PID algorithm questions - Printable Version

Code:
-- 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 = self.params.manual and grp.getvalue(self.params.manual) or false
  -- 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