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.