Hi!I want to control a motor with PWM, reading a rotary encoder to get speed, and I thought PID would be a good fit. However, I'm confused about a couple things, mostly regarding the proportional term.First off, when the speed gets higher than the setpoint, the error gets negative, right? When then multiplied by the proportional constant, the output variable also is negative, but the PWM duty cycle is only 0-100% (or 0-255).Second, if my target speed is, say, 120, and the measured speed is 50, the error is 70 - which, when multiplied by the proportional constant, comes out to (for instance) 40% duty cycle, and the speed never reaches the set point.Surely it can't be the integral term that helps bring the output variable to appropriate levels, simple proportional control should be more or less functional as well.What am I missing?
Thanks in advance! Just look at the general block diagram of a closed loop automatic control system. It has a subtraction module. A differencer.If the error signal becomes negative. Then the negative value at the output circles around.and comes back around to the negative input terminal. What happens when you subtract a negative number?. The result is the same as adding the absolute value of that negative number.aka adding a positive value.The typical negative feedback system.
When designed properly.works toward equalising the setpoint and the signal on the -ve input terminal.Integral is for cases where the error (setpoint minus -ve terminal value) is stuck at a small value.such as when friction stops a motor from reaching the desired position. The integral of a constant is a linearly increasing quantity (with respect to time). So the integral helps a system to get a 'constant' error unstuck (so that the control system can keep moving ahead to drive the error toward zero, or get at least some time averaged error of zero).Sure.
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At first glance, the PWM appears to be limited to POSITIVE PWM voltages. But that's where the H-bridge comes in.
The digital controller handles the 'error' value. If the error is zero or positive, then the digital controller can make the h-bridge work in 1 state. If the error becomes negative, then the digital controller can make the h-bridge flick over to the other state. Hi tmladek,Your desired speed is the PID controller's 'setpoint' and your encoder is the 'process variable' that's measuring the actual speed. Both your desired and actual speed should be the same unit of measurement, be it degrees/s, radians/s, RPM or whatever.The proportional part of the PID controller simply subtracts the actual speed from the desired speed (to give your error) and multiplies it by a gain factor:PID proportional output = (desired speed - actual speed). gainWhat took me some time to understand was the fact that the PID output is just a value, it doesn't have to be the same units as the inputs.
It's just used to drive your 'process' or in your case a motor. You just need to map and constrain the PID output to drive whatever it is you're trying to control, again in your case mapping and constraining it to a value between 0 and 255.The thing to think about here is feedback. The PID system is trying to drive the actual motor speed as measured by your encoder so that it matches your desired speed, in effect it's trying to minimise the error. This error is always about and with respect to your setpoint (desired speed). Naturally the error will always go sightly postive and negative about your setpoint, as the PID system never quite reaches equilibrium. In an extreme case, if you set your gain too high you can get overshoot or even worse oscillations, as your error swings uncontrollably between negative and positive.
No, often when you control a motor the PID output is a signed value, positive for forward, negative forbackwards - then you can do full control by position or speed, forwards or backwards or stationary.Even for one-direction speed control you forget the integrating term which 'remembers' the normalsteady state output from the loop, so the the proportional error term is merely correcting a difference fromthis normal output. The integrating term slowly adapts to changes in operating point and nulls out theerror to zero in steady state.However the complexity arises when the system as a whole is not linear (a varying load, or usingslow-decay mode in the motor controller), then you often have to turn down the PID gains forstability and lose control performance.
Great though the I term is it reduces stability, so is alwaysrather a compromise.There's no substitute for actually playing with a PID loop and tuning it and seeing what these thingsmean in practice - a lot of the theory assumes linear system with constant load, but real systems aremessier. More advanced control strategies can help, but they are more complex.And lastly don't forget that feed-forward is often very useful, not just feed-back - this could be assimple as adding a fixed offset onto the output of the PID loop to compensate for the normal load.
Hello,i'm a Newbie so forgive me for the Q? But i am new to this whole PID thing but i'm trying to get my mind around it. I'm using a pneumatic system(2 solenoids) so i will need to control 2 outputs with 1 input, is this even possible to do?
I mean, i know people are doing it all the time but i cant find any real help.Don't i need 2 outputs in the 'define Variables' here? If anyone can bring insight that would be much appreciated thanks.//Define Variables we'll be connecting todouble Setpoint, Input, Output;PID myPID(&Input, &Output, &Setpoint,2,5,1, DIRECT). PID refers to 'proportional plus integral plus differential' control. It is part of automatic control systems theory. The system is typically 1 input and 1 output. And 'PID' theory typically is based on 'second order systems' and 'second order system response'.The combination of the parameters associated 'PID' (such as proportional gain Kp, integral gain Ki, and derivative gain Kd) sets a particular system behaviour (in response to a particular kind of input - such as an input that might immediately change from 1 value to another value; or an input that might increase gradually from value to another etc. The 3 parameters will determine how response or unresponsive the system is, and how quickly the system will react in order to make the output reach a desired level; and other response features.To begin to understand PID, it is usually helpful to begin with basic automatic control theory.
And like many theories involving movement and activity, some maths knowledge can come in handy - like understanding integrals and integration, derivatives and differentiation.
Version Test if the X Input extension is available and return the version number of the program and the version supported by the server. This option does notrequire a device name.-list -short -long -name-only -id-only device If no argument is given list all the input devices. If an argument is given, show all the features of device.
If -long is provided, the outputincludes detailed information about the capabilities of each devices. Otherwise, or if -short is provided, only the device names and some minimal informationis listed.
If -name-only is provided, the output is limited to the device names. One device name is listed per line. Note that the order the devices arelisted is undefined.
If -id-only is provided, the output is limited to the device IDs. One device ID is listed per line. Note that the order the devices arelisted is undefined.-get-feedbacks device Display the feedbacks of device.-set-pointer device Switch device in core pointer. This option does nothing on X servers 1.5 and later.-set-mode device ABSOLUTE RELATIVE Change the mode of device.-set-ptr-feedback device threshold num denom Change the pointer acceleration (or feedback) parameters of device. The (1) man page has more details.
For X.Org Server 1.7 and above,there are additional device properties pertaining to pointer acceleration. These do not replace, but complement the pointer feedback setting.-set-integer-feedback device index value Change the value of an integer feedback of device.-set-button-map device mapbutton1 mapbutton2. Change the button mapping of device.
The buttons are specified in physical order (starting with button 1) and are mapped to the logical buttonprovided. 0 disables a button. The default button mapping for a device is 1 2 3 4 5 6 etc.-query-state device Query the device state.-list-props device device.
Lists properties that can be set for the given device(s).-set-int-prop device property format value Sets an integer property for the device. Appropriate values for format are 8, 16, or 32, depending on the property. Deprecated, use-set-prop instead.-set-float-prop device property value Sets a float property for the device. Deprecated, use -set-prop instead.-set-prop -type= atom float int -format= 8 16 32 device property value. Set the property to the given value(s). If not specified, the format and type of the property are left as-is. The arguments are interpreted according tothe property type.-watch-props device Prints to standard out when property changes occur.-delete-prop device property Delete the property from the device.-test -proximity device Register all extended events from device and enter an endless loop displaying events received.
If the -proximity is given, ProximityIn andProximityOut are registered.-test-xi2 device Register for a number of XI2 events and display them. If a device is given, only events on this device are displayed.-create-master prefix sendCore enable Create a new pair of master devices on an XI2-enabled server with the given prefix. The server will create one master pointer named ' prefixpointer' and one master keyboard named ' prefix keyboard'. If sendCore is 1, this pair of master devices is set to send core events (default). Ifenable is 1, this master device pair will be enabled immediately (default).-remove-master master Floating AttachToMaster returnPointer returnKeyboard Remove master and its paired master device.
Attached slave devices are set floating if Floating is specified or the argument is omitted. Ifthe second argument is AttachToMaster, returnPointer specifies the master pointer to attach all slave pointers to and returnKeyboardspecifies the master keyboard to attach all slave keyboards to.-reattach slave master Reattach slave to master.-float slave Remove slave from its current master device.-set-cp window master Set the ClientPointer for the client owning window to master.
Master must specify a master pointer.-map-to-output device crtc Restricts the movements of the absolute device to the RandR crtc. The output name must match a currently connected output (see(1)). If the NVIDIA binary driver is detected or RandR 1.2 or later is not available, a Xinerama output may be specified as 'HEAD-N', with N beingthe Xinerama screen number. This option has no effect on relative devices.-enable device Enable the device.
This call is equivalent to xinput -set-prop device 'Device Enabled' 1 -disable device Disable the device.