Reversing a DC motor.

Thread Starter

Mikedixon

Joined Jan 21, 2021
8
Hi there,
I am working on a project in which a vehicle will drive to a wall, the impact with the wall will then toggle a switch or press a button that will reverse the polarity of the motor and the vehicle will drive backwards away from the wall.
So far the only way I can see to do this is using a DPDT switch however I am worried that since the middle setting on a DPDT switch is off, the vehicle might simply switch itself off rather than reversing the polarity of the motor.
Is there a way of using a two-way switch that will reverse the polarity or using a button? Or is there even a way of using a DPDT switch but ensuring it won't turn the motor off?
Thanks in advance!
 

djsfantasi

Joined Apr 11, 2010
7,689
Not all DPDT switches are center off. What specific switch are you using?

Once it hits the wall and goes backward, then what happens?

Reversing a motor without pausing between forward and backward could damage the motor. Just saying...
 

Thread Starter

Mikedixon

Joined Jan 21, 2021
8
Not all DPDT switches are center off. What specific switch are you using?

Once it hits the wall and goes backward, then what happens?

Reversing a motor without pausing between forward and backward could damage the motor. Just saying...
Thanks for your reply!
I haven't decided on a specific switch at the moment, I'm kinda new to all this and a DPDT switch is the only way I can find of reversing the motor once it hits the wall, so if you can think of a better way I would appreciate your help!
The reason I need to do this is for a competition where you make a robot that has to travel to a wall and travel back to the start position on its own while only using analogue components.
In regard to damaging the motor are there any ways to get around this? Are we talking long term damage or immediate because realistically the motor only has to be used a few times.
Thanks again for your reply!
 

ericgibbs

Joined Jan 29, 2010
12,884
hi Mike,
Consider using micro-switches for the end of travel detection.
The uSwitch's will Set/Reset motor drive direction of the motor.

E
 

Thread Starter

Mikedixon

Joined Jan 21, 2021
8
hi Mike,
Consider using micro-switches for the end of travel detection.
The uSwitch's will Set/Reset motor drive direction of the motor.

E
Brilliant, thanks for your reply, I'll look into that!
Will the U switches have the problem of damaging the motor if it switches straight from forwards to reverse?
Thanks again
 

ericgibbs

Joined Jan 29, 2010
12,884
hi,
It will be possible to use say a simple resistor/capacitor to give a short delay from uSw operation to relay operation.

What is the 'onboard' DC power supply voltage to the motors.?
E
 

Thread Starter

Mikedixon

Joined Jan 21, 2021
8
hi,
It will be possible to use say a simple resistor/capacitor to give a short delay from uSw operation to relay operation.

What is the 'onboard' DC power supply voltage to the motors.?
E
Again, I haven't decided on that for certain but I am planning on using simple AA or 9V batteries.
Thanks again for all your help!
 

ericgibbs

Joined Jan 29, 2010
12,884
hi,
Before deciding on the battery type, you have to determine the required power rating of the motor that is capable of moving the mass of the remote vehicle at its operating velocity between the two End Stop points.

Simply how big , how heavy and and how fast will the vehicle have to be.?


E
 

Thread Starter

Mikedixon

Joined Jan 21, 2021
8
hi,
Before deciding on the battery type, you have to determine the required power rating of the motor that is capable of moving the mass of the remote vehicle at its operating velocity between the two End Stop points.

Simply how big , how heavy and and how fast will the vehicle have to be.?


E
OK, I guess I will have to do some more development of the design before I start choosing specific components so I can more confidently estimate how heavy it will be. This is a lot more complicated than I anticipated! Haha, the specification for the vehicle is 400mmx400mmx400mm so I don't suppose it's going to be ridiculously heavy. In terms of speed slower is probably better because it has to accurately return to its starting position
 

ericgibbs

Joined Jan 29, 2010
12,884
hi,
How accurate a return position.?
Have you decided on the type of motor, regarding the speed of the mass, perhaps a geared motor.?

E
I have done a VERY rough draft of the circuit, you must check to see if it meets the required operation
 

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Thread Starter

Mikedixon

Joined Jan 21, 2021
8
Hi, thank you for taking the time to draw up that circuit diagram however I am very new to all this and I'm afraid I don't really understand it. In regards to return position it just has to be as accurate as possible, my current idea is to have the vehicle slowly turn a cog or belt as it moves, then when the polarity of the motor is reversed the cog/belt will turn in the opposite direction and turn the motor off once it returns to its starting position (and thus meaning the vehicle has travelled the same distance forwards as it has backwards). The only real specification I have to make this to is it has to be smaller than 400mmx400mmx400mm and it can only use analogue electronics. As I'm sure you can tell I am somewhat out of my depth with the electronics aspect of the task!
 

ericgibbs

Joined Jan 29, 2010
12,884
hi,
About 15 inch cube, should be enough volume for the project.
Work out how you are going to fit the motor to the base of the 'buggy' [ got to call it something]
Also the type of wheels and their diameter, rubber rimmed of course.

Post what you draw up.
E

BTW: is the Spire still crooked.?;)
 

Thread Starter

Mikedixon

Joined Jan 21, 2021
8
hi,
About 15 inch cube, should be enough volume for the project.
Work out how you are going to fit the motor to the base of the 'buggy' [ got to call it something]
Also the type of wheels and their diameter, rubber rimmed of course.

Post what you draw up.
E

BTW: is the Spire still crooked.?;)
Ok will do, thanks for yor help! And is the spire still crooked? I'm sorry, I don't follow hahaha
 

ericgibbs

Joined Jan 29, 2010
12,884
hi,
I would not buy any items until you have figured out the mechanics of the drive system.
This will determine the motor size and battery required.
E
Clue: Spire > Church
 

Bernard

Joined Aug 7, 2008
5,788
Some ramblings: Distance measurement via all threaded rod like 4-40 or 6-32 geared down to match travel distance, bar magnet with reed switch. Magnet travels between two iron end plates spaced for required breakaway force. Reed SW controls a DPDT relay. Speed about 36" / sec. Magnet moves about 1/4 " Rod traveler on returning home opens start SW releasing relay.
 

Thread Starter

Mikedixon

Joined Jan 21, 2021
8
Hi, sorry for the long period of silence from me, I've had a bit of work to catch up on. Some good ideas coming up I reckon but also I am part of a group working on this so it is taking a while to completely finalise the designs and generally agree on the main components/features. I am from England and this is a design task for my first year engineering degree.
 

Bernard

Joined Aug 7, 2008
5,788
I used 2 uncased relays with weights attached to armatures as decelloration switches. Momentary closure with about 50 cm /sec on hitting the wall.
 
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