Siemens Simatic S7 Series System Manual

Program instructions
7.9 PID
S7-200 SMART
System Manual, 09/2015, A5E03822230-AC
275
Since the digital computer must calculate the output value each time the error is sampled
beginning with the first sample, it is only necessary to store the previous value of the error
and the previous value of the integral term. As a result of the repetitive nature of the digital
computer solution, a simplification in the equation that must be solved at any sample time
can be made. The simplified equation is:
Output = Proportional term + Integral term + Differential term
M
n
= K
C
* e
n
+ K
I
* e
n
+ MX + K
D
* (e
n
- e
n-1
)
where:
M
n
Calculated value of the loop output at sample time n
K
c
Loop gain
e
n
Value of the loop error at sample time n
e
n-1
Previous value of the loop error (at sample time n - 1)
K
I
Proportional constant of the integral term
MX
Previous value of the integral term (at sample time n - 1)
K
D
Proportional constant of the differential term
The CPU uses a modified form of the above simplified equation when calculating the loop
output value. This modified equation is:
Output = Proportional term + Integral term + Differential term
M
n
= MP
n
+ MI
n
+ MD
n
where:
M
n
Calculated value of the loop output at sample time n
MP
n
Value of the proportional term of the loop output at sample time n
MI
n
Value of the integral term of the loop output at sample time n
MD
n
Value of the differential term of the loop output at sample time n

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.