真心求教，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！