S7-300/400 Tip PID Tip No. 2
Copyright 1996 by SIEMENS page 3 / 15 Pid.doc
Status: 11/96 Version 1.0
SIMATIC S7-300/400 customers have free use of the application tips. These tips are only a general approach to using the S7-300/400 with various
applications. Your specific application may be different. It is your responsibility to use the SIMATIC S7-300/400 properly in your applications.
The variable which represents the state of the system being controlled is called the ‘Process
Variable.’ In our example above, you can see that the rate at which the steam spins the turbine
is a good indicator of the event that we are trying to control: the speed at which the water is
being boiled off. The output is the variable which, being altered by the controller, can affect the
process variable by different degrees based on its intensity -- By turning the hot plate up, the
water boils more quickly, more steam is produced, and the turbine’s speed increases.
Therefore, when a variable that accurately reflects the state of the process and an adjustable
control which proportionally affects the process variable, then it is possible to use a PID
controller. Common systems using PID controllers are air conditioning systems, solution mixing,
heaters, etc.
Auto Mode vs. Manual Mode自动模式和手动模式
There are two settings available on our PID controller. Putting a controller in Manual mode
causes the PID loop do nothing, so that the user can directly control the output. The second,
Auto, is the mode in which the PID loop is actually controlling the system. For the rest of this
text, it will be assumed that the controller is in Auto mode.
What does the PID controller do, and how does it do it? PID控制器作些什么?如何去做?
Quite simply, a PID controller adjusts the value of its output to try and balance the value of the
process variable to a given ‘setpoint.’ To calculate the output value for a given instance, the
controller finds the value of three different terms (using its user defined Sample time, Gain,
Rate, and Reset values along with the calculated Error value): a Proportional term, an Integral
term, and a Derivative term.
Output = M
P
+ M
I
+ M
D
Formula 2.1
What are the Sample, Gain, Rate, and Reset, and where do they come from?
The sample rate is the cycle time (in milliseconds) at which the PID loop recalculates the output.
The gain controls the sensitivity of the output calculation by affecting the influence of all the
terms. The reset is a time given in milliseconds which is used to increase or decrease the
influence of the Integral term in the equation. Finally, the rate value is used to control the
influence of the Derivative term in the equation. Each of these values needs to be preset by the
user before the PID controller starts.
If the user does not want integral action (no I in the PID calculation), then a value of infinity or a
value of 0 should be specified for the integral time. If the user does not want derivative action
(no D in the PID calculation), then a value of 0 should be specified for the derivative time. If the
user does not want proportional action (no P in the PID), then a value of 0 should be specified
for the gain (gain is normally a multiplier in the integral and derivative coefficient calculation, but
is removed from the coefficient calculation, if gain = 0, to allow I, ID, or D loop control).