Bluetooth door opener, Choose a stepper motor or a servo motor?

Thread Starter

Benjamin Peere

Joined Nov 29, 2018
33
Hello everyone,
I need the help of the Community,
I am working on a project, I need to create a device that can open or close a door lock via Bluetooth, this device works on batteries and it is just a simple adapter that you plug on your existing door lock, you just put the key Inside and it should be able to turn the key to open the door.
I have made a lots of research and tested a lots of things, at this moment, for prototyping I am using a Bluetooth chip nrf51822 to control a stepper motor via Bluetooth, the stepper motor is this one : https://www.pololu.com/product/2267,
and the driver I use to control it is this one : https://www.pololu.com/product/2966
I thought it would be enough and that I will have enough power to turn the key in the door lock, but obviously not, I tried on a door prototype with a new lock that works very smoothly and it has some difficulty to work, I power the system with 2 Li ion batteries (https://www.tinytronics.nl/shop/nl/...650-li-ion-batterij-2600mah-5.2a-icr18650-26j) that should give enough power I think.
When I noticed that the motor didn't give enough torque to turn the key I printed in 3D a planetary gearbox with a 5:1 ratio, so the torque is multiply by 5, but there again it doesn't work well. (I tried on a normal door and it was just impossible to open)
most of all, using a planetary gearbox reduce the speed of the motor drastically and I'm afraid that it taked hours to open the door.
some device exist on the market that require only 4AA batteries and it looks like they can open every doors, I Wonder how it works.

So my question is the Following one :
Should I use a stepper motor? Maybe I should use a servo motor? is it because of the stepper motor I use that I can't open the door?

The thing is I wanted to use a stepper motor because I can do multiple turns with a stepper motor with very few angle, some dorrs needs to turn the key 6 time to be open, whereas than with a servo I can't, it is limited to a certain angle and I can't do multiple turns. Also I Don't know how I could say to the servo how to stop when the door is open if I can't turn it on a precise number a turn.(I hope you understand what I mean ^^)

So if you I have any idea or suggestion, everything is welcome!
Thank you!
 

Alec_t

Joined Sep 17, 2013
14,280
a simple adapter that you plug on your existing door lock, you just put the key Inside and it should be able to turn the key to open the door.
I'm puzzled as to why you don't just turn the key by hand?
Be that as it may, I think neither type of motor is ideal. Have you considered using a simple DC gear-motor, such as found in car door lock actuators?
 

Thread Starter

Benjamin Peere

Joined Nov 29, 2018
33
That is more complicated than turning the key with the hand^^ this device will have an interface to give access to a house remotely and to avoid giving the key everytime you want someone to access your house, just by providing them a code that will give the right to the open via a dedicated application.

I Don't know, I Don't really have knowledge in motors, which kind of motor should I use? do you have an example of model?
It should work with a low voltage (max 8.4V provided by the two Li ion batteries) and the dc motor looks to need more
 

Alec_t

Joined Sep 17, 2013
14,280
Another option would be to use a conventional electric door strike. Simpler than providing a rotary action.
 

Alec_t

Joined Sep 17, 2013
14,280
Yes but the device shoul a simple device, easy to use and easy to install
A standard electric strike is pretty easy to install and has the advantage that it is inside the property. Your proposed key-turner would have to be outside and thus exposed to the weather and a target for would-be burglars/vandals. To combat that it would need to be very robust and very securely attached to the door; so hardly simple or easy to install :( .
what is the necessary torque to turn the key in the door lock?
That's something you would have to measure. It could vary wildly from one lock to another.
 

Thread Starter

Benjamin Peere

Joined Nov 29, 2018
33
A standard electric strike is pretty easy to install and has the advantage that it is inside the property. Your proposed key-turner would have to be outside and thus exposed to the weather and a target for would-be burglars/vandals. To combat that it would need to be very robust and very securely attached to the door; so hardly simple or easy to install :( .

That's something you would have to measure. It could vary wildly from one lock to another.
The device would be Inside the house and will be contrôle via Bluetooth, ne need to leave it outdoor, by leaving it Inside it prevent all the things you said and also provide the possibility to open the door with a regular key in case there is no more battery or your phone is out of battery too.

how could I measure the torque? I have no tool to measure it, I looked on internet to find an estimation but I couldn't find, I know now that 3.7Kg.cm and even 15kg.cm (the torque generated with my 5:1 ratio planetary gearbox) is not enough
 

Alec_t

Joined Sep 17, 2013
14,280
Inside it prevent all the things you said and also provide the possibility to open the door with a regular key
Many secure locks of the Yale or Euro type won't work with two keys inserted. Will yours?
how could I measure the torque?
Insert key in lock. Clamp one end of a lever to the key so that the lever extends radially with respect to the key rotational axis. Pull on the other end of the lever with a spring balance and measure the force needed to operate the lock. Torque = force x length of lever.
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
28,619
It sounds to me you are attempting to complete a task with a power source that does not have the capability to achieve it.
BTW, the term for the alternative to a stepper motor in your case is a RC Servo, just using the term servo motor includes industrial servo motors of all varieties.
Max.
 

Thread Starter

Benjamin Peere

Joined Nov 29, 2018
33
Doesn't look likely if you found 15Kg.cm was inadequate.

Edit: Using the post #12 method, I've just measured the key torque required for opening a Euro lock as ~ 3.5Kg.cm (48 oz.in).
So it means there must be Something wrong with my setup, because the stepper motor that I have here is supposed to deliver 3.5kg.cm(51Oz.in) without a gearbox, so normally even without a gearbox it should work, and with the gearbox it should be really easy, maybe I am not getting all the power from the motor, but I on't know what I am doing wrong, do you think I would need more power from the batteries?
 

Thread Starter

Benjamin Peere

Joined Nov 29, 2018
33
It sounds to me you are attempting to complete a task with a power source that does not have the capability to achieve it.
BTW, the term for the alternative to a stepper motor in your case is a RC Servo, just using the term servo motor includes industrial servo motors of all varieties.
Max.
I thought that 2 Li ion batteries, that provide a maximum of 4.2V each when fully charged and 5.2Amp together would be enough for this stepper motor, what would be your recommendation?
 

Alec_t

Joined Sep 17, 2013
14,280
the stepper motor that I have here is supposed to deliver 3.5kg.cm(51Oz.in) without a gearbox
That is the rated holding torque, not the torque exerted when trying to move from one step to the next. Your motor would need a gearbox with a substantial reduction ratio.
What sort of lock do you have? A domestic door lock is a lot different from the lock on a mediaeval castle :). Measure the actual torque needed, rather than rely on guesswork.
 

Thread Starter

Benjamin Peere

Joined Nov 29, 2018
33
That is the rated holding torque, not the torque exerted when trying to move from one step to the next. Your motor would need a gearbox with a substantial reduction ratio.
What sort of lock do you have? A domestic door lock is a lot different from the lock on a mediaeval castle :). Measure the actual torque needed, rather than rely on guesswork.
it is a recent door lock that I just bought for testing, so all new and really smooth^^ I can't measure the actual torque because I Don't have spring balance at the moment, I will buy one but really difficult to find in a shop so I have to order it on internet and it will take some time, so for the moment I will take the measure you did as a basis because it must be really close from what I need.
how do I know the torque exerted when trying to move from one step to the next? do you think that a 9:1 ratio would be enough? it is already slow with a 5:1 ratio and the higher the ratio will be, the slower it will be, also I can use Micro stepping with the stepper motor to increase the torque, but it is not as strong as I hope.
 
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