11 PID

PID/PID calculation

PID

This instruction is used to perform PID control that changes the output value according to the amount of input change.

-[PID (s1)(s2)(s3)(d)]

Content, range and data type

Device used

Features

This instruction is to complete the PID operation and is used to control the parameters of the closed-loop control system. PID control has a wide range of applications in mechanical equipment, pneumatic equipment, constant pressure water supply and electronic equipment, etc. among them:

 is the target value of PID control;

 is the measured feedback value;

 The starting address of the buffer area for setting parameters required for PID operation and saving intermediate results, occupies a total of 26 variable units in the subsequent addresses, the value range is D0 to D7974, it is best to specify the power failure retention area, which will remain after the power is OFF Set the value, otherwise the buffer area needs to be assigned value before starting the operation for the first time. The function and parameter description of each unit are described in this section;

 is the storage unit of the PID calculation result. Please designate  as a non-battery holding area, otherwise it needs to be initialized and cleared before starting the calculation for the first time.

Programming example

The parameter description is as follows:

What is stored in D9 is the target value of PID adjustment, and D10 is the closed-loop feedback value. Note that D9 and D10 must be of the same dimension, such as both 0.01MPa units, or 1℃ units, etc.;

A total of 26 units of D200 to D225 are used to store the set value and process value of PID operation. These values must be set item by item before the first PID calculation;

The D130 unit is used to store the calculated control output value to control the execution of the action.

The function and setting method of the parameter value of each unit about starting of  are described in the following table:

Unit	Features	Setting instructions
	Sampling time (TS)	The setting range is 1 to 32767 (ms), but it needs to be greater than the PLC program scan period
+1	Action direction (ACT)	bit0: 0 = positive action; 1 = reverse action bit3: 0 = unidirectional; 1 = bidirectional bit4: 0 = auto-tuning does not work; 1 = auto-tuning is executed, others cannot be used.
+2	Maximum ascent rate (DeltaT)	Setting range 0 to 32000 is the threshold of integral increment
+3	Proportional gain (Kp)	Setting range 0 to 32767, note that this value is enlarged by 256 times, the actual value is Kp/256
+4	Integral gain (Ki)	Setting range 0 to 32767, Ki=16384Ts/Ti, Ti is the integral time
+5	Differential gain (Kd)	Setting range 0 to 32767, Kd≈Td/Ts, Td is the derivative time
+6	Filtering (C0)	Setting range 0 to 1023, integral part filtering
+7	Output lower limit	Recommended setting range -2000 to 2000 When bit3 of S3+1=0, set to 0; When bit3 of S3+1=1, set to -2000;
+8	Output upper limit	Recommended setting value 2000
+9	Reserved	Reserved
︙	︙	︙
+25	Reserved	Reserved

Auto tuning example

✎Note:

● When multiple instructions are used, the device number of (d) cannot be repeated.

● During the execution of auto-tuning, the (s3) parameter space cannot be modified.

● The instruction occupies 26 point devices from the device specified in (s3).

● PID instruction can be used multiple times in the program and can be executed at the same time, but the variable area used in each PID instruction should not overlap; it can also be used in step instructions, jump instructions, timing interrupts, and subroutines, in this case When executing the PID instruction, the (s3)+9 cache unit must be cleared in advance.

● The maximum error of the sampling time Ts is -(1 operation cycle +1ms) +(1 operation cycle). If the sampling time Ts ≤ 1 operation cycle of the programmable controller, the following PID operation error (4D86H) will occur, and the PID operation will be executed with TS = operation cycle. In this case, it is recommended to use constant scan mode or use PID instruction in timer interrupt.

Error code

Example

See manual.

CCPID/CCPID calculation

CCPID

This instruction is used to perform PID control that changes the output value according to the amount of input change.

-[CCPID (s1) (s2) (s3) (d)]

Content, range and data type

Device used

Features

After setting target value (s1), measured value (s2), parameter (s3) to (s3) +12 and executing the program, the calculation result (MV) will be stored to the output according to the first sampling time (s3) in the parameter Value (d). For details, please refer to the user manual of "Wecon CC Series ccpid Function Description v1.4".

✎Note:

It can be executed multiple times at the same time (there is no limit to the number of loops), but please note that the device numbers (s3) and (d) used in the calculation cannot be repeated.

The instruction occupies 52 points of devices starting from the device specified in (s3).

During the execution of auto-tuning, the (s3) parameter space cannot be modified.

Error code

Example

See "CCPID Instruction Manual".

FPID/FPID calculation

FPID

The function of this instruction is to adjust PID control parameters by fuzzy algorithm.

-[FPID (s) (d1) (d2) (d3)]

Content, range and data type

Device used

Features

This instruction needs to be used in conjunction with the PID instruction. It completes the fuzzy calculation of the adjustments of the three parameters of PID, Kp, Ki, and Kd. By passing in the three parameters of the PID, the new three parameters are calculated and substituted into the PID for output control.

Parameter Description:

✎Note:

The instruction starts from the device specified in (d1) and occupies 38 points of the device, and initializes the parameters. Normally, it only needs to be initialized once before calling (some parameters are fixed) (occupies 38 words space).

The instruction starts with the device specified in (d2) and occupies 12 points of the device, input parameters, and input the first 6 parameters, where Kp, Ki, Kd are the initial values of the PID control parameters (occupies 12 words space) .

The instruction starts from the device specified in (d3) and occupies 8 points of soft elements and output parameters, among which Kp, Ki, Kd are the parameter values after fuzzy adaptive calculation, which can be input to the designated position of PID (occupy 8 words space).

The FPID instruction occupies 58 words. The address of each operand must have a specified interval interval, which cannot be occupied by other instructions.

Error code

Example

1. Parameter d1

2. Parameter d2

3. Invoke FPID

CCPID instruction introduction manual

Background and purpose

(1) Background:

PID (proportion, integral, derivative) controller has been the earliest practical controller for nearly a hundred years, and it is still the most widely used industrial controller. The PID controller is simple and easy to understand, and does not require precise system models and other prerequisites in use, making it the most widely used controller.

(2) Purpose:

You might not be familiar with the parameter settings in the new series of CCPID for the first time, this manual could let you quickly understand the meaning of each parameter in the CCPID and the influence on the control effect, so that you can quickly learn the CCPID.

Description of the host CCPID instruction

Instruction description

Content, range and data type

Name	Features	Bits (bits)	Whether pulse type	Instruction format	Step count
CCPID	PID Operation	16	No	CCPID	9

Device used

 is the target value (SV) of PID control;

 is the measured feedback value (PV);

 is the start address of the buffer area for setting parameters required for PID operation and saving intermediate results, occupying a total of 52 variable units of subsequent addresses (recommended to reserve 100 continuous spaces).The value range is D0 to D7,948, it is better to specify power failure retention, and the setting value remains after power supply is off. Otherwise,the buffer needs to be assigned value before starting the calculation for the first time. The function and parameter description of each unit are described in this section;

 is the storage unit (MV) of the PID calculation result. Please specify it as a non-battery retentive area, otherwise it needs to be initialized and cleared before the first start of calculation.

Programming example

The parameter description is as follows:

In D9, the target value of PID adjustment is stored, and D10 is the closed-loop feedback value. Note that D9 and D10 must be of the same dimension, such as both 0.01MPa units, or 1℃ units, etc.;

A total of 52 units of D200 to D224 are used to store the set value and process value of PID operation. These values must be set item by item before the first PID calculation;

D130 unit is used to store the calculated control output value to control the execution of the action.

The functions and setting methods of the parameter values of each unit used by  are described in the following table:

 to  +14 is the parameter range that can be set (parameters set when CCPID is executed).

 +15 to  +21 is the space used internally by CCPID control.

 +22 to  +51 is the parameter space used in the auto-tuning process.

1) The auto-tuning process occupies the space of S3+22 to S3+51. When the auto-tuning is successful, the adjusted parameters will be written into the space of S3+2 to S3+21.

2) +2 filter coefficient α: Processing in first-order inertial filter

Formula: T_now=(100-α)×T_α+α×T_old

T_α is the currently measured temperature. T_old is the temperature that participated in the PID calculation last time. T_now is the temperature used for the current PID calculation. α is the filter coefficient (when α=0, no filtering is performed, and the range of α is 0 to 100.(If there is a temperature with a small overshoot but a long stabilization time, the parameter can be set to 80, and analyze the specific problems in detail)

3) +6 work range: Twork(example: 170 represents 17℃)

4) +9 working mode:

0: Working mode that allows overshoot

1: Slight overshoot or no overshoot working mode

2: Custom settings; to achieve by setting +10, +11, +12 three coefficients.

5) +1 bit2 self-tuning transition zone switch: (upper limit 1℃, low limit 0.5℃)

The transition zone description in forward control:

In the heating process, when PV≤SV+1℃, 100% power output; when PV＞SV+1℃, no output.

In the cooling process, when PV＜SV-0.5℃, 100% power output; When PV≥SV-0.5℃, no output.

The transition zone description in reverse control:

In the cooling process, when PV≥SV-1℃, 100% power output; when PV＜SV-1℃, no output.

In the heating process, when PV＞SV+0.5℃, 100% power output; When PV≤SV+0.5℃, no output.

The transition zone description in bidirectional control:

In the heating process, when PV≤SV+1℃, 100% power heating output; when PV>SV+1℃, 100% power cooling output.

In the cooling process, when PV＜SV-0.5℃, 100% power heating output. When PV≥SV-0.5℃, 100% power cooling output

Programming case

CCPID application configuration

(1) Parameter setting

(2) CCPID control process setting

(3) Bidirection control

✎Note:

1. CCPID is a special instruction for operation control. CCPID operation will be executed only after the sample time is reached.

2. There is no limit to the number of times the CCPID instruction can be used, but+51 cannot be repeated.

3. Before CCPID instruction is executed, CCPID parameters need to be set.

Case analysis

(1) Control requirements

The control environment of this example is a kettle. The configuration is controlled by PLC-5V2416 host with 4PT module, and PI8070 screen is used for data storage and process curve viewing.

(2) Sample program

(3) Parameter description

(4) Parameter control effect description

1) Boiling water experiment

① Auto-tuning process and control process (no transition zone setting), take two-stage auto-tuning as an example

Figure 1 Auto-tuning process curve without transition zone

When the control system is a single temperature control system or a system where environmental interference does not cause large fluctuations. Generally the automatic tuning without transition zone is selected, so that the self-tuning process can be completed more quickly than the method with transition zone.

②Self-tuning process and control process (transition zone setting)

Figure 2 Self-tuning process curve with transition zone

It is more suitable in a two-way control system with transition zone self-tuning process. The transition zone has a range of 1.5°C. The upper limit is 1°C, and the lower limit is 0.5°C.

2) Difference in working interval setting

Figure 3 Process curve under different working interval parameters

Figure 4 Process curve without different working interval parameters (heating process diagram)

It can be seen from the partially enlarged graph that the parameters of the working interval have a certain influence on the overshoot and the stable time. In the case of allowing overshoot, setting the working interval parameters can make the overshoot smaller. This is because the deviation E of PID starting to work is relatively small, and the integration accumulation will not quickly saturate.

3) Result of filter coefficient setting

Figure 5 Process curve under different filtering parameters

The figure above is the experimental result under the small overshoot coefficient, the sample time is 1s. The coefficients of the first-order inertial filtering are (20, 50, 70, 80, 90). After adding the inertia coefficient, the stability time of system control is greatly accelerated, and it is increased by about 6 minutes for the boiling water experiment. The overshoot is about 1.2°C to 1.7°C.

Therefore, the introduction of first-order inertial filtering could greatly improve the PID environment where the temperature fluctuates to a certain extent and increase stabilization time.

✎Note: This parameter of filter coefficient is helpful for systems with not very large hysteresis or the control effect of the phenomenon that the control amount fluctuates back and forth has been greatly improved.

4) The difference in mode selection

0: Overshoot allowed(ukd = 100)

1: Small overshoot or no overshoot (ukd = 300)

Figure 6 Process curves in different working modes

When selecting mode 1 (small overshoot or no overshoot), the stable temperature may be slightly higher than the set temperature (fluctuates above the set temperature).

5) The function of the coefficient

Figure 7 Process curve under dynamic setting

When selecting working mode 2, there are three corresponding adjustable parameters: ukp[S3+10], uki[S3+11], ukd[S3+12]. Usually, the default parameters can be used for ukp and uki. Adjust the value of ukd could achieve the control effect.

Ukp is adjusted when the value of Kp reaches the maximum value, and the default value is usually 100.

Uki is adjusted when periodic oscillations occur. Gradually increase the value of uki to track the control effect.