Drive Height Adjustment

With the wheels now sorted, it's time to design the lifting mechanism.
Here I had two options again;
Have two motors, one for the front and one for the rear, with sensors to synchronize them.
Have one motor with different gears to front and rear to keep it level.

I went for one motor, this will ensure both wheel move relative to each other, they will need different gear ratios on each, because the front is just an arm rotates about a centre. The rear however is a wishbone configuration, because I need to keep the rear wheel assembly in a vertical alignment.


There is no actual need for this to raise and lower, I saw it some were on an other robot.
I thought is would be a good exercise to see if I can do it, all my projects are challenges to see what I can do.
My first idea to drive the raise system was with an other stepper motor so I could set it a what ever level I wanted.
I started drawing it with a worm pinion and gear.



Then I had a sleep, the next day I thought, the breakout board I am going to use to control the stepper motors has only 3 drive slots.

I need two for the front wheels and I will need one to rotate the torso, so I decided to use a DC motor with gearbox, I will add sensors to determine the position of the wheel arms.
I did some shopping around on eBay and found this:






Had to wait for it to arrive, so I could draw it accurately. During that time, while I was trying to print a perfect worm pinion to fit a gear, I came up with a better idea, I decided to use a worm pinion and rack, a lot easier to print.




Because I decided to use a DC motor I need to add some sensors to determine the position of the wheel arms.
I am going to have to use two photo interrupters, one to indicate fully up, and one to count increments.
This means that I will have to write into the program a sequence where it will reset its count to zero when it is fully up on switch on, so that its known position is correct.

You can see from the animation that there are two gears under the rack that goes back and forth, the front gear originally drove the front wheel arms, but after modifying it to make it narrower, so that it would traverse my furniture, both the front and rear wheel are raised and lowered from the rear gear.
The front gear now just supports the rack and shutters for the position sensor.




The lower sensor counts the position, the upper sensor detects when the rack reaches the end and sets the count to zero.
I used these sensors from eBay. They have an LM393 comparator so the signal is digitised.
Also on the shutters, I used a indelible marker pen to coat the sides with black ink. This is because PLA plastic is quite transparent to infrared light.
Here is a demonstration using a light sensor without a comparator to show how the ink makes a difference:

I am now able to raise the bot to whatever position I choose.

After a While.

As I keep adding to the robot (I can call it a robot now, It does things on its own).

It is getting heavier and the riser motor struggles to lift it up and down.

I had a thought, if I make the worm drive out of PTFE, may be it would reduce some of the friction on that drive.

This required another strip down.

I decided to replace the original red worm drive shown above, with the new blue one with PTFE thread shown below.

I also tried a 5 volt motor and gearbox to see if this improved it, but the 5 volt motor was not up to the job.

Replacing the 12 volt motor with the PTFE gear did improve it, but not enough.

I decided to replace the 12 volt motor with one that has a slower reduction gearbox, so I can still have this function working.
Having a gearbox with a lower reduction speed also put less strain on the motor which let it run at a faster speed, so the speed at which the body of the robot raises did not alter too much.

If it starts to struggle again, I may have to come up with a different design to raise the body of the robot.

If you like what I have done, or want to know more.

Please leave a comment below.