneously and the solenoid pulls current through the diode  70 , which causes the diode  70  to be forward biased. This causes the cathode of the diode  70  to be clamped to its forward voltage of approximately −1.0 volts, which in turn causes Vi to be clamped to approximately −1.0 volts, shown in  FIG. 6  as a low of signal  94 . 
   A third diode  74  and its current limiting first resistor  75  are coupled to the negative input of the op amp  72 , with the third diode  74  also coupled to ground. This arrangement limits the positive voltage at the negative input of the op amp  72 . 
     FIG. 7  shows the signal  100  at node V−, which is at the negative input of the op amp  72 . During the high  92  of the Vi signal  90  (in  FIG. 6 ) the op amp  72  is saturated at 0.0 volts and the third diode  74  limits the voltage at V− to a maximum of 0.4 volts. This protects the op amp from destructively high input voltage. When the Vi signal  90  changes to its low  94 , the op amp  72  cannot immediately move from saturation at 0 volts to operating in its linear mode. This results in a −1.0 volts spike  103  in the low  104  of V− signal  100 . As the op amp  72  moves out of saturation the V− low  103  ramps up to the low  104  of 0 volts, at which point the op amp  72  is in its linear mode. 
     FIG. 8  shows the waveform  110  of the signal at Vo, which is the output of the op amp  72 . When V− high  102  (0.4 volts) is at the op amp&#39;s negative input, the op amp  72  goes to low saturation of Vo at ground  112 . With 0.4 volts at V− and ground at Vo, the second diode  65  is reverse-biased such that the charge in the capacitor  64  is blocked from leaking. The charge accumulated in capacitor  64  during the highs of the PWM signal  62  remains in the capacitor  64  during the lows of Vo. Accordingly, after the capacitor  64  is fully charged, a generally high voltage Vc can be read by a processor to determine if the solenoid is properly connected. 
   When V− is at 0.0 volts the op amp  72  is in its linear mode. In the circuit  60 , the supply voltage for op amp  72  is preferably 5.1 volts so that when the op amp  72  is in its linear mode, its output is between 0.0 and 5.1 volts, regulating the attempt to maintain V- equal to the positive input (0.0 volts) of the op amp. However, as mentioned above, the op amp cannot come out of saturation instantaneously, and just as the V- signal  100  ramps up to a low  104  of 0 volts, the Vo signal  110  ramps up to its high  114  of 5.1 volts. During Vo high  114  and V- low  104 , the second diode  65  is forward biased and current flows into the capacitor  64  to charge it if it is not already fully charged. 
     FIG. 9  shows the waveform for the signal  120  at Vc after the capacitor  64  has been fully charged: Vc is not a DC high but varies between 3.7 and 5.1 volts, although this range of voltages can be read as a high signal. This fluctuation is the result of V− and Vc being capacitively coupled, so that changes at V− are reflected at Vc. For example, when V− changes from its V− high  102  to its V− low  104  it first experiences a spike  103 . This is reflected in the Vc signal  120  at spike  123 , which has the same magnitude as V− spike  103 . As the V− signal  100  ramps up to its low  104 , the Vc signal  120  also ramps up to its low  124  (4.7 volts). When the V− signal goes back to its V− high  102  (0.4 volts), the Vc signal returns to its high  102  (5.1 volts). The high  102  is approximately equal to the high  114  of Vo as shown in FIG.  8 . This assumes an approximately ideal second diode  65 . If the second diode  65  is less than ideal, an voltage drop across the second diode  65  will be reflected in the high  102  of Vc. For example, if the voltage drop across the second diode  65  is 0.4 volts, the high  102  of Vc will be 4.7 volts. To obtain a more constant Vc signal  120 , a resistor-capacitor circuit can be included as more fully described below. 
   Many different components can be used in the circuit  60  as shown in  FIG. 4 , and the components can be arranged in different ways according to the invention. A preferred first capacitor  64  is 0.1 μF capacitor, a preferred first resistor  75  is 1.2 Kohm resistor, and the second and third diodes  65 ,  74  are commercially available Schottky diodes. Many different amplifiers can be used for op amp  72  such as, without limitation, AD8054 and AD822, both available from Analog Devices, Inc. 
   A second resistor  76  can be included in those embodiments where it is desirable to prevent the op amp  72  from saturating at its positive rail. The voltage provided at the output Vo is then set by the first resistor  75  divided by the second resistor  76 . 
   Circuits according to the present invention are adapted to sensing many different failures related to driving inductive loads like a solenoid. Some of the failures include the solenoid power being disconnected, the FET failing, the PWM signal drive failing, and the solenoid failing either open or shorted. During these failures the FET  68  is still operating but the low at Vi is 0 volts instead of approximately −1.0 volts. V− is 0 volts when Vi is 0 volts so no current is flowing through the first resistor  75 . As a result, zero current flows through the capacitor  64  and the second resistor  76  and Vo remains at 0 volts. At this state the capacitor  64  is not charging and it is allowed to dissipate. When the capacitor bleeds off from 5.1 volts to 0 volts, the output Vc will be 0 volts, which indicates that the solenoid lost its power connection. 
     FIG. 10  shows a RC circuit  130  that can be coupled to the output Vc of the circuit  60 . The circuit  130  further filters the output Vc to provide a more ideal constant voltage output. The circuit  130  includes a second capacitor  132  and a third resistor  134 , with the resistor  134  coupled between Vc and the capacitor  132 , and the capacitor  132  also coupled to ground. Many different capacitors and resistors can be used, with a preferred second capacitor  132  being a 0.1 μF capacitor and a preferred second resistor  134  being a 4.7 Kohm resistor. 
   The circuit  130  can also include a fourth resistor  136  that allows for capacitor  64  to discharge when the solenoid is disconnected. Without resistor  136 , the first capacitor  64  will essentially be prevented from discharging by the second diode  65 . The resistor  136  provides a quicker means for discharging the first capacitor  64  so that the circuit  60  can detect intermittent disconnects of the solenoid&#39;s power connection. The fourth resistor  136  can have many different values with a suitable resistor  136  being a 47 Kohm resistor. 
   The present invention provides a simple and inexpensive way for sensing whether a load is being driven by its PWM drive signal. One particular embodiment is particularly applicable to verifying that a solenoid is properly connected to its power connection by sensing whether a solenoid is being driven by its PWM drive signal. 
   Although the present invention has been described in considerable detail with reference to certain preferred configurations thereof, other versions are possible. Many different circuit components can be used and can be arranged in many different ways. The circuit according to the present invention can be used as a sensor for many different types of PWM driven inductive loads.