Abstract:
The invention provides a method for limiting the power rate requirement by electronically advantaging the torque or force of a motor. The method is comprised of a local electrical energy storage reservoir, a function to charge the energy reservoir at a controllable rate, and a power controlled motor driver, and an energy management motion controller. This is an electrical circuit, which achieves a similar ‘speed verses force’ tradeoff as a mechanical reduction transmission. The circuit may be used to move cellular antenna repositioning motors where the motive power source may be less than the continuous rating of the repositioning motor.

Description:
[0001]    This is a non-provisional patent application claiming the priority of provisional patent application Serial No. 60/314,469, filed Aug. 23, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates to positioning motors where the motor will not have a tendency to back drive or drift while the power is off. In such applications, power may be applied to the motor in bursts that would exceed the power available from the power supplying the motor if required on a continuous basis. This application has applicability especially where motors are located in remote locations. In these situations, power transfer per unit distance may be a higher cost. One such application is providing power and control of motors for repositioning cellular phone antennas on towers.,  
           [0003]    In many applications this invention:  
           [0004]    allows use of a smaller less expensive motor to perform the application  
           [0005]    permits selection of a smaller gauge wire for remotely powered motors over long cables  
           [0006]    limits maximum power draw from the power supply by electronically extending the positioning time  
           [0007]    Generally this engineering tradeoff is exercised by using a mechanical transmission e.g. Gearbox, that is to provide a mechanical reduction transmission which will divisibly scale the output shaft rotation rate but multiply the shaft torque by the transmission ratio.  
           [0008]    With a given mechanical transmission and a given motor, this invention provides an additional electronically scalable speed verses torque tradeoff similar to that achieved by the mechanical transmission or gearing.  
         OBJECT OF THE INVENTION  
         [0009]    One general object of the invention is to provide an alternate system of achieving a speed versus torque tradeoff that can be used in place of or in conjunction with a mechanical transmission. A second general object of the invention is to provide a method of power rate verses time of motion tradeoff. An ancillary objective is to provide circuit to drive a motor with a power source rated below the motor&#39;s standard rating. This objective being useful for operating a remote motor such as that on the top of a cellular antenna tower for adjusting aspects of a cellular smart antenna system.  
           [0010]    The motor driver circuit of this invention achieves the above objectives as well as others not mentioned. The motor driver circuit of this invention includes a power source connected to charge a power storage component. The output of the power source and the power storage component may be electrically connected to a motor. A processor controls the rate of charge and discharge of the power storage device as well as the passage of power from the power source on an intermittent basis. The processor&#39;s intermittent control allows the motor to receive energy from the power source and from the power storage device simultaneously. As such the power source may be rated below the suggested continuous power requirement of the specific motor.  
           [0011]    As way of example, consider that 5 pounds of force is required of an electric motor linear motion actuator using a lead screw to translate the mechanical rotation to mechanical linear travel and the required travel is executed within 15 seconds and require 6 watts. Consider a second case, where the same total energy is used if the 5 pounds of force is applied for 30 seconds. However in the second example, the rate of power required is cut in half to 3 watts. An electrical measure of power rate is the watt. A system that accomplishes its task in the example above and takes the longer time will require a power supply of lower wattage.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a functional block diagram of the elements of an embodiment of the invention.  
         [0013]    [0013]FIG. 2 is a simplified buck converter configured to regulate the charge current of the energy storage element and therefore the input power.  
         [0014]    [0014]FIG. 3 a simplified boost-converter to provide the higher desired motor driver voltage.  
         [0015]    [0015]FIG. 4 is a logic diagram for a programming example for the processor of FIG. 1.  
         [0016]    [0016]FIG. 5 is a cellular phone tower with a motor drive circuit made in accordance with this invention installed. 
     
    
     DETAILED DESCRIPTION  
       [0017]    The motor driver circuit  1  of this invention includes a power source  2  connected to charge a power storage component  10 . The output of the power source  2  and the power storage component  10  may be electrically connected to a motor  13 . A processor  15  controls the rate of charge and discharge of the power storage device  10  as well as the passage of power from the power source  2  on an intermittent basis. The processor&#39;s  15  intermittent control allows the motor  13  to receive energy from the power source  2  and from the power storage device  10  simultaneously. As such the power source  2  may be rated below the suggested continuous power requirement of the specific motor  13 . See FIG. 1.  
         [0018]    Power is used at a limited rate by the motor  13  from the power source or supply  2 . For example if the input power is limited to 2 watts, then this invention will manage the motor drive system to never exceed that power draw limit.  
         [0019]    The motor driver circuit  1  consists of an input power limiting stage and an output motor driving stage.  
         [0020]    The input stage is comprised of current limiter  6  and power or energy storage component  10  and its associated elements. The energy storage component  10  may be a battery, a capacitor, including a super capacitor. The control goal of this input stage is to draw no more than the specified limited power level from power source input  2 . The voltage of power source  2  may be higher or lower than the voltage at which the energy is stored in the storage component  10  when the embodiment of this invention employs switching power conversion technique.  
         [0021]    The output stage consists of motor commutation function  12  and storage component  10 . The voltage at which the energy is stored in the power storage component  10  may be higher or lower than the voltage delivered to the motor  13  when the embodiment of this invention employs switching power conversion technique.  
         [0022]    The circuit input power limit goal is ‘embedded in’ or ‘communicated to’ the processor which may be referred to as an energy management microprocessor or DSP  15 , using communication path  3 .  
         [0023]    The motor circuit  1  may be viewed as performing multiple processes. In the energy charging processes, the energy management processor  15  includes an analog to digital converter, which permits it to measure the input power supply voltage  2 . The microprocessor program asserts control  5  to current limiter  6  such that the design current limit value is established. This controlled current level will result in the commanded power supply current or wattage load when computed in conjunction with the measured supply voltage. The commanded current is made available to power bus  8  and it begins to charge Energy Storage element  10  via path  9 . As current is supplied the voltage of bus  8  rises. The energy management microprocessor or DSP  15 , via analog input  7  monitors the voltage of the Energy Storage element  10 . Once the energy stored in  10  reaches its limit as indicated by the maximum voltage allowed for the circuit, microprocessor  15  turns off current limiter  6  via its control output  5 . This constant power input limited energy storage process is operating continuously, and if the voltage on bus  8  drops below a chosen threshold the process will again begin to operate to recharge the energy storage device  10  but never exceeding the stated input power load goal.  
         [0024]    One simplified embodiment of a switching current limiter is illustrated in FIG. 2. A transistor switch  22  is modulated using a PWM pulse width modulation waveform to its control gate. When switch  22  is ‘ON’, energy flows from the power supply input  2  through EMI filter  21 , the switch  22  and inductor  23  into Energy Storage capacitor  10  and current sensing resistor  26 . The current passing through sensor  26  is amplified by  27  and monitored by current regulation control-block  25  and compared against the current limit command  5 , which was asserted from the energy management microprocessor  15 . By this means the current regulation control block can build the desired energy storage level in the storage capacitor  10  while not exceeding the design current draw limit. Other limiting circuits may be used including inefficient linear current limiting circuits or components.  
         [0025]    Another of the processes of the motor driver circuit  1  is the motor driver process. In the motor driver process, the energy management microprocessor  15  may employ several different algorithms to accomplish the function of this invention. These are described here as mode  1  and mode  2 . Although there is a continuum between the functionality of mode  1  and mode  2 , the differences in the algorithm performance and behavior are described for the two cases where the energy storage element  10  is small or where the energy storage element  10  is large.  
         [0026]    In mode  1 , a small electrolytic capacitor is employed as the power storage element  10 . The motor  13  is run a small distance at its full rated current, then de-powered. It is important that the mechanical aspects of the motor  13  to not back drive the motor rotor during the time it is de-powered. The motor  13  needs to have a capacitance to sustain the small rotor displacement for a short period. In the case where the motor is driven with a constant current motor drive the required capacitance is calculated with the equation below.  
       C   =       I   ·   t     V                           
 
         [0027]    Where I is full motor drive power current, t is time motor drive time, and V is design accepted voltage drop in C.  
         [0028]    The charging function will charge the energy storage  10  element quickly due to its small size. This mode can appear to run the motor  13  continuously or in slightly more course steps. The distance to drive the motor  13  between power off periods may be optimized to match the motor magnetic detent period. For example if the motor is a step motor, every 4 full steps is a natural power off magnetic detent torque position, where the motor rotor is stable during the power off period. In such a system the powered off period may be just a few ten&#39;s of milliseconds.  
         [0029]    In mode  2 , the energy storage device  10  may be a very large capacitance in the multi-Farad range. As soon as power is applied to the system, the capacitor  10  begins to charge.  
         [0030]    However in this case enough energy is accumulated to drive the motor  13  for a few seconds or tens of seconds, depending on the electrical size of the capacitor and the draw of the motor  13 . In this case the input current is limited and when the motor command is received the entire positioning may be done at full speed.  
         [0031]    If this energy requirement is less than the energy available in Energy Storage  10 , then the move can be initiated at 100 percent duty cycle. An optimization of this algorithm will take into consideration the power available via path  2 . For example if the motor required 4 watts and the power limit parameter in force is 1 watt, then the algorithm may draw at a rate of 3 watts from energy storage  10  and 1 watt from supplied power source  2 .  
         [0032]    If the power required completing the motion exceeds the power stored in storage device  10  plus that available from power source  2 , then the motion may be segmented. Such a move will be short of the commanded position, then the charging process is permitted to recharge energy storage device  10  to a point where the control algorithm can complete the move or accomplish the next segment of the move.  
         [0033]    In many applications moves may be short of full travel, and in such applications this system will complete the move as quickly as a system, which was not, power limited. An advantage over a gearing system is that this system can be charged prior to receiving the positioning command, whereas the gear system obtains its torque speed tradeoff only during the move.  
         [0034]    The aforementioned segmented-move (mode  2 ) may be aesthetically unpleasing in some applications. An improvement in such applications will be achieved by advancing the motor  13  by its natural electrical pole cycle. This is a very short usage of power which will discharge energy storage only a little but will drive motor  13  at its full rating and therefore achieving its full torque for a short move. Then the motor will be de-energized or switched to a low holding torque power level, awaiting some recharge of energy storage  10  via current limiter  6 . Based on a computed time and duty cycle first algorithm or on the energy threshold second algorithm the motor will be run through the next pole cycle sequence or possibly multiple pole sequences. A 15-degree step motor, for example has a 4 pole cycle characteristic between natural power off detents. This operational strategy causes the motor to appear to be moving continuously, while limiting the average power (mode  1 ).  
         [0035]    The processor  15  may control the motor  13  and a motor power-supply  12  via enable control signal  11  and commutate signal  14 .  
         [0036]    One example of a current limiter  6  of FIG. 1 is shown in FIG. 2. It would be possible to use a linear current regulator to fulfill this function it would result in great inefficiencies. Several switching power supply topologies already known in the art circuits can be adapted to this purpose.  
         [0037]    For a system that would ‘step up’ the voltage to the storage element  10  a boost, fly-back or forward converter topology could be used.  
         [0038]    [0038]FIG. 2 depicts a simplified buck converter configured as a current rather than the more typical voltage regulator for charging the energy storage element  10  to its full capacity. Furthermore energy storage element  10  may be specified as a very high farad capacitor, sometimes referred to as Super Capacitors. High value capacitors can provide an advantage over chemistry based batteries as they will operate over a wider temperature range, and can be charged and discharged rapidly and many times. The maximum voltage tolerance of such components is typically 2-5 volts at this time, although technology advancement may change this.  
         [0039]    The power source  2  if filtered by the inductor and capacitor circuit  21  to reduce EMI electromagnetic interference as is good engineering practice.  
         [0040]    The circuit depicted by FIG. 2 regulates the current charging the energy storage element 10 by switching between two states at high frequency, such as 20-over 100 kHz.  
         [0041]    Switch S 1  typically an electronic transistor is closed by the control circuit  25  and current begins to rise through inductor  23  and flows into the multi-farad capacitor energy storage element  10  and through the low ohm current sensing resistor  24 . Control circuit  25  monitors the charge current via amplifier  27  by sensing the I*R drop and when it reaches the current limit computed by the control circuit and delivered via input  5  the control circuit turns off switch  22 . Then the energy stored in inductor  23  continues to flow into storage element  10  through current sensing resistor  24 , and via the common ground connection to diode  24  to complete the circuit for the inductor  23  discharge. This process continues until the charge voltage of capacitor  10  indicates it is full at which time this charge circuit is shutdown by microprocessor  15  via control signal  5 .  
         [0042]    [0042]FIG. 3 depicts a power conversion stage known in the art as a boost converter topology circuit to accept the low voltage of storage element  10  in this embodiment, and if necessary, raises the voltage to a level more suitable for the motor driver circuit  1 .  
         [0043]    This converter uses the energy storage element  10  as its input power. Switch  32  is closed and builds energy in boost inductor  31 . After the on period of the switchmode cycle, switch  32  is opened and the energy in the inductor flows through diode  33  and builds voltage in the capacitor  34 . The voltage is monitored by boost switchmode control circuit  35  and as voltage  34  raises to the threshold specified by controller  35  input  11 .  
         [0044]    The block  12  from FIG. 1 may include both a motor driver or commutation stage in addition to the power, conversion stage depicted in FIG. 3. Various types of motors can be used with this invention including but not limited to BLDC, stepper, induction, and brush DC motors. In fact any motor with an appropriate drive method can be used.  
         [0045]    The processor  15  may be programmed as shown in FIG. 4 to implement mode  1  control. The processor initially checks whether it has a new command such as a move command. If no, it waits to check for commands. If yes to the query of whether there is a new command, the processor  15 , checks if the last move is completed, if yes, then the processor continues to check for new commands. If the last ordered move is not completed, the processor checks if the power storage component  10  has sufficient power to supplement the power source  2  to move the motor  13 . If there is not sufficient energy, the processor waits and allows further charging of the power storage component  10 , subsequently checking if the move is complete. If the power storage component  10  has sufficient energy to supplement the power source for the next electrical cycle of the motor  13  on the last ordered move, the processor applies the power of the power source  2  and the energy storage device  10  to the motor  13 . This moves the motor  13  one electrical cycle and then the processor  15  turns ‘OFF’ the power from the combination of the power source  2  and the energy storage device  10  and subsequently checks if the ordered move is complete.  
         [0046]    One useful application for the motor driver circuit is to drive motors located a distance from a power source. One example would be motors for repositioning apparatus of smart cellular phone antennas. An example is shown in FIG. 5. These motors  13  are necessarily located at or near the top of antenna towers  121  and are for repositioning antenna appartus  119 . The motor driver circuit  1  of this invention can be used to drive such antenna  119  repositioning motors  13  with the power source  2  below that which the repositioning motors  13  are rated and reduce the required cable sizes and voltage drops incurred in such applications. In the example shown the power source  2  is on the ground and connected to the driver circuit  1  via cabling  120  with the power source receiving external supply  123 . The driver circuit  1  could have also been on the ground level with the motor  13  in the tower  121 .  
         [0047]    As described above, the motor driver circuit  1  and motor driver circuit in combination with an antenna apparatus  119  repositioning motor  13  provides a number of advantages, some of which have been described above and others of which are inherent in the invention. Also modifications may be proposed to the motor driver circuit  1  and motor driver circuit  1  in combination with an antenna apparatus  119  repositioning motor  13  without departing from the teachings herein.