Siemens Simatic S7 Series System Manual

Program instructions
7.9 PID
S7-200 SMART
System Manual, 09/2015, A5E03822230-AC
277
To avoid step changes or bumps in the output due to derivative action on setpoint changes,
this equation is modified to assume that the setpoint is a constant (SP
n
= SP
n-1
). This results
in the calculation of the change in the process variable instead of the change in the error as
shown:
MD
n
= K
C
* (T
D
/ T
S
) * ((SP
n
- PV
n
) - (SP
n-1
- PV
n-1
))
or just:
MD
n
= K
C
* (T
D
/ T
S
) * (PV
n-1
- PV
n
)
where:
MD
n
Value of the differential term of the loop output at sample time n
K
C
Loop gain
T
S
Loop sample time
T
D
Differentiation period of the loop (also called the derivative time or rate)
SP
n
Value of the setpoint at sample time n
SP
n-1
Value of the setpoint at sample time n - 1
PV
n
Value of the process variable at sample time n - 1
PV
n-1
Value of the process variable at sample time n - 1
The process variable rather than the error must be saved for use in the next calculation of
the differential term. At the time of the first sample, the value of PV
n - 1
is initialized to be
equal to PV
n
.
Selecting the type of loop control
In many control systems, it might be necessary to employ only one or two methods of loop
control. For example, only proportional control or proportional and integral control might be
required. The selection of the type of loop control desired is made by setting the value of the
constant parameters.
If you do not want integral action (no "I" in the PID calculation), then a value of infinity "INF",
should be specified for the integral time (reset). Even with no integral action, the value of the
integral term might not be zero, due to the initial value of the integral sum MX.
If you do not want derivative action (no "D" in the PID calculation), then a value of 0.0 should
be assigned for the derivative time (rate).
If you do not want proportional action (no "P" in the PID calculation) and you want I or ID
control, then a value of 0.0 should be specified for the gain. Since the loop gain is a factor in
the equations for calculating the integral and differential terms, setting a value of 0.0 for the
loop gain will result in a value of 1.0 being used for the loop gain in the calculation of the
integral and differential terms.

Siemens Simatic S7 Series Specifications

General

CPU	Varies by model (e.g., S7-300, S7-400, S7-1200, S7-1500)
Communication Interfaces	PROFIBUS, PROFINET, Ethernet, MPI
Programming	TIA Portal, STEP 7
Programming Languages	LAD, FBD, STL, SCL, GRAPH
I/O Modules	Digital, Analog, Specialty modules available
Operating Temperature	0°C to 60°C (standard models)
Mounting	DIN rail mounting
Operating Voltage	24V DC or 120/230V AC (varies by model)
Protection Class	IP20 (standard models)
Memory	Varies depending on the S7 model. Includes work memory (RAM) for program execution, load memory (EEPROM or Flash) for program storage, and system memory for operating system functions. Memory sizes range from kilobytes to megabytes.