I am currently doing research on progressive freezing and thus require a system that will lower a vessel into a cooling bath a continual rate. From my research it seems that a linear actuator is the best option as they are fairly simple and reasonable priced.
The rate of lowering will approximately need to be 0.5 to 2 cm/hr, and be constant at the set rate. I was wondering if this was possible with a linear actuator as it is a fairly slow rate of movement? The weight of the vessel will approximately be 10 lbs at the maximum.
Is this set up possible with the given criteria? Will a controller be required?
And using the servo you suggested, mounted above the guide rail, to lower the vessel using the weight of the vessel (approximately 10 lbs)?
Do you have any more information regarding the life of servos? Say approximately 4 hours per run, with 100 runs, roughly 400 hours. Is this something reasonable to expect?
Would a manual servo controller be able to control the device at that low of speeds?
Do you have any more information regarding the life of servos? Say approximately 4 hours per run, with 100 runs, roughly 400 hours. Is this something reasonable to expect?
Would a manual servo controller be able to control the device at that low of speeds?
You can try these guys, “http://www.gpf-engineering.co.uk/” Thestandard driver that should be able to do it, although if you connect it to a lead screw you are introducing a lot of inefficiencies. You should take a look at doing it with pulleys, I usually find HPC Gears are good.
The stepper motor will last as long as the bearings can withstand (as there is no other contact), if I were you to prolong the life of the bearing I would use an Idler Pulley to take majority of the side load.
A linear actuator at that speed will likely produce a lot of “jittering” because of static friction changing to kinetic friction and back. A DC linear actuator is essentially a DC gear motor connected to a lead screw.
Your better choice is actually a DC geared motor, DC motor controller and pulley system. You would start with a low RPM gear motor and use a pulley system to reduce the speed even more. Choose cabling that does not stretch. The motor controller can be used to electronically control the speed as well. The one catch is that there is the possibility for side to side motion.
A final option (and also likely the cheapest) would be to use the HS-785 winch servo, and R/C servo controller and a string. This is very similar to the pulley idea, but instead has the servo on at all times (definitely reduce the life of the servo having it powered for hours on end).
The guide rail would certainly make it straight, but you’d need to watch for areas where the rail might snag because of friction - not sure how important it is for you to get an exact and continuous speed.
None of the manufacturers have expected life because these servos were almost all originally intended to be used intermittently on R/C vehicles (for example the winch servo is meant to be used on an R/C sailboat as a winch to retract and release the main sail. Almost all actuators have a duty cycle, but we have yet to see any data for servos.
Let’s calculate: the radius of the pulley is about 2cm, and the actuator can rotate from 0 to 1.38 seconds per 60 degrees (max). There are 100 different speeds possible in either direction, so it’s in 0.138 second per 60 degree increments. This means the lowest theoretical speed would be 0.828 seconds per 360 degrees. This would release pi*d length of cable = 12.566cm per 0.828 seconds (~15cm/s). Therefore you’d still need a pulley system to slow it down to 0.5cm/hr.
The highest gear ratio motor we offer is the Tamiya Planetary Gear box: the ratio is 400:1, and you can add additional stages by buying additional gear boxes. Therefore in theory, you can add two gearboxes together to get a 4:1 x 4:1 x 4:1 x 4:1 x 5:1 x 5:1 x 5:1 x 5:1 = 160000:1 gearbox. The motor’s nominal rpm is 8300, so it would reduce it to 0.05rpm (= 3 revolutions per hour). Add a small pulley with the wire, and a bearing block on either side (to support the weight) and you should be ok. Note that you’d need really long screws to hold that gear box together. If you add even a 2:1 pulley, the load on the motor is half, as is the speed of decent.
Most servos were never required to run for 4 hours continuously, and robotics is pushing the limits of what they can do. Most people use the servo for about 1 hour at most and let it cool down, but we would not be surprised if others have used them for longer. Metal gear, brushless servos (most are also digital) are more expensive than plastic / brushed (mostly analog) servos and will last longer. We’ll contact one of our manufacturers to see if they can provide some guidelines or input.
Note that a servo has (in theory) ~2000 unique positions which correspond to 180 degrees of rotation. Although a servo has a maximum rate of rotation, you usually control a servo’s position rather than its speed. Therefore when controlling a hobby servo with a servo controller, you will be determining how long it remains at each position (stop and go) which loosely corresponds with an angular rotation. However, as we indicate in the last reply, a servo controller on its own can’t reduce the speed to what’s needed, so he’d need pulleys. Servos might work but are not the best option.
Geared steppers are a good option, but require programming.
