VB 增量式 PID 算法的一个问题

Option Explicit

Dim EK1 As Single, EK2 As Single

Dim U As Single

Dim Uk As Single, Uk1 As Single, A As Single, B As Single, C As Single, Ek As Single

Dim Ti As Integer, Td As Integer, Kp As Integer

Dim SP As Single, PV As Single

Private Sub Command1_Click()

Timer1.Enabled = True

End Sub

Private Sub Command2_Click()

End

End Sub

Private Sub Timer1_Timer()

Ti = Val(Text1) '读取积分时间值

Td = Val(Text2) '读取微分时间值

Kp = Val(Text3) '读取比例值

SP = Val(Text4) '设定值

Text5.Text = Val(VScroll1.Value)

PV = Val(Text5) '模拟的反馈值

If Ti = 0 Then

A = Kp * (1 + Td)

Else

A = Kp * (1 + 1 / Ti + Td)

B = Kp * (1 + 2 * Td)

C = Kp * Td

Ek = SP - PV

Uk = A * Ek - B * EK1 + C * EK2

Text6.Text = Uk '输出显示

End If

EK2 = EK1

EK1 = Ek

End Sub

求助各位版主，这是我编写的PID控制程序，EK表示当前的测量误差，EK1是上次测量误差，EK2是上上次误差，然后PID算法根据这三次误差输出控制量UK，正确的功能应该是根据系统设定值和实际输出值求得Ek = SP - PV,然后将这个的EK值传递给EK1，下一次测量值编程Ek，以此类推！我现在的问题是，无法将当前的测量误差传递给上一次，按着我编的程序，这三次误差实际上都等于当前的误差，求各位版主看一下问题出在哪儿！