End effectors and tooling integration

The end effector is the part of a robot cell that touches the product, and it is usually the part that is custom-designed for each project. Robots are bought from a catalogue; grippers, welding torches, dispensing heads and vacuum tools are often engineered or at least configured for the specific parts. ROB-400 analysed grasping from first principles: contact mechanics, force closure, finger geometry and the basic sizing of grip and suction forces. This lesson takes the integrator's view of tooling: how a complete tool stack is assembled on the robot flange, how its mass and inertia are checked against the robot's limits, how compressed air and electric tools are supplied and controlled, how tool changers and compliance devices work, and how process tools such as welding torches and dispensers are integrated. A good end effector is light, stiff, simple to maintain, and safe when power or air is lost.

The tool stack

Between the robot's mechanical interface and the part, several components are usually stacked. The robot flange follows a standard bolt pattern, commonly ISO 9409-1, which defines the pitch circle, bolt holes and locating pin for each flange size. On it may sit an adapter plate, a collision sensor or breakaway, a tool changer master plate, a force-torque sensor, the tool changer tool plate, and finally the gripper or process tool with its fingers or nozzles. Each item adds mass, length and cost, and each joint adds a little compliance and a little positioning error.

The tool centre point (TCP) is the point on the tool whose position and orientation the robot program controls, such as the centre between gripper fingers, the tip of a welding wire or the centre of a suction cup face. Lesson 4 explains how the TCP is defined and calibrated. A long tool stack moves the TCP far from the wrist, which magnifies small angular errors in the wrist joints into larger position errors at the tip and raises the static and dynamic torques on the wrist axes.

Checking mass, centre of gravity and inertia

Lesson 1 checked static wrist torque. A complete check also includes the combined centre of gravity of the tool and part, and the moment of inertia about each wrist axis, which limits how quickly the wrist can accelerate the load. Robot manufacturers publish allowable moments of inertia for axes 4, 5 and 6 in kilogram square metres, and their controllers need correct payload data to plan motions; a wrong payload setting causes poor path accuracy, vibration, false collision detection or premature gearbox wear.

The combined centre of gravity of several components along the flange axis is the mass-weighted average of their individual positions. The moment of inertia about a wrist axis is the sum, for each component, of its own inertia about its centre of gravity plus its mass times the square of the distance from the axis, the parallel axis theorem.

# Payload data from a list of tool components (CAD-style mass properties)
import numpy as np
# name, mass kg, centre of gravity (x, y, z) mm from flange, own inertia about cg (Ixx, Iyy, Izz) kg m^2
parts = [("changer", 1.2, (0, 0, 25), (0.0010, 0.0010, 0.0015)),
         ("gripper", 4.5, (0, 15, 110), (0.0200, 0.0150, 0.0120)),
         ("part",    6.0, (0, 0, 220), (0.0300, 0.0300, 0.0100))]
m = sum(p[1] for p in parts)
cg = sum(p[1] * np.array(p[2]) for p in parts) / m
I = np.zeros(3)
for name, mi, c, Icg in parts:
    d = (np.array(c) - cg) / 1000                    # m, offset from combined cg
    I += np.array(Icg) + mi * np.array([d[1]**2 + d[2]**2, d[0]**2 + d[2]**2, d[0]**2 + d[1]**2])
print(f"mass {m:.1f} kg, cg (x, y, z) = ({cg[0]:.0f}, {cg[1]:.1f}, {cg[2]:.0f}) mm")
print("inertia about combined cg (Ixx, Iyy, Izz) kg m^2:", np.round(I, 3))

mass 11.7 kg, cg (x, y, z) = (0, 5.8, 158) mm
inertia about combined cg (Ixx, Iyy, Izz) kg m^2: [0.106 0.101 0.024]

Most robot controllers can also estimate payload automatically by moving the wrist through a test motion and measuring motor torques. This is useful for checking the entered data, but the estimate is only as good as the robot's dynamic model and the test motion, so integrators still calculate the expected values from CAD.

Pneumatic and electric grippers

Most industrial grippers are pneumatic: compressed air at typically 4 to 7 bar drives a piston that opens or closes the jaws. Pneumatic grippers are simple, robust, powerful for their size and inexpensive. Electric grippers use a motor and screw or a servo drive; they cost more but allow programmable force, position and speed, report jaw position for part verification, and avoid the energy losses of compressed air. Compressed air is one of the most expensive utilities in a plant, because compressors convert only a small fraction of their electrical input into useful work at the tool.

Vacuum tools are the other dominant gripper family, especially in packaging, palletizing, sheet metal and glass handling. Vacuum is produced either by venturi ejectors, which use compressed air and need no moving parts, or by electric vacuum pumps and blowers. Venturi ejectors are simple and fast but consume considerable compressed air; air-saving ejectors switch off the supply once vacuum is reached and a check valve holds it, restarting only if vacuum decays.

Fail-safe behaviour and pneumatic circuits

What happens if the air supply or power fails while the robot holds a part? With a single-solenoid, spring-return valve, the gripper may open when the solenoid loses power and drop the part. With a double-solenoid detented valve, the valve stays in its last position, and the gripper stays closed as long as pressure remains in the cylinder. Pilot-operated check valves on the gripper ports, or grippers with internal springs, hold the part even if air pressure is lost. For vacuum tools, a check valve and a vacuum reservoir can hold the part briefly, and the robot can be programmed to lower it before release. The right answer depends on the risk assessment: dropping a heavy part may be dangerous, but a gripper that cannot be opened during a rescue is also a hazard. Designers document the intended behaviour on power loss, air loss and emergency stop for every actuator.

# Hold time of a vacuum tool after the supply stops, with a reservoir and a leak
import numpy as np
V = 0.004            # m^3 of volume under vacuum (cups, hoses, reservoir)
p_atm = 101_325.0    # Pa
p0 = p_atm - 60_000  # starting absolute pressure (60 kPa vacuum)
leak = 1.0e-4        # m^3/s of air leaking in through the porous carton (assumed constant)
area, mass = 0.04, 18.0
p_needed = p_atm - 2 * mass * (9.81 + 5.0) / area   # SF 2 with 5 m/s^2 upward acceleration
# isothermal: mass inflow raises pressure; dp/dt = p_atm * leak / V
dpdt = p_atm * leak / V
t_hold = (p_needed - p0) / dpdt
print(f"vacuum needed for SF 2 while accelerating: {(p_atm - p_needed)/1000:.1f} kPa")
print(f"time until that limit after supply loss: {t_hold:.1f} s")

vacuum needed for SF 2 while accelerating: 13.3 kPa
time until that limit after supply loss: 18.4 s

With these assumed numbers the tool keeps a safe grip for about 18 seconds after the supply stops, which is long enough for a programmed set-down if the robot can still move, but not long enough to leave a carton hanging during a lengthy stop. Real leakage through corrugated board varies widely with board grade and humidity, so integrators measure it. A vacuum switch that warns the robot when the level falls toward the limit lets the program react before the carton drops.

Tool changers

Automatic tool changers let one robot use several end effectors: different grippers for different parts, or a gripper and a deburring spindle. A changer consists of a master plate on the robot and a tool plate on each tool. Locking is usually by pneumatically driven balls or a cam, designed so that the lock holds mechanically if air is lost. Modules on the sides pass compressed air, vacuum, electrical signals, power, fieldbus or coolant. Each tool sits in a tool stand with sensors confirming its presence, and the robot program checks that the correct tool is locked before continuing.

Tool changers add mass and cost, and a changeover takes several seconds, so they make sense when the alternative is a second robot or a long manual changeover. Manual quick-change systems, which lock with a lever but still provide repeatable location and utility connections, are a cheaper option for changeovers a few times a shift. For tool identification, many systems include coded pins or an electronic ID chip, so that the PLC can confirm the correct tool and load the matching TCP and payload data automatically.

Compliance and collision protection

Robots are stiff position-controlled machines, and parts and fixtures are never perfectly placed. Compliance devices let the tool give a little when it meets resistance. A remote centre compliance (RCC) device places its centre of compliance near the tip of a peg, so that lateral and angular misalignment during insertion is corrected by contact forces rather than causing jamming. Floating units and spring-loaded fingers serve similar purposes. Active compliance, using a force-torque sensor and force control as described in ROB-300, provides programmable behaviour at higher cost.

Collision protection limits damage when something goes wrong. A breakaway unit holds the tool rigidly with spring or air pressure until an overload, then deflects and opens a switch that stops the robot. Many current robots instead detect collisions from unexpected motor currents, which needs no extra hardware but depends on correct payload data. Neither is a safety function for protecting people unless it is specifically rated as such; Lesson 8 explains the difference.

Process tools

Not every end effector grips. Arc welding robots carry a torch connected to a wire feeder and power source through a cable called a hose package or dress pack, routed through or along the arm. Torch cleaning stations ream spatter from the nozzle, trim the wire and apply anti-spatter fluid at intervals. Through-arm torches on hollow-wrist robots reduce cable wear and interference. Dispensing tools for adhesives and sealants use metering pumps whose flow must be coordinated with robot speed so that the bead cross-section stays constant even when the robot slows at corners. Spindles for deburring and polishing need compliance to follow edges. Screwdriving tools monitor torque and angle to verify each joint.

Dress packs deserve design attention. Hoses and cables flex millions of times over a robot's life, and failures of cables at the wrist are among the most common causes of downtime. Integrators use dress pack systems with retractors and protective sleeves, simulate cable motion where possible, and avoid programs that wind axis 6 repeatedly past its full rotation.