Patent Publication Number: US-2023163714-A1

Title: A missile servo actuation system and method of controlling a servo actuation system in a missile

Description:
FIELD OF THE INVENTION 
     The invention concerns a missile servo actuation system. The invention also concerns a method of controlling a servo actuation system in a missile. 
     BACKGROUND OF THE INVENTION 
     Electric servo motors in missile actuation systems may be required to operate over a wide temperature range; however, control of such motors can be difficult since key parameters such as electrical resistance and torque constant often vary significantly with temperature. Control of electric servo motors over a range of temperatures can be improved by compensating for temperature-dependent variations in the missile actuation system control systems. 
     It is known to measure motor temperature in a missile actuation system using a temperature sensor, and then to adapt the control signals according to the measured temperature values. A disadvantage of this approach is the complexity of the hardware and expense of the additional components. 
     It would be desirable to provide an improved missile servo motor control system which overcomes the aforementioned drawbacks. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the invention provides a method of controlling a servo actuation system, according to claim  1 . 
     In an advantage over systems and methods of the prior art, the present invention provides an estimated motor temperature value for input to motor control algorithms to enable compensation for motor temperature in motor control signals, without requiring temperature sensor hardware. The present invention provides savings on system complexity and cost, and a reduction in weight which is particularly desirable in missile design. 
     Aside from feeding in to the motor control algorithms, the motor temperature estimated from the one or more parameters may be used in a variety of other applications. For example it may be used for plant characterisation or detection of manufacturing faults, model validation, controller adaptive tuning, improved fault detection during operation potentially allowing graceful degradation, and/or self-protection against over-temperature. 
     In a first step of the method, the motor resistance may be estimated from motor voltage divided by motor current. Motor voltage may include an adjustment for the product of the motor velocity and the velocity constant. The estimated motor resistance may be equated to the motor voltage minus the product of the estimated velocity constant and the motor velocity, all divided by the motor current. 
     In a second step of the method, once the motor resistance and torque constant have been estimated, the motor temperature may be derived for example, by comparing the estimated motor resistance and/or torque constant against known values at a reference temperature (for example 20° C.). For example, motor temperature may be equated to the ratio of estimated motor resistance to motor resistance at 20° C., minus one, all multiplied by ( 1 /0.00393), and added to 20. Alternatively or additionally, motor temperature may be equated to the ratio of estimated torque constant to torque constant at 20° C., minus one, all multiplied by (-1/0.0012), and added to 20. 
     In a second aspect, the invention provides a servo actuation system according to claim  11 . 
     It will of course be appreciated that features described in relation to one aspect of the invention may be incorporated into other aspects of the invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings of which: 
         FIG.  1    is a schematic diagram showing a servo actuation system in a missile, according to the example embodiment of the invention; 
         FIG.  1   a    is a schematic diagram of the components of the servo actuation system of  FIG.  1   ; 
         FIG.  2    is a schematic diagram of the functional modules of the servo actuation system of  FIG.  1   ; 
         FIG.  3    is a flow diagram of a method of controlling the motor in the servo actuation system of  FIGS.  1 - 3   . 
     
    
    
     DETAILED DESCRIPTION 
     According to the example embodiment of the invention, a guided missile  2  has four tail fins  4  (only two of which are visible in  FIG.  1   ) which are actuated by a servo actuation system  1  to rotate the tail fins  4  with respect to the missile body  6 , through a range of deflection angles ( FIG.  1   ). In an alternative embodiment, a missile may have several servo actuation systems used for actuating different mechanical components of the missile such as wings, or a fuel pump, in addition to the tail fins. 
     The servo actuation system  1  of the example embodiment of the invention includes a processor  8 , an inverter  5 , a servo motor  7 , a current sensor  11  and a position sensor  13  ( FIG.  1   a   ). The processor  8  is connected in series with the inverter  5  and the servo motor  7 . The current sensor  11  is located downstream of the inverter  5  and upstream of the motor  7 . The position sensor  13  is located downstream of the motor  7 , on the motor rotor (not shown). 
     In the example embodiment of the invention, the motor  7  is a brushless DC servomotor with a construction known in the art and not discussed further herein. The inverter  5  is in this example embodiment is a three-phase MOSFET-based inverter. 
     The inverter  5  imposes AC motor voltage demands  10  on the motor  7  terminals by switching the motor  7  terminals between the high and low sides of the power supply in a controlled switching sequence ( FIG.  2   ). The rotor (not shown) of the motor  7  moves according to the demanded voltage  10 , and the output shaft (not shown) actuates the fins via a gear-box (not shown). The electro-mechanical operation of the motor  7  is known in the art and not discussed further herein. 
     The processor  8  contains software modules, including a controller  3 , a state observer  9  and a parameter estimator  15 . In an alternative embodiment the software modules could be implemented in hardware, for example using FPGAs). 
     The controller  3  receives a demanded motion (or position) input  12  relating for example, to a missile fin position, and outputs a DC demanded voltage signal  14  to the inverter  5 . 
     The state observer  9  provides a feedback pathway from the motor  7  to the controller  3 . The state observer  9  estimates parameters of the motor  7  from measured motor input and/or output values, and feeds back information on the motor condition to the controller  3  such that the control (demanded voltage) signals  14  can be adjusted accordingly. For example, in operation, the state observer  9  estimates motor velocity and acceleration based on the rotor position measurements of the position sensor  13 , and outputs the estimated values along with the measured rotor position values to the controller  3 . 
     The state observer  9  also outputs to the parameter estimator  15 , where further parameters of the motor  7  are calculated. The parameter estimator  15  calculates motor temperature for example based on the motor velocity and acceleration values already inferred by the state observer  9 , the measured motor current values from the current sensor  11 , and estimates of the motor voltage. The motor temperature estimates and/or other parameters are output to the controller  3  and the state observer  9 . A further feedback pathway is therefore provided from the motor  7  to the controller  3  via the state observer  9  and the parameter estimator  15 . The parameter estimator  15  contains a first estimator  16   a , a second estimator  16   b  and a comparator  16   c . 
     In this example embodiment, the motor temperature is calculated as follows: 
     In a first step  17  of the method of the example embodiment of the invention ( FIG.  3   ), the first estimator  16   a  in the parameter estimator  15  estimates the motor resistance and torque constant (i.e. the velocity constant), using a recursive least squares algorithm:             R     est       =       voltage   −           vel const       est        x vel           current               R est  = motor resistance (estimated)   vel = motor velocity   vel const est  = velocity constant (estimated) i.e. torque constant (estimated)   current = motor current   voltage = motor voltage   

     The motor current is measured using the current sensor  11 . In this example embodiment, the motor voltage and motor velocity are each estimated online (i.e. in real-time during operation) by the state observer  9 ; the motor velocity is derived from position measurements made using the position sensor  13  located on the rotor (although in an alternative embodiment it may be measured directly). 
     The motor true voltage is assumed to follow the demanded voltage after scaling for the inverter duty cycle and compensating for deadband non-linearity in the inverter  5 . The motor voltage is calculated by the state observer  9  as the product of the demanded voltage and inverter duty cycle. A correction is made to the inverter duty cycle, to compensate for deadband introduced by the inverter  5 . The method of deadband compensation is known and described in J.W.Choi et al. “Inverter output voltage synthesis using novel dead time compensation”, IEEE Transaction on Power Electronics, Vol  11 , No.2, March 1996, pp. 221-227. 
     In a second step  19  of the method of the example embodiment of the invention, once the motor resistance and torque constant have been estimated, then the motor temperature is derived by the comparator  16   c  by comparing the estimated motor resistance and/or torque constant against known values at a reference temperature (in the example embodiment, 20° C.): 
     
       
         
           
             Temperature 
             = 
             
               
                 
                   
                     
                       
                         
                           R 
                           
                             est 
                           
                         
                         / 
                         
                           R 
                           
                             20 
                             deg 
                             C 
                           
                         
                       
                     
                     − 
                     1 
                   
                 
                  x  
                 
                   
                     1 
                     / 
                     0.00393 
                   
                 
               
             
             + 
             20 
           
         
       
     
     
         
         Temperature = motor temperature 
         R est  = motor resistance (estimated) 
         R 20degC  = motor resistance at 20° C. And/or: 
         
           
             
               
                 Temperature 
                 = 
                 
                   
                     
                       
                         
                           
                             
                               
                                 kt 
                               
                               
                                 est 
                               
                             
                             / 
                             
                               
                                 kt 
                               
                               
                                 20 
                                 deg 
                                 C 
                               
                             
                           
                         
                         − 
                         1 
                       
                     
                      x  
                     
                       
                         − 
                         1 
                         / 
                         0.0012 
                       
                     
                   
                 
                 + 
                 20 
               
             
           
         
         Temperature = motor temperature 
         kt est  = torque constant (estimated) 
         kt 20degC  = torque constant at 20° C. 
         Where kt is the torque constant. 
       
    
     The inventors have recognised that resistance and torque constant, as electrical parameters, are good indicators as they exhibit high repeatability (i.e. the way that temperature affects the motor resistance and torque constant is repeatable). Other parameters could be used but are less preferable. Temperature affects friction, allowing some inference to be made from the mechanical dynamic performance of the motor; however, the effect is variable and is heavily disturbed by variable mechanical loads. The electrical parameters are immune to the mechanical load conditions. 
     The motor resistance has been found to be easier to estimate accurately if there is a large range of current in the underlying data; the torque constant has been found to be easier to estimate accurately if there is a large range of speed. The inventors have recognised that the temperature estimation function can be adjusted to place more weight on one or other of the estimated parameters (motor resistance and torque constant). 
     In the example embodiment the weighting is chosen dynamically, based on the richness of the data. The parameter with the richer data is more likely to be accurate, and so is given a higher weighting in the temperature estimation calculation. If the voltage/ current data is richer than the velocity data then the motor resistance is given a higher weighting than the torque constant, and vice versa. Use of a dynamic weighting provides a more accurate temperature estimate. This has various benefits including making it more likely that the protection against overheating triggers at the correct time; that any controller gain scheduling is more effective; and model validation data is more accurate. 
     The richness comparison also enables cross-checking: if both the voltage/current data and velocity data are rich, but the parameter estimates are not consistent with the same temperature, then that is a useful indication that a fault has developed, such as an inter-winding short-circuit, fractured magnet, or cable/connector damage. 
     In another example embodiment of the invention, the temperature could be estimated from the motor resistance and torque constant using constant weightings, for example a constant weighting of 50:50. In another example embodiment of the invention, 100% weighting could be placed on one of motor resistance or torque constant. 
     If there is little range in either the current or velocity data, then optionally high frequency current pulses can be introduced that have negligible net effect on the motor torque/acceleration, but provide sufficiently rich voltage/current data for the motor resistance, and then temperature, to be estimated. 
     The introduction of sharp current pulses in order to get richer data also increases the effectiveness of the system, making the temperature estimate more accurate. 
     In a third step  21  of the method of the example embodiment of the invention, once the motor temperature is estimated, it is output from the parameter estimator  15  to the controller  3  (or in an alternative embodiment could be fed back via the state observer  9 ). The motor control algorithms then take account of the temperature to adjust the control signals which are provided to the inverter  5 . 
     In the example embodiment of the invention, the motor control algorithms are adapted to include gain-scheduling so as to maintain current, velocity and position control performance despite variations in motor torque constant and resistance due to temperature changes. In other words, the proportional and integral gains change in order to maintain the desired closed-loop transfer function of the system, counteracting changes in the plant parameters. Furthermore, in the example embodiment of the invention, the motor control algorithms are adapted to include a cut-out function so that when the motor temperature exceeds a pre-determined threshold, the motor drive is disabled, thus protecting the motor from overheating. 
     Use of dynamic weightings and/or the introduction of sharp current pulses provides a more accurate temperature estimate, which has various benefits including making it more likely that the protection against overheating triggers at the correct time; that any controller gain scheduling is more effective; and model validation data is more accurate. 
     In a further advantage of the example embodiment of the invention, the torque constant derived by the first estimator  16   a  can be used to derive further useful parameters of the motor. 
     In an optional fourth step  23  of the method motor inertia and friction are estimated by the second estimator  16   b  in the parameter estimator  15  from the estimated torque constant and the measured motor velocity and motor current data, using a using a recursive least squares algorithm: 
     
       
         
           
             
               
                 kt 
               
               
                 est 
               
             
             = 
             
               
                 
                   
                     
                       
                         inertia 
                       
                       
                         est 
                       
                     
                      x acc 
                   
                 
                 + 
                 
                   
                     
                       
                         visc fr 
                       
                       
                         est 
                       
                     
                      x vel 
                   
                 
                 + 
                 
                   
                     coul fr 
                   
                   
                     est 
                   
                 
               
               
                 current 
               
             
           
         
       
     
     
         
         Kt est  = torque constant (taken from first estimator) inertia est  = motor inertia (estimated) 
         acc = motor acceleration 
         viscous fr est  = viscous friction (estimated) 
         vel = motor velocity 
         coul fr est  = coulomb friction (estimated) 
         current = motor current 
       
    
     The motor acceleration is estimated online (i.e. in real-time during operation) by the state observer  9 . The motor acceleration is derived from the motor rotor position measurements, but in an alternative embodiment may be measured directly. The estimated motor inertia and friction, as useful parameters, can then be used in a variety of other applications to improve motor performance. 
     Whilst the invention has been described and illustrated with reference to a particular embodiment, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described. 
     The example embodiment of the invention concerns a missile; however, the technique applies to other motor drive systems too, such as vacuum cleaners, cars, and industrial drives. The estimated temperature may be used in a variety of applications, by way of example including: plant characterisation / detection of manufacturing faults; model validation; controller adaptive tuning; and improved fault detection in operation, potentially allowing graceful degradation. 
     In plant characterisation, if temperature is inferred using the first estimator and the second estimator is used to infer the friction, then the temperature dependence of friction can be estimated. In manufacturing pass-out tests, if the estimated torque constant and resistance are inconsistent with a single temperature value, then that is an indicator that something is wrong in the hardware build. If the estimated resistance and torque constant parameter values are outside their expected ranges, then that indicates a problem. If the friction estimate produced by the second estimator is unexpectedly high then that could indicate that the bearing grease is wrong, or that the pre-loads are too high. In model validation, the estimation data can inform the model. The estimated temperature may be used to validate thermal models of the motor and the environment local to the motor for example to determine and report the temperature of the motor in operational or test environments where the use of discrete temperature sensors is impractical, or to determine the ambient temperature prior to operation of the motor after a period of being inactive. In adaptive tuning, system performance can be maintained despite significant variation in the plant parameters. In fault detection, if the parameter estimates are either not credible, or their rate of change is not credible, or the temperature implied by the two parameter estimates (resistance and torque constant) is not consistent, then a fault may be detected. 
     Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.