Abstract:
A control system for a linear actuator having an electric motor drawing a variable current level during operation. The control system includes a current level sensor for determining an operational current level of the linear actuator and a controller for generating a drive signal and a force request signal representative of a desired current level of the linear actuator. The drive signal remains constant during a predetermined time interval of the controller. The control system further includes a current limiting component for receiving the force request signal, the current level of the linear actuator and the drive signal. The current limiting component minimizes the current level of the electric motor in response to a comparison between the force request signal and the desired current level within a time interval substantially smaller than the predetermined time interval of the controller.

Description:
RELATED APPLICATION  
       [0001]    This application claims the benefit of priority, pursuant to 35 U.S.C. §119(e), from copending U.S. Provisional Patent application Ser. No. 60/202,587 filed May 9, 2000, incorporated by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention is directed to a control system for linear actuator devices, and more particularly to a control system for linear actuator devices utilized upon a floor maintenance machine.  
         BACKGROUND OF THE INVENTION  
         [0003]    For purposes of convenience, the invention will be described in conjunction with a presently preferred implementation thereof embodied in an electric linear actuator. It will be understood, however, that the principles of the invention may apply equally as well to devices of analogous structure.  
           [0004]    The design of automatic floor cleaning equipment often involves a considerable amount of rotary and/or linear motion actuation and control. Positioning of structures such as cleaning heads and squeegees must be accomplished quickly and transparently to the operator. The traditional method of controlling motion on cleaning equipment utilizes limit switches or other proximity switches that either directly control the power to one or more linear actuators, e.g., via relay switches, etc., or indirectly control linear actuators via a signal sent to a CPU indicating the position of the actuators. These switches introduce negative reliability and assembly issues into the design of the machine. For example, an actuator or linkage could be damaged if a jam occurs in mid stroke of the actuator as current would continue to be supplied to the actuator. Additionally, limit switches may become contaminated or damaged through the operation of the machine. The switches may also be misaligned during the assembly of the machine. Any of these situations can cause the actuator to stall, overheat, and/or damage the linkage or associated structure coupled thereto.  
           [0005]    In mobile equipment systems that include a plurality of electric and or hydraulic devices, such as servo actuators, motors and pumps, it is conventional practice to couple all of such devices to a remote master controller for coordinating or orchestrating device operation to perform a desired task. Motors and actuators may be employed, for example, at several coordinated stages of a surface cleaning machine for automated control of fluids and surface working devices. In accordance with conventional practice, the master controller may comprise a programmable controller or the like coupled to the various remotely-positioned devices. Feedback from the remote devices may be provided via control signals therefrom. For closed-loop operation, a sensor may be coupled to each device for sensing operation thereof, and feeding a corresponding signal to the master controller through an analog-to-digital converter, etc.  
           [0006]    Thus, in a system that embodies a plurality of electric and/or hydraulic devices, a substantial quantity of electrical conductors must be provided for feeding individual control signals to the various devices and returning sensor signals to the master controller. Such conductors interfere with system design and operation, and are subject to failure. The bank of D/A and A/D converters for feeding signals to and from the master controller add to the expense and complexity of the overall system. Perhaps most importantly, system performance is limited by capabilities of the master controller. For example, a programmable controller may require one hundred milliseconds to scan a device sensor signal, compute a new control signal and transmit such control signal to the remote device. An overburdened programmable controller may not perform acceptably in high performance applications that may require a ten millisecond response time, for example, at each of a plurality of remote devices.  
         SUMMARY OF THE PRESENT INVENTION  
         [0007]    The present invention relates to a linear actuator control system exhibiting improved performance. To solve some of these limitations associated with the prior art devices, a control system has been implemented in which the speed and force from the actuator can be independently controlled from a control processing unit (CPU). In a system according to the present invention, the CPU can monitor the force being delivered to the actuator and that information can be used to deduce the force and/or position of the actuator. This information can also be used to determine that the actuator has reached the end of its stroke. A system according to the present invention has the ability to reduce or terminate the power being delivered to the load device in order to prevent damage to the device. The reaction time of this protection circuitry is short enough to prevent damage to the load and the energy control circuitry. Importantly, such a system can eliminate the position sensing devices normally used in this type of machine.  
           [0008]    The present invention relates to a control system for one or more linear actuator devices, such as present on a surface maintenance machine. One aspect of the invention is to provide a linear actuator control system for use on a surface maintenance machine, such as a scrubber or sweeper, which utilizes a comparison circuit in which a signal representative of the load current in an linear actuator is modified by a signal representative of the desired load current to maintain applied load current at a desired level.  
           [0009]    Another aspect of the present invention provides a control system which automatically limits the current load to a linear actuator in the event of an abnormal condition, e.g. linkage jamming, obstruction contact, etc.  
           [0010]    Another aspect of the present invention provides a control system for automatically controlling one or more linear actuators of a surface maintenance machine which may be applied to various types of surface maintenance machines having different surface maintenance tools and providing for different surface maintenance functions.  
           [0011]    A linear actuator control system in accordance with a further aspect of the invention includes a linear actuator having an electric motor component. The electric motor component is connected to drive circuitry that includes a solid state switch, preferably a FET, that is connected between one terminal of the electric motor, with the other terminal being connected to electrical ground. The control switch circuit receives a switch control signal from the microprocessor-based control electronics, and is connected to the control electrode (gate) of the FET for setting the switch circuit and controlling power to the electric motor of the linear actuator through the FET in response to the control signal. Feedback circuitry is responsive to the current through the electric motor for resetting the switch circuit and interrupting application of power to the electric motor. The feedback circuitry is responsive to a voltage drop across a shunt resistor.  
           [0012]    It is therefore a general object of the present invention to provide a linear actuator control system that exhibits a fast response time necessary for high performance applications, while at the same time reducing cost and complexity that are inherent in prior art system of the character described above. In furtherance of the foregoing, a more specific object of the invention is to provide a system of the described character wherein each of the system linear actuators embodies microprocessor-based control adapted to communicate with a central or master controller and for thereby distributing, at least partial, control of the several linear actuators while maintaining overall coordination thereamong.  
           [0013]    Another object of the present invention is to provide a linear actuator control structure in which all control components, including current level detectors and microprocessor-based control electronics, are fully integrated into compact inexpensive packages, and which may be readily employed in a wide variety of system applications.  
           [0014]    Yet another object of the invention is to provide a linear actuator of the described character with enhanced robustness of hardware, including the elimination of limit switches or other position detection devices within or in association with the linear actuator.  
           [0015]    Still another object of the present invention is to provide a system for controlling a linear actuator device, with control electronics that limit current overload as compared with prior art devices of a similar character, and that have enhanced capabilities for protecting the linear actuator against damage due to structure obstruction, contact, etc. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    Preferred embodiments of the invention will be described in detail hereinafter with reference to the accompanying drawings, in which like reference numerals refer to like elements throughout, wherein:  
         [0017]    [0017]FIG. 1 is a perspective of a typical walk-behind surface maintenance machine which may utilize the control system of the present invention;  
         [0018]    [0018]FIG. 2 is a block diagram illustrating the control system for a linear actuator according to the present invention;  
         [0019]    [0019]FIG. 3 is a simplified schematic circuit illustrating a preferred embodiment of the present invention; and  
         [0020]    [0020]FIGS. 4A and 4B together illustrate a preferred embodiment of the control system of FIG. 1 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0021]    For purposes of convenience, the invention will be described in conjunction with a presently preferred implementation thereof embodied in an electric linear actuator. It will be understood, however, that the principles of the invention may apply equally as well to devices of analogous structure.  
         [0022]    In FIG. 1, a vehicle such as a floor scrubbing machine  10  is indicated generally and may be of a type manufactured by Tennant Company of Minneapolis, Minn., assignee of the present invention. Such a device is disclosed in U.S. Pat. No. 4,757,566, the entire disclosure of which is incorporated by reference herein for all purposes. The scrubber  10  may include a housing  12  and a rear operating control  14  which is used by the operator to control vehicle  10  speed and direction. A control device  16  is used to control functions of the machine  10 . There may be a pair of rotating brushes or pads  18 . A linear actuator  20  may be utilized to control the position, and hence the downward force, of the brushes  18 . A squeegee  22  is normally positioned at the rear of the vehicle  10  and is effective, as is known in the art, to squeegee the floor and remove any standing water. Normally, there will be a vacuum device  24  attached to the squeegee  22  which will apply suction to remove standing water collected by the squeegee.  
         [0023]    In one embodiment of the present invention, there may be one or more surface working tools such as sweeping brushes, scrubbing brushes or polishing pads, and there may be one or more electric actuators  20  controlling the position of said surface maintenance tools  18 . In other embodiments of the present invention, there may be one or more hopper or debris containers (not shown), and there may be one or more linear actuators  20  controlling the lifting of the hopper during a hopper dumping procedure. Linear actuators  20  may comprise an electric DC motor as the motive element. Those versed in the art are aware that in an electric DC motor the current which the motor draws is proportional to the load on the motor.  
         [0024]    Although the invention will be described in connection with a scrubber  10 , it should be clear that the control structure according to the present invention has application to other types of vehicles using surface maintenance tools, such as a sweeper or a polishing or burnishing machine.  
         [0025]    Referring to FIG. 2, a block diagram is provided to explain functional interrelations between various elements of a control device  16  according to the present invention. The control device  16  is utilized to control the linear actuator  20 . Control device  16  includes a central processor unit  30  (CPU) which receives input from elements of the control system and provides output signals to elements of the control system. Control device  16  includes the additional elements: maximum current level converter  32 , high speed current limit  34 , power control device  36 , current measurement element  38 , current level converter  40 . Additional elements or components would be appreciated by those skilled in the relevant arts.  
         [0026]    CPU  30  may be a dedicated controller or may be part of a larger controller for operating additional functions of a maintenance machine. CPU  30  may be a programmable logic controller (PLC). CPU  30  provides a speed request signal  42  to the high speed limit block  34 . The speed request signal  42  may be an analog or digital signal. In one embodiment, the speed request signal  42  is an analog signal comprising a voltage level representative of the speed request. CPU  30  also provides a maximum force request signal  44  which is converted by the maximum current level converter  32 , which may be D/A converter, into a maximum current level signal  46 . Maximum current level signal  46  is provided as another input signal to the high speed current limit block  34 .  
         [0027]    CPU  10  receives a signal  48  from the high speed current limit block  34  indicating whether or not a maximum designated current of the linear actuator  20  has been exceeded. The CPU  10  utilizes this information to determine if the actuator  20  has reached the end of its stroke, or if it has come in contact with an obstacle. The CPU  10  can utilize internal timers to estimate the position of the actuator  20  during a move. It can use this information to adjust the speed and maximum force of the actuator  20  as the actuator movement progresses. For instance, the CPU  10  may request high speed and high current to start the actuator moving, high speed and medium current through the bulk of the movement, and low speed, low current to minimize the impact at the end of actuator stroke. The feedback signal  48  could be a current level, rather then the maximum force signal. In that case, the CPU  10  could also use the load information to make decisions as to actuator  20  speed, position, or stroke length. For example, this would be useful if a hopper lift height of a particular machine should be limited by load. This could also be used to estimate the speed and position of the actuator  20  using back EMF calculations or changes in mechanical advantage as the actuator  20  progresses through its stroke.  
         [0028]    The maximum current level conversion block  32  converts the force request  44  from the CPU  10  to a maximum current level  46  that can be interpreted by the high speed current limit block  34 . The high speed current limit block  34  uses the maximum current level signal  46  from the maximum current level conversion block  32 , the speed request signal  42  from the CPU block  10 , and a current level signal  50  from the current level conversion block  40  to generate an energy level control signal  52  for the power control device  36 . In one embodiment, the energy level control signal  52  is a pulse width-modulated signal used to control the gate of a Field Effect Transistor (FET) within the power control device  36 .  
         [0029]    The output of the high speed current limit block  34  will reflect the duty cycle of the speed request  42  unless the maximum current level is exceeded (current limit mode). When in current limit mode, block  34  will signal the power control device  36  to limit the current of the linear actuator  20  in order to prevent overheating of the device or other damage. Also when in current limit mode, the high speed current limit block  34  will send a maximum force exceeded signal  48  to the CPU  10  indicating that the maximum allowable current has been exceeded. The CPU  10  can then utilize this information to terminate operation of the load. Because the high-speed current limit  34  acts prior to CPU  10  direction to reduce the current to the load  20 , the time delay induced waiting for the CPU  10  to directly terminate operation of the load  20  is less critical.  
         [0030]    The power control device  36  receives the control signal  52  from the high speed current limit block  34 , and uses it to control power flow from a battery  54  to the load. In one embodiment, the power control device  36  is a Field Effect Transistor (FET). The current measurement block  40  provides a voltage level  50  proportional to the level of current flowing to the load. In this design, the current measurement device  38  is a shunt resistor. The current level conversion block  40  receives the raw current level information from the current measurement block  38 , and transforms it into a format that can be received by the high speed current limit block  34 .  
         [0031]    [0031]FIGS. 3 and 4 illustrate preferred embodiments of the present invention. A microprocessor controller (or CPU)  110  is utilized in the control structure. Those skilled in the relevant arts will recognize that the controller  110  can receive a variety of inputs and control a variety of outputs. Specific to illustrated embodiment of the present invention, the outputs of the controller  110  include a PWM (pulse-width-modulated) drive signal  112  and a “force request” signal  114 . An input to the microprocessor  110  includes a comparison signal  106 .  
         [0032]    The force-request signal  114  is received by a D/A converter  122  which calculates a maximum current level corresponding to the force request signal  114  and outputs an analog signal representing the maximum current level  124  to a threshold comparator  160 . The other input  130  to the comparator  160  is received from a motor current signal circuit  142 , as described hereinafter. The comparator output  136  is provided both to a NAND device  162  and as a comparison signal  106  to the microprocesser  110  (as a feedback signal). The PWM drive signal  102  and comparator output  136  are received as input signals into the NAND device  162 , the output of which is used to control the power control device (FET)  138 . The linear actuator power control switch  138  is a FET having primary current-conducting source and drain electrodes connected in series with the linear actuator  120  and a current sensing shunt resistor  166  between ground. The motor current signal  130  (to the comparator)  160  is obtained via a motor current signal circuit  142  vis-à-vis an amplified shunt resistor voltage. The output  130  from an amplifier  143  is a voltage indicative of load current in the linear actuator  120 . The motor current sensor  142  utilizes a shunt resistor  166 , with the voltage drop across the shunt  166  used as an indicator of the current flow to the motor  120 . Alternative current sensors  142  maybe used, however. For example, a toroidal core or other non-contact type of sensor may be utilized.  
         [0033]    In operation, the microprocesser  110  generates a PWM drive signal  102  and a force request signal  104 . In under-load current conditions (the comparison signal  106  not high), the PWM signal  102  is passed through the NAND device  162  to the FET switch  138  to control the duty cycle of the linear actuator  120 . Conversely, when under excessive current load condition (the comparison signal  106  is high), the NAND device  162  blocks the PWM signal  102  from activating the FET switch  138 .  
         [0034]    After the PWM drive signal  102  is generated, the force request signal  114  is generated and passed for further processing by the D/A converter  122 . The threshold comparator  160  is used to detect over current conditions (the motor current signal  130  exceeds the D/A output signal)  124 . A comparison signal  136  is generated and fed back to the microprocessor  110 .  
         [0035]    [0035]FIG. 3 includes additional aspects of the present invention, include a multiplexer and the FET-based bridge for implementing the control system for a pair of linear actuators.  
         [0036]    Various modifications of the above-described embodiment of the invention will be apparent to those skilled in the relevant arts, and it is to be understood that such modifications can be made without departing from the scope of the invention.