Braking confusion

5 years 11 months ago #29529361 by MattPye
Braking confusion was created by MattPye
Hello reader,

I am new to automation. I'm just gonna explain my project. I work in scenery for television. I need to motorize a platform that carries scenery from point A to B and then back. I want to use the Mag. tape and sensor set up on a cart. My understanding is I will need 2 Mag. sensors (one for each direction). I also plan on using a roboteq duel motor controller. 2 motors for a left wheel and a right wheel. The plan was building a sub cart to go inside the platform. (a generic cart that can be used in different applications) The platform will have swivel caster taking the load and the automated cart will essentially push/turn it from the inside. So my specific question was about braking. Can brushless or brush DC motors be used to act as a brake or "park" when it is in a stationary position? (someone would be stepping on and off the platform) And can the roboteq control achieve this? OR do I need to use a digital out on the controller for a separate break system?

All advice, for any part of this project is welcome.
Thank you

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5 years 11 months ago #29529362 by Griffin Baker
Replied by Griffin Baker on topic Braking confusion
You should only need 1 mag sensor. The sensor will output a PWM signal which you will apply to the motor controllers pulse input. You use a dual channel controller which will run a mix mode for tank steering. You would need to write a script for the AGV to tell the controller how to handle the pwm signal from the sensor. There is a sample script in the link below under application notes.

dev.roboteq.com/dev1/index.php/support/documentation


You can add braking motors to the digital outputs(24v is fine, max 1A). The controller provides regenerative braking. So if the motor command is at 0, there will be resistance on the motor output that would try to prevent the motor from moving. There is also additional information regarding progressive regenerative braking in the link provided above.

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5 years 11 months ago #29529363 by MattPye
Replied by MattPye on topic Braking confusion
Very helpful thank you. As far as the Mag sensor goes. I can use only one sensor if I want the cart to go in reverse over the tape it just went forward on, not doing a loop that includes merges or forks. My tape will be a single straight line about 20' long. Essentially I want it to retrace it's own steps? I'm just clarifying because the how to posted on the roboteq website at the link below shows an example of "center drive and casters" chassis expresses a need for two sensors?

dev.roboteq.com/dev1/index.php/applicati...tic-track-guided-agv

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5 years 11 months ago #29529364 by Griffin Baker
Replied by Griffin Baker on topic Braking confusion
How will it know when it needs to start retracing it's steps?

Do you plan to use some sort of tape markings that says to stop moving forward and then go backwards?

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5 years 11 months ago #29529365 by Griffin Baker
Replied by Griffin Baker on topic Braking confusion
I'd imagine that with the 1 mag sensor, you would simply adjust the script so that output is inverted.

So throttle a - value and steering would be also be inverted.

So here is a copy of the starter script. This one tells the sensor to move in the forward direction. So if you were to have some sort of marking that says you are done going forward, then it could go to another loop where it does the opposite and then return to the forward direction upon reaching its' destination.

option explicit

' This script provide basic control for an AGV.
' Motor will turn on upon the presence of a track and stop when track disapears.
' The track position information is used to provide left/right steering.
' At forks, the AGV will follow the left or right track depending whether the last
' marker detected was on the left or right side of the track.
' Make sure you precede merges with a marker so that the AGV remains on the main track.
'
' Sensor track and marker information are received via the RS232 port
' Controller must be configured in mixed mode and Script Autostart.

' declare variables
dim Gain as integer
dim DefaultThrottle as integer
dim MaxTurn as integer
dim TapeDetect as boolean
dim MarkerLeft as boolean
dim MarkerRight as boolean
dim Throttle as integer
dim Tape_Position as integer
dim LineSelect as integer
dim Steering as integer
dim MagSensorStatus as integer
dim OnSwitch as boolean

' initialize constants
Gain = -7 ' Use negative value to invert steering command
MaxTurn = 300
DefaultThrottle = 250
LineSelect = 1 ' Use left track by default

' Print Log header
print("\rTd\tTp\tMl\tMr\tTh\tSt")

' main loop to repeat every 10ms
top:
wait(10)
' read sensor data

TapeDetect = getvalue(_MGD)
MarkerLeft = getvalue(_MGM, 1)
MarkerRight = getvalue(_MGM, 2)
MagSensorStatus = getvalue(_MGS)
OnSwitch = getvalue(_DI, 2)

' Use TapeDetect to apply throttle or not
if (TapeDetect)
Throttle = DefaultThrottle
else
Throttle = 0
end if

' Check Marker presence to select Left or Right track
if (MarkerLeft) then LineSelect = 2
if (MarkerRight) then LineSelect = 1

Tape_Position = getvalue(_MGT, LineSelect)

' use tape position multiplied with gain as steering
Steering = Tape_Position * Gain

' prevent over steering
if(Steering>MaxTurn) then Steering = MaxTurn
if(Steering< -MaxTurn) then Steering = -MaxTurn

' Send throttle and steering to controller Configured in Mixed mode
setcommand(_G, 1, Throttle)
setcommand(_G, 2, Steering)

' Log output
print("\r", TapeDetect,"\t", Tape_Position,"\t", MarkerLeft,"\t", MarkerRight,"\t", Throttle,"\t", Steering,"\t",OnSwitch)

goto top

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5 years 11 months ago #29529368 by MattPye
Replied by MattPye on topic Braking confusion
Good morning,

I plan on using a wireless push button control. Would it be better/easier to program it for a single button that basically tells the script "next" and it will move from A---B or B---A whichever the opposite position is. Or should there be two buttons one for position A and one for position B. Which would be easier? I really appreciate the help.

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5 years 11 months ago #29529370 by Griffin Baker
Replied by Griffin Baker on topic Braking confusion
Either one is fine.

You could set it your script that when the digital input changes state, that it goes to the loop that tells the motors to move forward or reverse. This would likely be a latching switch.

If your switch is in the on position, then the script can tell the agv to move forward on the tracks. When the switch goes low, you can have script move to a different loop that tells the agv to go backwards.

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5 years 11 months ago #29529372 by MattPye
Replied by MattPye on topic Braking confusion
Alright, I think that's everything I need for now. Thanks again.

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5 years 11 months ago #29529373 by MattPye
Replied by MattPye on topic Braking confusion
One more quick after thought. Could the Roboteq motor controller control a stepper motor? That would achieve the braking effect I am looking for. So that when the cart is in a position an outside force couldn't move it. Or is there another idea?

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5 years 11 months ago #29529374 by Griffin Baker
Replied by Griffin Baker on topic Braking confusion
Our controllers do not currently support stepper motors. There are smaller braking motors that can be used as people have been using them with stuff like wheel chair applications.

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