Patent Publication Number: US-7904196-B2

Title: Method and apparatus for controlling pressure in electric injection molding machine

Description:
TECHNICAL FIELD 
     The present invention relates to a method and apparatus for controlling pressure in an electric injection molding machine. 
     BACKGROUND ART 
     A typical electric molding machine employed in the art senses pressure using a pressure sensor in a control target (such as mold open/close, extrusion, and nozzle touch) and, based on a signal from the pressure sensor, configures a closed-loop control circuit to control the propelling power. 
     Specifically, for control of injection pressure, a load cell is arranged at the root of a screw, for example, to sense a force that pushes the screw (a forward force) in the form of pressure by the load cell. Then, based on the sensed pressure, feedback control is applied such that the pressure to be sensed at the load cell reaches a desired pressure value, thereby controlling the propelling power of the screw. 
     A measured signal output from a general pressure sensor such as the load cell is a weak analog signal. An electric injection molding machine includes a large number of motorized instruments that also act as noise sources. Accordingly, noises caused from the motorized instruments may superimpose on the weak analog signal output from the load cell. In this case, the propelling power may not be controlled well as a phenomenon. Therefore, devices such as multistage noise filters are located on an analog signal line from the load cell to prevent a noise-caused control failure. Nevertheless, it is extremely difficult to completely eliminate such the control failure. 
     For adjustment of the load cell, manual works such as zero-point adjustment of an amp and span adjustment are required. Therefore, depending on adjusters, adjusted conditions may differ subtly and result in individually different controlled conditions as a problem. 
     For that reason, based on an angular velocity or rotational angle and a drive current or torque of an electric motor, a current melt pressure value for use in melt pressure control is estimated using state equations. Such a sensorless melt pressure estimating method has been disclosed (in Patent Document 1: U.S. Pat. No. 6,695,994). In the melt pressure estimating method, state equations indicative of the force exerted on the resin (melt) from forward movement of the ram may be given as shown in Expression 1 (see  FIGS. 9-13 ).
 
 E 1 : P   MELT   =F   inj   /A   BARREL  
 
 E 2 : F   inj =(2 πe   S   e   B   N   SP   /lN   MP )[( T   2   −J   TOT α)− T   U   ]−F   LOSS  
 
E3: α=ω′  [Expression 1]
 
     In this expression, E1 is a melt pressure equation, E2 is an injection force equation, and E3 is a motor acceleration equation, with P MELT : Melt pressure value, F inj : Injection force, A BARREL : Barrel area, e S : Ball screw efficiency, e B : Belt efficiency, N SP /N MP : Diameter ratio of Transmission pulleys at Ball screw and Motor, l: Ball screw lead, T 2 : Measured torque value, J TOT : Inertia moment, α: Motor angular acceleration, T U : Support bearing frictional torque, F LOSS : Loss force, and ω: Angular velocity. 
     In the melt pressure estimating method disclosed in the above-described Patent Document 1, for estimation of the melt pressure, the obtained torque command value and angular velocity associated with the motor are employed to directly solve the state equations shown in Expression 1 to obtain the melt pressure P MELT . Accordingly, the Expression includes a differential term denoted with E3, which lowers the resistance against the noises. As a result, a precise melt pressure control is made difficult as a problem. 
     The present invention has been made in consideration of such the problem and has an object to provide a method and apparatus for controlling pressure in an electric injection molding machine, which is capable of achieving precise propelling power control without the use of a pressure sensor such as a load cell. 
     DISCLOSURE OF THE INVENTION 
     The present invention provides a method of controlling pressure in an electric injection molding machine, comprising: detecting an angular velocity ω of a motor operative to propel forward a screw in an injection molding machine; deriving an estimated melt pressure value δ^, based on an observer, from the detected angular velocity ω of the motor and a torque command value T CMD  given to the motor; and controlling the motor such that the estimated melt pressure value δ^ follows a melt pressure setting δ REF . 
     The “observer (observed state value)” defined in the present invention is an equation for obtaining an estimated value of a state variable by solving a differential equation expressed to estimate a state variable (converge at a state variable) such that a control target output coincides with a model output. The “observer” of the present invention thus made by previously solving the differential equation is not required to execute differentiation on actually obtaining the estimated melt pressure value δ^. 
     The observer may be represented by Expression 2, which is depicted in  FIG. 8 , 
     where 
     
         
         
           
             ω^: Estimated value of Angular velocity of Motor 
             d 1 , d 2 : Certain coefficients 
             J: Inertia moment over Injection mechanism 
             F(ω) : Dynamic frictional resistance and Static frictional resistance over Injection mechanism 
           
         
       
    
     The screw in the injection molding machine and the motor may be coupled together via a belt suspended around pulleys mounted on respective rotation shafts. In such the case, the observer can be represented by Expression 3, which is depicted in  FIG. 9 , 
     where 
     
         
         
           
             d 1 -d b : Certain coefficients 
             J M : Inertia moment at Motor side 
             ω M : Angular velocity of Motor 
             R M : Pulley radius at Motor side 
             F: Tension of Belt 
             K b : Spring constant of Belt 
             J L : Inertia moment at Screw side 
             ω L : Angular velocity at Screw side 
             R L : Pulley radius at Screw side 
             F d (ω L ) : Dynamic frictional resistance at Screw side 
             K w : Elastic modulus of Resin 
             K wd : Coefficient of Viscosity of Resin 
             σ: Force of Screw pushing Resin 
           
         
       
    
     The present invention provides another apparatus for controlling pressure in an electric injection molding machine, comprising: an observer arithmetic unit operative to derive a value, an estimated melt pressure value δ^, based on an observer, from an angular velocity ω of a motor operative to propel forward a screw in an injection molding machine and a torque command value T CMD  given to the motor; and a torque arithmetic unit operative to calculate the torque command value T CMD  for the motor based on the above Expression 3 using the estimated melt pressure value δ^ derived at the observer arithmetic unit and feed back the torque command value to the motor. 
     The “melt pressure” as in the estimated melt pressure value δ^ and the melt pressure setting δ REF  is defined as the force of the screw in the injection molding machine that pushes the melt (resin), which differs from the force detected at the load cell in the art that pushes the screw. In a word, the control target in the control of screw propelling power is different in the art from the present invention. 
     In accordance with the present invention, an angular velocity ω of the motor operative to propel forward the screw in the injection molding machine is obtained. An estimated melt pressure value δ^ is derived from the obtained angular velocity ω using the observer theory. The motor is then controlled such that the estimated melt pressure value δ^ follows the melt pressure setting δ REF . Thus, the melt pressure can be controlled precisely without the use of any pressure sensor such as a load cell. 
     In addition, in the present invention a higher resistance against noises is achieved because the equation for calculating the estimated melt pressure value δ^ contains no differential term. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a controller for an electric injection molding machine according to an embodiment of the present invention. 
         FIG. 2  is a detailed block diagram of the controller. 
         FIG. 3  is a graph showing variations over time in torque command value for the controller and in estimated propelling power value. 
         FIG. 4  is an illustrative view of a transmission system in an electric injection molding machine according to another embodiment of the present invention. 
         FIG. 5  is a detailed block diagram of a controller for the electric injection molding machine according to the embodiment. 
         FIG. 6  is an illustrative view of a method of acquiring a velocity-dependent component of the dynamic frictional resistance in the electric injection molding machine. 
         FIG. 7  is an illustrative view of a method of acquiring a load-dependent component of the dynamic frictional resistance in the electric injection molding machine. 
         FIGS. 8-15  depict different expressions or equations referenced herein. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The embodiments of the present invention will now be described based on the drawings. 
       FIG. 1  is a block diagram showing a configuration of a pressure controller for an electric injection molding machine according to an embodiment of the present invention. 
     A control target or motor  1  is an injection motor operative to move a screw back and forth in an injection cylinder, not shown. The motor  1  is equipped with an encoder  2 , which detects positional information (rotational angle) θ of the motor and provides it to external. The positional information θ from the encoder  2  is converted at a differentiator  6  into an angular velocity ω, which is then fed to an observer arithmetic unit  3 . The observer arithmetic unit  3  estimates the propelling power of the screw (melt pressure) δ based on the output ω from the differentiator  6 . Based on a melt pressure setting δ REF  set at a melt pressure setting unit  5  and an estimated melt pressure value δ^ derived at the observer arithmetic unit  3 , a torque arithmetic unit  4  obtains a torque command value T CMD , which is fed back to the control target or motor  1 . 
     The following description is given to operation of the controller. 
     In general, an equation shown in the following Expression 4 has been known as a state equation for an injection mechanism.
 
 Jω′=T   CMD   +δ+F (ω)  [Expression 4]
 
     In this equation,
         J: Inertia moment over Injection mechanism   ω′: Angular acceleration of Motor   ω: Angular velocity of Motor   T CMD : Torque command value   δ: Melt pressure   F(ω): Dynamic frictional resistance and Static frictional resistance over Injection mechanism       

     In consideration of the above Expression 4 and on the assumption that δ′ (differentiated value of δ) is equal to zero, the following Expression is given. 
     
       
         
           
             
               
                 
                   
                     
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                     Expression 
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     The observer (observed state value) is herein defined as the following Expression 6. 
     
       
         
           
             
               
                 
                   
                     
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                     Expression 
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                     6 
                   
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     In this equation,
         ω^: Estimated value of Angular velocity of Motor  1     δ^: Estimated value of Melt pressure (Propelling power)       

     The above Expression 6 gives the following Expression 7.
 
 dω^/dt=d   1 ω^+(1 /J )δ^+(1 /J ) T   CMD   −d   1 ω+(1 /J ) F (ω)
 
 dδ^/dt=d   2   ω^−d   2 ω  [Expression 7]
 
     When a sampling (or processing) period is represented as dt, and data x at a sampling (or processing) period immediately before is represented as x −1 , the following is given.
 
 dω^/dt =(ω^−ω^ −1 )/ dt  
 
 dδ^/dt =(δ^−δ^ −1 )/ dt   [Expression 8]
 
     Accordingly, the estimated angular velocity ω^ and the estimated melt pressure value δ^ are represented as the following Expression 9.
 
ω^=ω^ −1 +( dω^/dt ) dt  
 
δ^=δ^ −1 +( dδ^/dt ) dt   [Expression 9]
 
     Substitution of Expression 7 into Expression 9 yields the following Expression 10. 
     
       
         
           
             
               
                 
                   
                     
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     This replacement gives the following.
 
 dx/dt=Ax  
 
x=e At   [Expression 12]
 
     If the eigenvalue of A has a negative real number, then x→0 when t→∞. Namely, the following is given.
 
ω=ω^
 
δ=δ^  [Expression 13]
 
     In this case, measured values of ω, δ coincide with estimated values thereof. Accordingly, d 1 , d 2  may be determined such that the real number in the eigenvalue of A becomes negative. 
       FIG. 2  is a block diagram showing details of the observer arithmetic unit  3 . 
     The positional information θ output from the encoder  2  is differentiated at the differentiator  6  into the angular velocity ω. This angular velocity is then subtracted at both adders  31  and  32  from an estimated angular velocity value ω^ −1 , obtained at immediately preceding processing, to provide (ω^ −1 −ω), which is sent through adjusters  33  and  34  and adjusted into amplitude corresponding to coefficients d 1  and d 2 . 
     On the other hand, a torque command value T CMD   −1  and an estimated melt pressure value δ^ −1 , obtained immediately before, are summed at an adder  35 . To this sum, the dynamic frictional resistance and static frictional resistance over the injection mechanism, F(ω), is further added at an adder  36 . The sum from the adder  36  is sent through an adjuster  37  and adjusted into amplitude corresponding to a coefficient 1/J. This adjusted value is added at an adder  38  with the output from the adjuster  33 . Thus, the content between brackets { } in the second term on the right side in the upper equation of the Expression 10 can be obtained. Similarly, the content between brackets { } in the second term on the right side in the lower equation of the Expression 10 can be obtained based on the output from the adjuster  35 . 
     These values are integrated at integrators  39  and  40 , respectively, to obtain the estimated values ω^ and δ^ based on Expression 10. 
     The estimated melt pressure value δ^ thus obtained is subtracted from a target pressure setting δ REF  at an adder  7  and the resultant difference is fed to the torque arithmetic unit  4 . The torque arithmetic unit  4  computes a torque command value T CMD  based on the following Expression 14 in the simplest, which is fed back to the motor  1 .
 
 T   CMD   =kp (δ REF −δ^)  [Expression 14]
 
     In this equation,
         δ REF : Melt pressure setting   kp: Certain constant       

     Preferably, the torque command value T CMD  may be computed based on the following Expression 15 and fed back to the motor  1 .
 
 T   CMD   =kp (δ REF −δ^)+ ki ∫(δ REF −δ^) dt   [Expression 15]
 
     In this equation,
         ki: Certain constant       

     As described above, in the present invention, the equation of motion in Expression 11 is solved using the observer theory to calculate the melt pressure δ^. Therefore, any pressure sensor such as a load cell is not required. In addition, the inertia moment J and the dynamic frictional resistance and static frictional resistance F(ω) herein employed are unique parameters of the injection mechanism. Accordingly, the control can be executed independent of the molding resin. 
     The dynamic frictional resistance can be derived from a relation between a torque of the motor and an advancing speed of the screw. Namely, the dynamic frictional resistance is derived from a torque command and a measured value of the injection speed (calculated from the encoder output) at the time when the screw is advanced under no load (without any resin). The tensed condition of the belt that couples the motor and the pulleys in the electric injection molding machine may vary the dynamic frictional resistance possibly. Accordingly, periodic recalculations and updates are desired. 
       FIG. 3  is a graph showing the torque command value T CMD  and the estimated melt pressure value δ^ obtained when the injection molding is performed actually using the controller according to the embodiment. The first half shows the injection step from the beginning of filling a resin into a mold until almost the completion of filling while the screw is velocity-controlled. The second half shows the retaining step after the mold is almost filled with the resin while the screw is propelling power-controlled. In the shown example, the estimated melt pressure value rises up to 600 N·m during the velocity control period and the estimated melt pressure value is retained at 100 N·m in the retaining step. 
     Thus, in accordance with the present invention, a highly precise pressure control can be achieved. 
     The above can be suitably used when a delay in the transmission system from the motor to the screw is negligible, for example, when the motor is directly connected to the screw, or when the motor is linked to the screw via a high-stiffness system such as gears. 
     In the actual electric injection molding machine, however, the delay in the transmission system from the motor to the screw may not be negligible. For example, as shown in  FIG. 4 , a rotational shaft  11  of the motor  1  is coupled to a pulley  12 , then the pulley  12  is linked via a belt  13  to a load-side pulley  14 , and a rotational shaft  15  of the pulley  14  rotates to drive the screw rotationally. In this case, it is required to consider the inertia moment J M , the angular velocity ω M  and the pulley radius R M  at the motor  1 ; the tension F and the spring constant K b  of the belt  13 ; and the inertia moment J L , the angular velocity ω L , the pulley radius R L  and the dynamic frictional resistance F d (ω L ) at the load. 
     State equations established in consideration of the transmission system are represented by the following Expression 16.
 
 J   M ω M′   =T   CMD   −FR   M  
 
 J   L ω L′   =FR   L   +δ+F   d (ω L )
 
 F′=K   b ( R   M ω M   −R   L ω L )  [Expression 16]
 
     The differentiated value δ′ of the melt pressure δ is defined as Expression 17 where the elastic modulus of the resin is herein represented as K w , the coefficient of viscosity as K wd  and the force of the screw pushing the resin as σ.
 
δ′= K   w ω L   +K   wd ω L′ +σ  [Expression 17]
 
     The Expressions 16 and 17 can be modified as Expression 18, which is depicted in  FIG. 10 . 
     The observer (observed state value) is herein defined as Expression 19, which is depicted in  FIG. 11 . 
     Also in this case, Expression 20, which is depicted in  FIG. 12 , can be derived in the same manner as the Expression 10 is derived from the Expressions 7-9. 
     When the differentiated value of the melt pressure δ is δ′=0, the Expression 16 can be expressed as Expression 21, which is depicted in  FIG. 13 . 
     The observer (observed state value) is herein defined as Expression 22, which is depicted in  FIG. 14 . 
     Also in this case, the Expression 23, which is depicted in  FIG. 15 , can be derived in the same manner as the Expression 10 is derived from the Expressions 7-9. 
     Also in this case, the estimated melt pressure value δ^ can be derived without containing any differential term. Accordingly, it is possible to realize a control system excellent in resistance against noises. 
     Expression 22 is replaced by the following Expression 24.
 
 {circumflex over (X)}′=D{circumflex over (X)}+Bu+Cv−Ey   [Expression 24]
 
       FIG. 5  is a block diagram showing details of an observer arithmetic unit  8  operative to execute the computation of Expression 24. 
     The positional information θ output from the encoder  2  is differentiated at the differentiator  6  into the angular velocity ω M , which is sent as y through an adjuster  81  and adjusted into amplitude corresponding to the coefficient E=(d 1 , d 2 , d 3 , d 4 ). 
     On the other hand, a torque command value T CMD   −1 , obtained immediately before, is adjusted as u at an adjuster  82  into amplitude corresponding to the coefficient B=(1/J M , 0, 0, 0). On the other hand, previously obtained v (=the dynamic frictional resistance F d (ω L )) is adjusted at an adjuster  83  into amplitude corresponding to the coefficient C=(0, 1/J L , 0, 0). This value is added at an adder  84  to the output from the adjuster  82 . In addition, an estimated value X^ −1 , obtained immediately before, is adjusted at an adjuster  85  into amplitude corresponding to the coefficient D. At an adder  86 , the output from the adder  84  is added to the output from the adjuster  85 . From the sum, the output from the adjuster  81  is subtracted to obtain the differentiated value shown in Expression 24 that is derived from X^ or the estimated value of X. This value is integrated at an integrator  87  to obtain the estimated value δ^ based on Expression 23. 
     The estimated melt pressure value δ^ thus obtained is subtracted from the target pressure setting δ REF  at the adder  7  and the resultant difference is fed to the torque arithmetic unit  4  to compute the torque command value T CMD , which is fed back to the motor  1 . 
     The dynamic frictional resistance F d (ω L ) is determined in a method (calibration method), which is described next with reference to  FIGS. 6 and 7 . 
     A dynamic frictional resistance model is herein defined as a sum of a velocity-dependent component and a load-dependent component. The velocity-dependent component can be derived from a relation between a motor velocity (or position) and a torque value (or current value) at the time of injection with no resin loaded. The load-dependent component can be derived from a relation between a torque value (or current value) and a pressure value at the time of injection with a plugged nozzle. 
     Velocity-Dependent Component 
     In steady state, as ω M′ =0 and ω L′ =0, the left sides in the first and second equations of Expression 16 both become zero. Therefore, the dynamic frictional resistance F d (ω L ) is represented as follows.
 
 F   d (ω L )=−( R   L   /R   M ) T   CMD −δ  [Expression 25]
 
     Accordingly, it depends only on the torque at the time of injection with no resin loaded (regarded as δ=0). Therefore, as shown in the left half of  FIG. 6 , the torque value  1 ,  2  is measured to obtain the dynamic frictional resistance  1 ,  2  when injection with no resin loaded varies the velocity as  1 ,  2  on forward movement for injection. This is plotted on the graph in the right half of  FIG. 6  to obtain a characteristic curve of the velocity-dependent component. The velocity-dependent component on backward movement for injection can be obtained similarly. 
     Load-Dependent Component 
     Next, a calibration tool is prepared with an attached sensor for measuring pressure (such as a pressure sensor and a strain gauge). Then, the torque value and the pressure value are measured on injection with a plugged nozzle tip to obtain the dynamic frictional resistance  1 ,  2  as shown in the left half of  FIG. 7 . This is plotted, on the graph in the right half of  FIG. 7  to obtain a characteristic curve of the load-dependent component. 
     These velocity-dependent component and load-dependent component in combination can be employed as the dynamic frictional resistance model of the injection mechanism for pressure estimation.