Acceleration is an increase of speed due to force. It is defined by the rate the speed increases. When you are in a car you feel the force when the car accelerates. If the acceleration is abrupt you feel like you’re being thrown back into the seat. When the car comes to a stop, your head jerks forward because you had braced yourself against the deceleration force. Stiffening your muscles against the deceleration force is like what a robot arm has inherently in its ‘stiffness.’
No machines are absolutely rigid; they all flex, due to a lack of absolute stiffness. This is one of the adjectives commonly used to describe a machine – its stiffness. This is critical in machines where there are high forces at play, milling machines for example. A laser cutter on the other hand needs less ‘stiffness’ as long and it is on a firm foundation as it merely moves the head around.
Robot arms also need a high degree of stiffness, not usually because they have to exert great forces like a milling machine, but because of inertia; like you in a car. The robot arm needs less stiffness the further away the parts of it are from the base, but it has an overall balanced stiffness. If the arm is stationary and extended and you try to push the manipulator at the end of the arm it may move a little bit. The more robust the arm is, like the ones that lift and manipulate a car door like they do in a car plant, the less they move, they all move a bit, even if you can’t detect it.
When the arm starts to move, the acceleration has the same effect on the arm as it has on you in a car, but you react much more because you are much less ‘stiff.’ To reduce the effect of these acceleration and deceleration forces the acceleration and deceleration could be reduced, but this slows the whole motion down, and as time is money the user wants it to go as fast as possible. Another way is to smooth the acceleration and deceleration to at least stop the jerks at the start and end of motion. This is where the S curve comes in.
If the acceleration and deceleration are slowly increased at the start, and slowly decreased at the end, but the average acceleration and deceleration is the same, the ‘jerking’ is minimized. The lag and overshoot do not go away and the stress on the machine is no less during the highest rates of acceleration and deceleration, but the jerks at the very start and end are much less pronounced and the perceived springiness will be greatly reduced. Why don’t all machines use this technique? The more expensive ones do, but it takes greater computing power to do the math and it also gets complicated. It’s all in the programming of the controller. The greater processing power available and the greater the skill of the coder, if the processing power is somewhat lacking, the lighter and less stiff a machine or robot arm has to be, to avoid shaking it and its load to bits, making it less expensive.