Force overshoot and other pressure disturbance mitigation in pneumatic force control devices
Pressure disturbances in a pneumatic robotic force control device—including force overshoot upon initial contact between a robotic tool and a workpiece—are mitigated by increasing the mass air flow in or out of a pneumatic chamber via one or more force overshoot mitigation air passages formed in the robotic force control device. The force overshoot mitigation air passages may connect the two chambers in air flow relationship, or may allow air flow from a chamber to the exterior. The force overshoot mitigation air passages may have a static or variable effective area. The optimal area may be calculated based on measured flow rates and pressures during typical use cases.
1 . A robotic force control device configured to be interposed between a robot arm and a robotic tool, comprising:
a carriage capable of linear motion along an axis and configured to be directly connected to the robotic tool;
a pneumatic cylinder containing a piston dividing the cylinder into first and second chambers;
a rod connected to the piston and protruding from the cylinder, the rod directly connected to the carriage outside of the cylinder;
a pneumatic control system connected in an air flow relation with at least one of the first and second cylinder chambers via corresponding first and second air lines; and
at least one force overshoot mitigation air passage configured to increase a mass air flow rate to or from at least one of the chambers and thereby reduce an effect of pressure disturbances in the piston chambers upon movement of the carriage, so as to minimize a force overshoot between the robotic tool and the object, upon initial contact between the robotic tool and the object.
2 . The device of claim 1 , wherein the force overshoot mitigation air passage comprises a through hole in the piston, connecting the first and second chambers in air flow relationship via the through hole.
3 . The device of claim 1 , wherein the force overshoot mitigation air passage comprises a through notch formed at the periphery of the piston, connecting the first and second chambers in air flow relationship via the through notch.
4 . The device of claim 1 , wherein the force overshoot mitigation air passage comprises an annular space between the periphery of the piston and the interior wall of the cylinder, connecting the first and second chambers in air flow relationship via the annular space.
5 . The device of claim 1 , wherein the piston is formed from a porous and permeable material, and wherein the force overshoot mitigation air passage comprises pores in the piston material which allow air flow through the piston, connecting the first and second chambers in air flow relationship.
6 . The device of claim 1 , wherein the force overshoot mitigation air passage comprises an annular space between the periphery of the rod and the passage through which the rod exits the cylinder.
7 . The device of claim 1 , wherein the force overshoot mitigation air passage comprises a static or variable leak in one or both of the first and second air lines.
8 . The device of claim 7 , wherein one or both of the first and second air lines includes a connector with a plug having a vent area embedded in it.
9 . The device of claim 7 , wherein one or both of the first and second air lines includes a 2-port valve that allows a vent area to be modified.
10 . The device of claim 1 , further comprising first and second air line connectors attached to the cylinder and configured to connect the respective first and second air lines to the respective first and second chambers, and wherein the force overshoot mitigation air passage comprises a static leak in one or both of the first and second air line connectors.
11 . The device of claim 1 , wherein an area of the at least one force overshoot mitigation air passage is determined using a thin-port flow model, based on measured flow rates and corresponding pressures.
12 . The device of claim 11 , wherein a plurality of force overshoot mitigation air passage areas are determined based on different measured flow rates and pressures, and a best fit force overshoot mitigation air passage area is determined from a least squares fitting of the plurality of determined areas.
13 . A method of operating a robotic force control device interposed between a robot arm and a robotic tool, the device comprising a carriage capable of linear motion along an axis and configured to be directly connected to a robotic tool, a pneumatic cylinder containing a piston dividing the cylinder into first and second chambers, a rod connected to the piston and protruding from the cylinder, the rod directly connected to the carriage outside of the cylinder, and a pneumatic control system connected in air flow relation with each of the first and second cylinder chambers via respective first and second air lines, the method comprising:
increasing a mass air flow rate to or from at least one of the chambers via at least one force overshoot mitigation air passage, and thereby reducing the effect of pressure disturbances in the piston chambers upon movement of the carriage; and
moving the robot arm such that the robotic tool contacts the workpiece, whereby the carriage moves relative to the pneumatic cylinder;
whereby the force overshoot mitigation air passage minimizes a force overshoot between the robotic tool and the workpiece, upon initial contact between the robotic tool and the workpiece.
14 . The method of claim 13 , further comprising:
measuring a plurality of mass air flow rates and corresponding pressures; and
determining an area of the at least one force overshoot mitigation air passage using a thin-port flow model, based on the measured flow rates and corresponding pressures.
15 . The method of claim 14 , wherein the thin-port flow model is {dot over (m)}=a vent f (P Hi , P Lo ) where
{dot over (m)} is the mass air flow through an orifice;
a is an area of the orifice;
f is a pressure-dependent, non-linear forcing function; and
P Hi and P Lo represent the absolute referenced high and low pressure values.
16 . The method of claim 15 , wherein the pressure-dependent, non-linear forcing function is
f
(
P
Hi
,
P
Lo
)
=
{
α
P
Hi
(
P
Lo
P
Hi
)
2
k
-
(
P
Lo
P
Hi
)
k
k
+
1
P
Hi
P
Lo
≤
θ
β
P
Hi
P
Hi
P
Lo
>
θ
}
where the constants α, β, and θ are given by:
α
=
C
2
M
ZRT
k
k
-
1
β
=
C
kM
ZRT
(
2
k
+
1
)
k
+
1
k
-
1
θ
=
(
k
+
1
2
)
k
k
-
1
where
T is the Temperature;
M is the Molecular Mass of Air;
R is the Universal Gas Constant;
Cd is the Discharge Coefficient;
Z is the Compressibility Factor; and
k is the Specific Heat Ratio of Air.