So I have a probe to be moved along the vertical axis, where it would meet a hindrance and would ramp the force at the rate of about 0-50gms in 5 seconds. The question was how to move the probe. I decided to use a stepper motor linear actuator, but, that is lost in transit and I have no time to order another. What are my options? I thought of a regular geared motor actuator, but am not sure if it can run that slow.
Only you know how slow.. You can slow it down using PWM. Or reducing supply voltage.
would it be able to go 0.1mm/sec? I don't know, never handled linear actuators, and can't locate anything from specs which would guide
That's only 0.7% of the nominal speed.
While possible with PWM, I wouldn't expect it running smoothly and reliably. You should find slower actuator.
Yes you need a slow geared actuator. All the linear actuators i have messed with (I am not saying ALL actuators, just the ones I have used) have DC brushed motors. You can change the speed via PWM and use the correct motor driver. But I have noticed they don't like to be driven below about 50% PWM and become very weak. Most times, if you try to run it with a low PWM, you have to "bump start" it briefly with a higher RPM to get it to start.
That tells me you have the physical structure already built that will fit one linear actuator. But now you are looking at other actuators. Are you going to build a new fixture to fit which ever actuator you finally get?
hadn't built any fixtures yet, so have that flexibility
Hi, @skyhigh123
How are you going measure force?
To control the rate you will need some feedback circuit.
Tom.... 
I think the key point here is not only how slowly the actuator can move, but how you intend to detect or control the applied force.
“0–50 grams in 5 seconds” sounds like a force ramp, not just a position ramp. The actuator itself will not directly know the applied force unless you use some form of feedback or force-limiting mechanism.
That feedback could be a load cell, a known spring between actuator and probe, or in some cases even motor current as a rough indication of load or obstruction. However, for a controlled low force such as 50 g, motor current may be rather indirect because friction, gearbox losses and starting torque can dominate.
So before choosing the emergency replacement actuator, I would first decide whether the setup is position-controlled with a spring, force-controlled with a sensor, or only needs a rough obstruction/load indication.
Does it truly need to increase the force as it moves, or are you trying to increase the holding force against a stationary object? Or can the actuator just simply be strong enough to move the load and hold it when stopped?
Are you truly talking about force? Or do you really mean speed?
When talking about grams, I am envisioning an incredibly small actuator. What are the physical size limitations of this project?
there is a load cell between the probe and the actuator. The probe meets a rigid flat surface and load cell + arduino measures how much force is being put to that surface as the actuator moves up.
The one lost in transit was a readymade compact unit. but I guess I can make my own actuator with the right components? Like a tiny stepper motor and some guide rods? Never made an actuator, can someone guide what components I would need?
A basic linear actuator would be nothing more than a motor attached to a drive screw, preferably via a gearbox. The drive screw has a nut but typically is fixed to a tube that fits over the drive screw. The end of the tube is fixed to the load to prevent the tube from turning while the drive screw is turning, thus producing an extension and retraction. Internal limit switches at the full retract and full extension are certainly useful to prevent completely unscrewing the tube or retracting to far.
I have also seen them fashioned up using a servo motor. Instead of the horn (arm) they had a rack and pinion. The pinion gear was mounted on the servo's shaft and drove the rack through some guide rails linearly.
If you happen to have a 3D printer you could probably print a plastic gearbox for your project. Along with the housing to put all the components in.
You need to consider that at the same time as you pick an actuator. The actuators are mostly designed to open/close a door when you see the ends of the actuator. Your fixture will have to ensure your probe does not pivot on the end of the actuator.
@scottcalv 3D printed cam and follower? I hardly need a 20mm stroke.
We don't know the physical limitations of your project until you tell us. You also mentioned that you've "never made a linear actuator. And can we guide you in that."
We don't have any idea of the specifications and constraints of your project nor do we know what tools you have to work with or have available to you.
A 3D printer can work well within the constraints of a 20 mm stroke. It was just a thought.
A picture is worth a thousand words.
This thread is wandering all over. Have you actually tried anything to report on?
The vague terms you've been using may have mislead me, but to me it sounds like your describing something very similar to the linear mechanism used in 5 1/4" and 3 1/2" floppy drives. If you have access to an old computer with such a drive, take a look. Sure, it'll be a bodge, and a hack, and whatever, but you certainly could make a 20mm sliding platform from the head mechanism on a 5 1/4" floppy, which is driven by a small stepper and has end travel sensors, or other limiting mechanisms.
Lots of info on the web about how to hack those old things, as well..
@skyhigh123 wasn't your original question about how to build an actuator? If you want .1mm / second, a threaded rod with a 1mm pitch turning at 6RPM should get you there. You could direct drive it from a stepper motor and still achieve good power.
I will hint again though. A picture is worth a thousand words.
You mentioned that about 20 mm of travel would already be sufficient. That is actually a very important piece of information, because the solution space for a short travel is quite different from that of a long travel.
However, the required behaviour is still unclear. Is this a quasi-static application where the probe moves slowly and force increases gradually, or is it a dynamic system that requires continuous feedback and closed-loop control? Those are completely different worlds.
At the moment there seems to be enough knowledge available in this thread, but not enough information about the actual problem.
So I think the next step is not more suggestions, but more details: photos, sketches, dimensions, force requirements, speed requirements, accuracy requirements, environmental conditions ... you name it.
The better the problem is defined, the better the proposed solutions will be.