PID script

[manos@dynamitec]

Hello Admin,

I am trying to realize a constant light control loop with the PID script you have on the website. 
I've tried to adjust the kp, ki, kd parameters to make the system stable but there are some issues.
Since this not about temperature control and fluctuations to the measured brightness value in the room can be pretty high due to sunny/cloudy day, the system is unstable. I would like to be able to set a min/max step (like +/- 3%) for the value that is written to the bus each control loop even if the PID calculation is resulting to a big difference from the previous value. In this way I would like to avoid positive or negative jumps in the written value. Another thing is to be able to add something like a deadband or hysteresis to the system so if the actual value = setpoint +/- 20 lux for exmple then don't calculate anything. Something similar of what is done now via the manual variable.
As my lua scripting skills are basic could you or anyone in this forum please help me figure this out and how the new script will be? Below I am attaching the script I am using now:

Code:

PID = {   -- default params   defaults = {     -- invert algorithm, used for cooling     inverted = false,     -- minimum output value     min = 10,     -- maximum output value     max = 100,     -- proportional gain     kp = 0.25,     -- integral gain     ki = 0.01,     -- derivative gain     kd = 0.1,   } } -- PID init, returns new PID object function PID:init(params)   local n = setmetatable({}, { __index = PID })   local k, v   -- set user parameters   n.params = params   -- copy parameters that are set by user   for k, v in pairs(PID.defaults) do     if n.params[ k ] == nil then       n.params[ k ] = v     end   end   -- reverse gains in inverted mode   if n.params.inverted then     n.params.kp = -n.params.kp     n.params.ki = -n.params.ki     n.params.kd = -n.params.kd   end   return n end -- resets algorithm on init or a switch back from manual mode function PID:reset()   -- previous value   self.previous = grp.getvalue(self.params.current)   -- reset iterm   self.iterm = 0   -- last running time   self.lasttime = os.time()   -- clamp iterm   self:clampiterm() end -- clamps iterm value function PID:clampiterm()   self.iterm = math.max(self.iterm, self.params.min)   self.iterm = math.min(self.iterm, self.params.max) end -- clamp and set new output value function PID:setoutput()   local t, object, value   self.output = math.max(self.output, self.params.min)   self.output = math.min(self.output, self.params.max)   value = math.floor(self.output)   local t = type(self.params.output)   -- write to output if object is set   if t == 'string' or t == 'table' then     if t == 'string' then       self.params.output = { self.params.output }     end     for _, output in ipairs(self.params.output) do       grp.write(output, value, dt.scale)     end   end end -- algorithm step, returns nil when disabled or no action is required, -- output value otherwise function PID:run()   local result   -- get manual mode status   local manual = false   if self.params.manual then     manual = grp.getvalue(self.params.manual)   end   -- in manual mode, do nothing   if manual then     self.running = false   -- not in manual, check if reset is required after switching on   elseif not self.running then     self:reset()     self.running = true   end   -- compute new value if not in manual mode   if self.running then     -- get time between previous and current call     local now = os.time()     self.deltatime = now - self.lasttime     self.lasttime = now     -- run if previous call was at least 1 second ago     if self.deltatime > 0 then       result = self:compute()     end   end   return result end -- computes new output value function PID:compute()   local current, setpoint, deltasc, deltain, output   -- get input values   current = grp.getvalue(self.params.current)   setpoint = grp.getvalue(self.params.setpoint)   -- delta between setpoint and current   deltasc = setpoint - current   -- calculate new iterm   self.iterm = self.iterm + self.params.ki * self.deltatime * deltasc   self:clampiterm()   -- delta between current and previous value   deltain = current - self.previous   -- calculate output value   self.output = self.params.kp * deltasc + self.iterm   self.output = self.output - self.params.kd / self.deltatime * deltain   -- write to output   self:setoutput()   -- save previous value   self.previous = current   return self.output end

Thank you in advance.