APMonitor

Example ARX in GEKKO

plt.figure(figsize=(7,4))

plt.tight_layout() plt.savefig('test.png',dpi=300)

plt.figure(figsize=(7,4))

plt.tight_layout() plt.savefig('test.png',dpi=300)

(:toggle hide gekko_siso button show="Python GEKKO SISO ARX":) (:div id=gekko_siso:)

(:source lang=python:)

1. see https://apmonitor.com/wiki/index.php/Apps/ARXTimeSeries

from gekko import GEKKO import numpy as np import pandas as pd import matplotlib.pyplot as plt

1. load data and parse into columns

url = 'http://apmonitor.com/do/uploads/Main/tclab_dyn_data2.txt' data = pd.read_csv(url) t = data['Time'] u = data['H1'] y = data['T1']

m = GEKKO()

1. system identification

na = 2 # output coefficients nb = 2 # input coefficients yp,p,K = m.sysid(t,u,y,na,nb,pred='meas')

plt.figure() plt.subplot(2,1,1) plt.plot(t,u,label=r'$Heater_1$') plt.legend() plt.ylabel('Heater') plt.subplot(2,1,2) plt.plot(t,y) plt.plot(t,yp) plt.legend([r'$T_{meas}$',r'$T_{pred}$']) plt.ylabel('Temperature (°C)') plt.xlabel('Time (sec)') plt.show() (:sourceend:) (:divend:)

(:toggle hide gekko_mimo button show="Python GEKKO MIMO ARX":) (:div id=gekko_mimo:)

(:source lang=python:) from gekko import GEKKO import numpy as np import pandas as pd import matplotlib.pyplot as plt

1. load data and parse into columns

url = 'http://apmonitor.com/do/uploads/Main/tclab_dyn_data2.txt' data = pd.read_csv(url) t = data['Time'] u = data'H1','H2'? y = data'T1','T2'?

m = GEKKO()

1. system identification

na = 2 # output coefficients nb = 2 # input coefficients yp,p,K = m.sysid(t,u,y,na,nb,pred='meas')

plt.figure() plt.subplot(2,1,1) plt.plot(t,u,label=r'$Heater_1$') plt.legend([r'$Heater_1$',r'$Heater_2$']) plt.ylabel('Heaters') plt.subplot(2,1,2) plt.plot(t,y) plt.plot(t,yp,) plt.legend([r'$T1_{meas}$',r'$T2_{meas}$', r'$T1_{pred}$',r'$T2_{pred}$']) plt.ylabel('Temperature (°C)') plt.xlabel('Time (sec)') plt.show() (:sourceend:) (:divend:)

• Time Delay

 GEKKO Usage: y,u = m.arx(p,y=[],u=[])

1. A (na x ny)

1. B (ny x (nb x nu))

A = np.array()

B1 = np.array([0.63212,0.18964]).T B2 = np.array([0.31606,1.26420]).T B = np.array(B1],[B2?)

C = np.array([0,0])

1. create parameter dictionary
2. parameter dictionary p['a'], p['b'], p['c']
3. a (coefficients for a polynomial, na x ny)
4. b (coefficients for b polynomial, ny x (nb x nu))
5. c (coefficients for output bias, ny)

p = {'a':A,'b':B,'c':C}

y,u = m.arx(p)

• MIMO Model Identification

Also see:

Also see

• Continuous State Space
• Discrete State Space

 GEKKO Usage: y,u = m.arx(A,B,na,nb,ny,nu)

 Data: A, B matrices

Example Model Predictive Control in GEKKO

Example Model in APMonitor

 Objects
   sys = arx
 End Objects

 Parameters
   mv1
   mv2
 End Parameters

 Variables
   cv1 = 0
   cv2 = 0
 End Variables

 Connections
   mv1 = sys.u[1]
   mv2 = sys.u[2]
   cv1 = sys.y[1]
   cv2 = sys.y[2]
 End Connections

 File sys.txt
   2 ! inputs 
   2 ! outputs 
   1 ! number of input terms 
   2 ! number of output terms 
 End File

 File sys.alpha.txt
   0.36788, 0.36788
   0.223, -0.136
 End File

 File sys.beta.txt
   0.63212, 0.18964
   0.31606, 1.2642
 End File

With n_a=3, n_b=2, n_u=1, and n_y=1 the time series model is:

There may also be multiple inputs and multiple outputs such as when n_a=1, n_b=1, n_u=2, and n_y=2.

$$y_{k+1} = \sum_{i=1}^{n_a} a_i y_{k-i+1} + \sum_{i=1}^{n_b} b_i u_{k-i+1}$$

(:title ARX Time Series Model:) (:keywords linear, ARX, Output Error, dynamic, multiple input, multiple output, MIMO, model predictive control:) (:description Autoregressive exogenous models are a linear representation of a dynamic system in discrete form. Examples show how to use a time series model in APMonitor, Python GEKKO, and Python Numpy.:)

APMonitor Objects

 Type: Object
 Data: A, B, C, and D matrices
 Inputs: Input (u)
 Outputs: Output (y)
 Description: ARX Time Series Model
 APMonitor Usage: sys = arx
 GEKKO Usage: y,u = m.arx(a,b,na,nb,ny,nu)

ARX time series models are a linear representation of a dynamic system in discrete time. Putting a model into ARX form is the basis for many methods in process dynamics and control analysis. Below is the time series model with a single input and single output with k as an index that refers to the time step.

$$y_{k+1} = \sum_{i=1}^n_a a_i \, y_{k-i+1} + \sum_{i=1}^n_b b_i \, u_{k-i+1}$$

With na=3, nb=2, nu=1, and ny=1 the time series model is:

$$y_{k+1} = a_1 \, y_k + a_2 \, y_{k-1} + a_3 \, y_{k-2} + b_1 \, u_k + b_2 \, u_{k-1}$$

There may also be multiple inputs and multiple outputs such as when na=1, nb=1, ny=2, and nu=2.

$$y1_{k+1} = a_{1,1} \, y1_k + b_{1,1} \, u1_k + b_{1,2} \, u2_k$$

$$y2_{k+1} = a_{1,2} \, y2_k + b_{2,1} \, u1_k + b_{2,2} \, u2_k$$

Time series models are used for identification and advanced control. It has been in use in the process industries such as chemical plants and oil refineries since the 1980s. Model predictive controllers rely on dynamic models of the process, most often linear empirical models obtained by system identification.

These models are typically in the finite impulse response, time series, or linear state space form. Once in APMonitor form, nonlinear elements can be added to avoid multiple model switching, gain scheduling, or other ad hoc measures commonly employed because of linear MPC restrictions.

Example Model in APMonitor

 Objects
   sys = arx
 End Objects

 Parameters
   mv1
   mv2
 End Parameters

 Variables
   cv1 = 0
   cv2 = 0
 End Variables

 Connections
   mv1 = sys.u[1]
   mv2 = sys.u[2]
   cv1 = sys.y[1]
   cv2 = sys.y[2]
 End Connections

 File sys.txt
   2 ! inputs 
   2 ! outputs 
   1 ! number of input terms 
   2 ! number of output terms 
 End File

 File sys.alpha.txt
   0.36788, 0.36788
   0.223, -0.136
 End File

 File sys.beta.txt
   0.63212, 0.18964
   0.31606, 1.2642
 End File

Example Model Predictive Control in GEKKO

(:source lang=python:) import numpy as np from gekko import GEKKO import matplotlib.pyplot as plt

na = 2 # Number of A coefficients nb = 1 # Number of B coefficients ny = 2 # Number of outputs nu = 2 # Number of inputs

1. A (ny x na)

A = np.array()

1. B (ny x nu x nb)

B = np.array()

1. Create GEKKO model

m = GEKKO(remote=False)

1. Build GEKKO ARX model

y,u = m.arx(A,B,na,nb,ny,nu)

1. load inputs

tf = 20 # final time u1 = np.zeros(tf+1) u2 = u1.copy() u1[5:] = 3.0 u2[10:] = 5.0 u[0].value = u1 u[1].value = u2

1. customize names

mv1 = u[0] mv2 = u[1] cv1 = y[0] cv2 = y[1]

1. options

m.time = np.linspace(0,tf,tf+1) m.options.imode = 4 m.options.nodes = 2

1. simulate

m.solve()

plt.figure(1) plt.subplot(2,1,1) plt.plot(m.time,mv1.value,'r-',label=r'$MV_1$') plt.plot(m.time,mv2.value,'b--',label=r'$MV_2$') plt.ylabel('MV') plt.legend(loc='best') plt.subplot(2,1,2) plt.plot(m.time,cv1.value,'r:',label=r'$CV_1$') plt.plot(m.time,cv2.value,'b.-',label=r'$CV_2$') plt.ylabel('CV') plt.xlabel('Time (sec)') plt.legend(loc='best') plt.show() (:sourceend:)

Also see Continuous State Space and Discrete State Space