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Altera_Forum
Honored Contributor I
2,006 Views

Implementation of Digital PID controller in FPGA

I'm trying to implement a PI controller in FPGA using schematic entry method in altera quartus II . I'm using the following equivalent PI controller equation, 

 

c(n) = c(n-1)+kp*(e(n)-e(n-1))+Ki*T*e(n-1) 

 

where, 

c(n) = controller output 

Kp= proportional constant 

Ki=integral constant 

e(n)=Reference -actual input 

T=sampling period 

 

I'm implementing it for 8 bit data. I couldnot implement c(n-1) in the above equation using schematic entry. Can someone help me implement this? I would be grateful if you can suggest me other ways of implementing PI controller like in VHDL.
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30 Replies
Altera_Forum
Honored Contributor I
147 Views

Looks like you will need a storage element. Perhaps a fifo would work or a D-flipflop with an enable.

Altera_Forum
Honored Contributor I
147 Views

gmpstr, 

Thanks a lot for response. I used D type flip flop and I sucessfully implemented 

kp*(e(n)-e(n-1))+Ki*T*e(n-1) 

 

but once I took the c(n) and pass it through D type flip flop to get c(n-1) and add to above, my problem started. The value of c(n-1) is always zero though it acutally is not and the result is just always equal to 

 

kp*(e(n)-e(n-1))+Ki*T*e(n-1). 

 

During synthesis, I got the following warning, 

 

Warning (14130): Reduced register "lpm_dff0:inst9|lpm_ff:lpm_ff_component|dffs[0]" with stuck data_in port to stuck value GND 

Warning: Output pins are stuck at VCC or GND 

Warning (13410): Pin "output[0]" is stuck at GND 

 

Maybe this will help you understand my problem. I hope to get feedback from you in this matter.
Altera_Forum
Honored Contributor I
147 Views

You need to figure out why the data_in port is being reported as stuck at GND. You could post a quartus archive of the design and I'm sure you would get feedback that would solve the issue.

Altera_Forum
Honored Contributor I
147 Views

 

--- Quote Start ---  

I'm trying to implement a PI controller in FPGA using schematic entry method in altera quartus II . I'm using the following equivalent PI controller equation, 

 

c(n) = c(n-1)+kp*(e(n)-e(n-1))+Ki*T*e(n-1) 

 

where, 

c(n) = controller output 

Kp= proportional constant 

Ki=integral constant 

e(n)=Reference -actual input 

T=sampling period 

 

I'm implementing it for 8 bit data. I couldnot implement c(n-1) in the above equation using schematic entry. Can someone help me implement this? I would be grateful if you can suggest me other ways of implementing PI controller like in VHDL. 

--- Quote End ---  

 

 

Hi, 

 

can you post your schematic ? 

 

If you don't what to use VHDL , maybe Verilog could be a choice ? 

 

Kind regards 

 

GPK
Altera_Forum
Honored Contributor I
147 Views

Some years ago, I started programming DSP applications with maxplus2 and schematic entry.You can achieve any kind of arithmetic utilizing the respective MegaFunctions. However, you have to assure, that c(n) sum doesn't overflow. Standard add and sub operators or megafunctions don't provide saturation logic, you have to add it separately.

Altera_Forum
Honored Contributor I
147 Views

Hi everyone, 

I've attached the archive of PI controller project. I've used altera megafunctions in this schematic entry. Pls help me solve this problem. I've not yet added saturation logic on it.
Altera_Forum
Honored Contributor I
147 Views

Warning (14130): Reduced register "lpm_dff0:inst9|lpm_ff:lpm_ff_component|dffs[0]" with stuck data_in port to stuck value GND 

Warning: Output pins are stuck at VCC or GND 

Warning (13410): Pin "output[0]" is stuck at GND 

 

These warnings make sense after looking at the schematic. The two multipliers before the add are doing a multiply by 2 so the result will always be even, thus bit 0 will always be 0. 

 

Also noticed that The two dff on the top left(inst1 and inst2) are 16 bit but should be 8bit. When I double click them, the megawizard shows them as 16 bit. Not sure if this is a problem. I fixed that issue and ran a modelsim simulation and the simulation shows data coming out of the dff of inst9. 

 

synthesized_wire_11 is the output of dff(inst9)
Altera_Forum
Honored Contributor I
147 Views

gmpstr, 

I looked on the simulated output of yours and it seems to work fine, But when I tried to simulate the schematic with the corrections you mentioned with a waveform file, I got correct result for 2 periods only. I've attached .vwf file . Could you pls tell me what went wrong?
Altera_Forum
Honored Contributor I
147 Views

 

--- Quote Start ---  

Also noticed that The two dff on the top left(inst1 and inst2) are 16 bit but should be 8bit. When I double click them, the megawizard shows them as 16 bit. Not sure if this is a problem. 

--- Quote End ---  

The width is automaticly set to 8 in compilation, even without causing a warning. This is a special feature of schematic entry evaluation, I think. It also infers a multi-bit register from a single bit one. 

 

 

--- Quote Start ---  

But when I tried to simulate the schematic with the corrections you mentioned with a waveform file, I got correct result for 2 periods only. 

--- Quote End ---  

I see, that your simulation output has 8 bit. I don't understand what you connected here, cause your design has a 16-bit output. 

 

Generally I see two problems. Controller deviation signal e is by it's nature of SIGNED type, no matter what your input signals are. To my opinion it's advisable, to use SIGNED for all signals throughout a PI controller, also for coefficients to avoid type confusion. 

 

The output stuck to ground refers to a problem of coefficient scaling and required signal widths in your design. I already mentioned the need to limit the integrator output. These points are not related to FPGA programming rather than basic DSP issues. They also arise with digital microprocessor controllers and can be examined (and solved) by pencil and paper methods. 

 

P.S.: As another supplement: Cause the sampling period T most likely is a multiple of the system clock period, the delaying FFs should have a clock enable to set the sampling period.
Altera_Forum
Honored Contributor I
147 Views

Hi everyone. I'm also triyng to implement a PID in quartus schematic. Does anyone knows ho to make a correct saturation for adders? i've thinked about use multiplexer with index bit carry and bit overflow. 

thank you for your help
Altera_Forum
Honored Contributor I
148 Views

I'm using a package of extended arithmetic functions for such purposes. Basically I leave it to the compiler to find an appropriate way to realize the intended behaviour. You may want to compare the achieved effectivity with handcoded saturation arithmetic, I'm satisfied with it up to now. 

FUNCTION SUM (X1,X2: SIGNED) RETURN SIGNED IS VARIABLE S:SIGNED(X1'length-1 downto 0); BEGIN S:=X1+X2; IF X1>=0 AND X2>=0 AND S<0 THEN S:= (others => '1'); S(S'left):='0'; ELSIF X1<0 AND X2<0 AND S>=0 THEN S(S'left-1 downto 0):= (others => '0'); S(S'left):='1'; END IF; RETURN S; END;
Altera_Forum
Honored Contributor I
148 Views

Thank you very much for your help!! The problem is that i've always used schematic blocks and i've never used vhdl language, and so i don't know how to integrate this code with the adders i've used. can you help me in that operation? thank you so much!!

Altera_Forum
Honored Contributor I
148 Views

Hi all, 

 

Te code of FvM is well written but I am not sure what is its purpose. 

You only need to clip (saturate) if you are going to discard one or more MSBs. 

If you discard LSBs then there will never be the need for saturation, possibly you may go for rounding.  

 

In short: 

Truncation of one LSB = divide by 2 so no clipping needed but rounding. 

Truncation of one MSB is relatively = multiply by 2 because you would discard one LSB bit less so it needs clipping 

 

To implement saturation in schematic or HDL: 

Just test the MSB you want to discard: if it equals the opposite of sign bit then fill up data with that opposite. 

 

Note however, in many real systems you should avoid clipping as it affects signal integrity.
Altera_Forum
Honored Contributor I
148 Views

Simply, when adding numbers without providing extra result bits, the result may overflow to opposite sign, e. g. in case of an integrator  

integ <= integ + x;  

In most real control applications, overflow results in fatal failure, e. g. with said PID controller. Saturation arithmetic prevents from this situation.  

integ <= SUM(integ,x);
Altera_Forum
Honored Contributor I
148 Views

Hi FvM, 

 

Thanks for the clarification, I agree totally that in cases of feedback overflow will occur and be fatal. This saturation is different from my clipping. We better say overflow protection. 

Apologies, I can see now what your function is doing, in effect precomputing to prevent overflow. it seems like DSP Vs control systems conflict.
Altera_Forum
Honored Contributor I
148 Views

Hi POWERGUI 

 

To sum up, I hope the following method will work: 

(we only need saturation if there is feedback. If there is no feedback then we can accommodate the result). 

 

Assume adder inputs are x1, x2 each 8 bits 2's complement and adder result is S of 9 bits. for feedback x2 = S(8) & S(6:0). i.e. discard S(7). This brings us back to the concept of MSB discarding algorithm, thus: 

 

if S(7) is opposite S(8) then fill up all S(or x2) with opposite of S(8).
Altera_Forum
Honored Contributor I
148 Views

s ur controller working fine now...im supposed to implement a PID controller in spartan kit...

Altera_Forum
Honored Contributor I
148 Views

Hi ! 

 

I want to implement PI Controller in FPGA SPARTRAN 3e kit, 

using VHDL. 

Can you please guide me . 

I am very new in this field. 

I studied VHDL 3 years ago, and now i have to implement this task. 

i have to brush bup my VHDL knowledge. 

Can you guide me step by step. 

i will be really obliged. 

 

also i have code in matlab for same PI controller but 

i have to convert that to VHDL. 

Now it depends should i convert that to vhdl. or else i should write a new code. 

 

Hope to listen from you. :)
Altera_Forum
Honored Contributor I
148 Views

me too i'm working on that but as i see no one response

Altera_Forum
Honored Contributor I
19 Views

 

--- Quote Start ---  

me too i'm working on that but as i see no one response 

--- Quote End ---  

I don't want to be unpolite, but asking for general help about PID implementations without a specific question seems to me like expecting others to write your private tutorial instead of looking into available text books first.  

 

When you are asking for help in the forum, you should clarify that you spent some effort to learn about the matter before. Apart from this reservation, you'll surely notice that forum members willingly answer any kind of beginners questions, even ignorant ones. 

 

Regards 

 

Frank
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