Balancer mechanisms

different type of counterbalance exist.

the simplest one is dead weight. disadvantage is that this adds weight (inertia) and requires more space since it relies on gravity to form see-saw.

this is why dead weight is not practical on axis 2 but may be used on axis 3. wrist axis motors are often used as a this since they already need to be there... sometimes additional metal block is added which depends on robot type.

your post seem to be concerned with axis 2 and on this axis deadweight is just not practical since weight would need to be even larger (making robot slow) and require robot to be higher above the floor. Therefore axis 2 counterbalance needs something else. traditionally they would use spring(s). More than one spring can be added)

Springs are simple but ... they are bulky and problematic for servicing, for example cannot be turned off or adjusted. Any adjustment requires changing spring(s) but this is coarse and in increments corresponding to spring type. On some robots all springs are enclosed into one "drum"....

but some designs have two smaller ones, one on each side of the arm.

And then there is a hydro-pneumatic counterbalance. gas is compressible and liquid is not. so using bladder and piston one can make an equivalent of a spring but - more compact, can be fine adjusted by simply using different pressure. can be "switched off" (depressurized) before transport to meet air travel regulation (if you need to get it fast) etc. But this type may be a problem in case of certain application due possible contamination in case of leak (food industry, cleanroom, nuclear).

older style of this counterbalance used single larger vessel. newer designs normally use equivalent volume but with two or more of smaller ones (workaround for regulations).

One thing to keep in mind is that axis 2 counterbalance is not the same for all mounting positions. if robot is to be used in a different mounting orientation (wall or ceiling) than designed, counterbalance must be replaced with version for that orientation.

i find all robot mechanics quite interesting... and the trend is to make robot arms slicker and more like human arm. without anything protruding or sticking out.

this means no external rods (parallelogram linkages used to be standard for palletizing robots), more compact or no counterbalance etc.

example of previous generation palletizers

compare that with new generation. compact axis 2 counterbalance, no linkages, nothing sticks out at wrist and elbow (no motors or deadweight protrudes behind shoulder), can work in smaller cells including ones with low ceiling. this is quite important since palletizers tend to be mounted on pedestals.