Do I need a microcontroller or no?

[MrSticky]

So, I'm new here and marginally new to electronics but I'd really like to build a powered gripper that I can wear around. My first model just used my fingers to bend some bicycle chain fingers that were welded into the right shape, but its rather weak and I'm ready to move to the next phase of the project. Motorized gripper.

I'm not too worried about the mechanical parts but I'm learning as I go about the electrical systems.
I was looking at these motor drivers:
http://www.solarbotics.com/products/50646/resources/   
http://www.solarbotics.com/products/k_smd/

And instead of using a mircocontroller just hook up two buttons to run the inputs. One for open, one for close. I was planning on just using gear ratios to set the open/close speed so I didn't think that I needed to worry about creating PWM in the signal. I also assumed a DC motor rather than a servo would be fine if I used a worm gear or rack/pinion setup to open/close the gripper. This would also eliminate/reduce the need for motor braking to maintain its grip.

Are my assumptions correct, or am I missing a some fundamental concept?
Any help or input would be greatly appreciated, as this project could look really good on my resume (looking for a career in prosthetics after I finish college).

-MrSticky

[Soeren]

Hi,

My first model just used my fingers [...] but its rather weak and I'm ready to move to the next phase of the project. Motorized gripper.

"It" is not weak, it's just relaying the power... It's the "motor" that needs a tune up... Just go to the gym on a regular basis (or get a "Kung-Fu Tiger Claw Trainer") and "it" will get stronger over time

Are my assumptions correct, or am I missing a some fundamental concept?

Mostly correct, but I'd recommend implementing an over current protection, to cut or retard the current when the fingers are locked around something (as the motors will burn their windings if continually being fed the stall current).
It will have to be able to supply a short burst of full (or nearly so) power for reversing, before judging the current too large, or it may be too weak to let go of objects once it holds them in a tight grip.

Force sensing might be good and a simple method of force sensing is measuring the motor current, which will increase with increased force.

Any help or input would be greatly appreciated, as this project could look really good on my resume (looking for a career in prosthetics after I finish college).

While it was a couple of years ago I was at the last convention with some of the very best prosthetic technicians/artists (those making prostheses for people missing limbs, not SFX people), I noticed that they stopped using bicycle chains for fingers, so you might wanna try other materials, if you hope to make a living out of it 

[MrSticky]

Hahaha. Touche, good sir. Touche. =P

And I appreciate the advice about cutting the current when its gripping. I'm not familiar with the hardware that would be able to do that and I was hoping to keep this limited to just a power source, motor driver, motor and switches to work the mechanism. Would adding that feature take much more then that? Also, I really like the idea of integrating a force senor (I was surprised at how cheap they are!) but I feel I would need a mircocontroller to make that work right, so that will probably have to wait until the MkIII hand. Or it will be at least a few weeks until I can wear down my resistance to spending more on this project and just buy everything I need to make it awesome. =P

The bike chain will definitely be replace on this next model. It was something cheap that I could start working with right away, but I've certainly found some limitations with using it. 

[Soeren]

Hi,

And I appreciate the advice about cutting the current when its gripping. I'm not familiar with the hardware that would be able to do that and I was hoping to keep this limited to just a power source, motor driver, motor and switches to work the mechanism. Would adding that feature take much more then that?

It should be made a part of the motor driver. I don't know what you had in mind for a motor driver but adding over current protection could be as little as a resistor and a small signal transistor.

What have you planned for the motor controller?

Also, I really like the idea of integrating a force senor (I was surprised at how cheap they are!) but I feel I would need a mircocontroller to make that work right, so that will probably have to wait until the MkIII hand.

If you just use the current as a measure of force, no microcontroller would be needed, but perhaps you should just go for the over current protection in the first version - learning stuff is easier if you don't try to take in too much at a time

[MrSticky]

I was planning on using an L293D chip. Probably just with the 'Secret' Motor Driver kit from solarbiotics (http://www.solarbotics.com/products/k_smd/). I only need to drive one motor at the moment, so I don't mind that it links the 2nd set of inputs/outputs to the first set. Its cheap, and if need be I can solder the parts on a bigger board to include, say, a transistor.
I've been trying to look into how to use a transistor to add over current protection, but most of what I've been finding has been using them as switches or how to protect THEM from over current. Would you mind giving me a reference where I can read up on this, or walk me through how to wire it up to the chip? And/or if you have any critiques of my choice of motor driver that would be cool too.

I really appreciate all your help. I'm really excited about this project, but I've got a fair bit to learn before I can get it to where I want it to be.

[Soeren]

Hi,

I was planning on using an L293D chip. Probably just with the 'Secret' Motor Driver kit from solarbiotics
[...]
I only need to drive one motor at the moment, so I don't mind that it links the 2nd set of inputs/outputs to the first set. Its cheap, and if need be I can solder the parts on a bigger board to include, say, a transistor.

1 L293D chip, 1 resistor, 1 LED and a small PCB for $12.95 and you call it cheap?

At SparkFun (which isn't particularly cheap), the SN754410 (slightly better) pin for pin equivalent chip is $2.35 and the L293D can be bought several places at under $3.
a 0.25W resistor and an LED is next to nothing, so you'd pay around $10 for a PCB that you already plan to abandon?
Get some strip board and just the chip alone.
The L293D can deliver only 600mA each "channel" (continuous current), but you can parallel two channels for twice the current. It has got 4 independent channels, but you need two to make an H-bridge, which you will need to be able to run your motor both forward and in reverse.

If you plan on 2 simple push buttons (for "close hand" and "open hand"), you need to put a switch in the power line, as the L293D uses power even if the motor isn't energized.

What battery voltage wll you be using?
How much current does your motor take, free running and stalled?

I've been trying to look into how to use a transistor to add over current protection, but most of what I've been finding has been using them as switches or how to protect THEM from over current. Would you mind giving me a reference where I can read up on this, or walk me through how to wire it up to the chip?

Don't know what you mean by "THEM"?

I can draw you a schematic if needed, but then I have to know the battery voltage.


You could do away with the L293D completely, by using a DPDT rocker switch with a ON-OFF-/ON layout and spring loaded to the center (OFF) position. That way you'd only need to add the simple over current circuit, to keep your motor alive.

[Conscripted]

I tried to do a Google search for schematics for over current schematics but only found conversations about adding fuses or circuit breakers to protect the motors. I'm assuming that isn't the type of solution you were talking about.

How would an over current circuit function? I assume that at a given current level it shunts some of the power someplace else but I have no idea how that would be accomplished.

Conscripted

[Soeren]

Hi,

[...] fuses or circuit breakers to protect the motors. I'm assuming that isn't the type of solution you were talking about.

Right.
A fuse should always be added, no doubt about that, but using a fuse to protect against stall conditions will be expensive and a pain in the general hip area. Even with resettable fuses it would grow old real fast.

How would an over current circuit function?

Very well I hope

I assume that at a given current level it shunts some of the power someplace else but I have no idea how that would be accomplished.

Keeping on topic, there's two scenarios:
With the L293D, you control the enable pins with a small signal transistor
With a switch, you need a transistor able to handle much more, as you use it more or less like a CCG, allowing a certain max current.

Imagine a 12V supply motor and a 2A limit - the transistor will then have to cope with up to a maximum of 24W (worst case).

With a bit more circuitry, it would be possible to cut the current completely if/when the transistor reached a certain temperature (like if a continually stalled motor was kept powered).

If the focus isn't so much on minimum circuitry, a PWM output could be controlled by sensing the current and when a given current is reached, the duty cycle can be lowered to keep the average current within specs.

[MrSticky]

I always have been terrible at putting prices into the right perspective. You make a valid point about the component-wise pricing.

Sorry about the confusing use of "THEM". I meant that the articles I found discussed how to protect transistors from over current, rather than using them to protect other components from over current.

And well, shoot! It never occurred to me to use something like a DPDT rocker switch. I've seen them before, but they certainly did not come to mind with with project! Thank you so much for suggesting that!!!

If you would be so kind, I would really appreciate it if you could draw up the circuit with a DPDT switch and the over current protection. Now that I'm not constrained by the chip anymore (that was what I was basing battery and motor choice on), I think that a 6 volt battery would work (well, a 6v battery pack, I don't want to wear one of those big single batteries) to go with this motor:
http://www.pololu.com/catalog/product/994
I'm looking for a high torque motor, preferably with a worm gear so it will self lock and I don't have to worry about stalling the motor, but the only worm gearbox I found was plastic and I question its durability. So I decided I'd just go with a motor that's already torque-y and deal with a rack and pinion setup.

[Soeren]

Hi,

If you would be so kind, I would really appreciate it if you could draw up the circuit with a DPDT switch and the over current protection.

Attached.

I'm looking for a high torque motor, preferably with a worm gear so it will self lock and I don't have to worry about stalling the motor, but the only worm gearbox I found was plastic and I question its durability. So I decided I'd just go with a motor that's already torque-y and deal with a rack and pinion setup.

Oh, you can have them - even in coaxial versions like this 45W output unit:

But they're really expensive!

Although not self locking, a large gear ratio should help anyway.

[MrSticky]

Sweet! I just have a question about F1, by the positive battery terminal. Is that a 1.5db attenuator or a 1.5 amp fuse? I've never seen the designation 'AT' before (again, I'm pretty inexperienced with circuitry).

Thank you so much for all of your help! I really appreciate you putting up with all of my n00b questions and getting me set on the right track.

[Soeren]

Hi,

Sweet! I just have a question about F1, by the positive battery terminal. Is that a 1.5db attenuator or a 1.5 amp fuse? I've never seen the designation 'AT' before (again, I'm pretty inexperienced with circuitry).

1.5AT means 1.5Ampere slow blow. The "T" is from the german word "träge" which means something quite close to "slow". The "F" used on fast blow fuses doesn't stand for "fast" either, but is from the German word "flink".

T-fuses is used where there should be headroom for short term loads at or near the nominal current - like in all mains transformer applications. It's main purpose is to protect the battery from direct shorts.
F-fuses is used where you need a more instant fusing and there's even some superfast fuses for protecting semiconductors.

It's calculated for around 1A. The motor you linked to has got a stall current of 1.5A, but if you need to lower the current, increase the resistor.
Oh, I forgot to mention...
R1 should be rated 2W (or more)
R2 should be rated 1W (or more)


BTW. the transistor will need a small heat sink if you plan to keep it pushing against something for more than a very few seconds. Even the simplest U-shaped TO-220 heat sink should do. A bit of 1..2mm aluminum will probably do, as long as you make it plane where the transistor mounts and a thin and even spread of heat sink compound on the transistor, will help transfer the heat better if needed.

Thank you so much for all of your help! I really appreciate you putting up with all of my n00b questions and getting me set on the right track.

You're welcome - let's know how it goes