AR Gripper - Open Hardware Adaptive Electric Parallel Robot Gripper

The AR Gripper is an open hardware parallel robot gripper designed for handling everyday items such as bottles and cups.

Specifications

• Grip: 3M gripping material with foam rubber pads
• Stroke: 103mm
• Weight: 570g
• Closing Speed: 80mm/s
• Drivers: Linux: Python, ROS, Windows possible but untested
• Type: Adaptive parallel electric
• 3D Models: Fusion 360, STEP, STL
• Material: 3D printed Colorfabb XT CF-20

Features

• Replaceable gripper fingers
• Fully customizable
• Mounting holes for additional extensions
• ROS drivers and simulation
• Adaptive: stops when an object is detected, torque configurable

The Story Behind the AR Gripper

I designed this gripper because I couldn't find any robot grippers that fulfilled my requirements for building my bartender robot "Roberto the Robot," which is cost-effective at the same time.

I first tried the Robotiq Hand-E because it was available to me. It is an adaptive electric parallel gripper with a stroke of 50mm. Unfortunately, the items I wanted to handle had a diameter of up to 90mm (big coke bottle), and the price is quite high (>3000$). However, I liked the simplicity and modular design of the gripper. For the first version of Roberto, I designed custom gripper fingers that made it possible to grip tools and bottles using a special counterpart. Another huge disadvantage of the Hand-E is its weight of 1kg, which is okay for the Borunte robot I used but can be a challenge for smaller robots.

Because I wanted to handle bottles and other tools without prior modification, I started to look for alternative grippers. My requirements were:

• Can grip objects ≥ 90mm diameter
• Payload ≥ 2kg
• Is adaptive
• Does not require pneumatics
• Costs less
• Grip force big enough to hold a 1.5l bottle made out of glass or plastic
• Ideally integrates with ROS

Most of the gripper products I found did not satisfy the cost requirements or were just too small. I also found a couple of open hardware designs, such as the open hand project, but they all looked discarded.

I then found the SAKE Robotics EZGripper, an underactuated electric gripper design, which satisfied all of my requirements. I used this gripper to improve Roberto the bartender, and the gripper mostly worked as expected. However, I needed to make some adaptations to the gripper pads to increase the friction between the bottle and gripper.

Later last year, I started to work on a smaller low-cost robot. Ironically, the EZGripper would have cost more than the rest of the robot, which seemed wrong and just did not satisfy my requirements.

Looking at the EZGripper and the Hand-E, I thought to myself: "How hard can it be?" The key ingredients of a gripper are a high torque actuator, a mechanical transmission, and sturdy gripper fingers. The hardest part was to find a suitable motor for the gripper. I first looked into the Dynamixel smart actuators because they are used by many fellow roboticists, and they come with good ROS drivers. However, the price of a suitable Dynamixel actuator would have been a few hundred dollars alone. So I ventured out on the internet and looked for alternatives. I figured that servo motors are used frequently in RC models. However, most of the servos used there are analog servos with a fixed angle; what I needed is a servo with free drive and feedback. After some research, I discovered the company Feetech with a good range of high torque smart servos.

This was the key ingredient for my venture in building a better and more cost-effective robot gripper. I first thought about building a simple underactuated gripper with just one finger, but I decided to go for a parallel gripper design as it is even simpler and should work well for my application. The rest was just execution; after some testing and destroying lots of gripper fingers, I improved the design and created something that's finally usable and, most importantly, open source.

Part List

3D Printed Parts

• Side Left
• Side Right
• Top
• 2x Sliders
• 2x Fingers
• Gear
• Mounting Adapter

Bolts

• Body
  • 6x M3x12 sunken head ISO7991
  • 6x M3x8 sunken head ISO7991
• Top
  • 10x M3x20 sunken head ISO7991
• Finger and Slider
  • 4x M3x20 sunken head ISO7991
  • 2x M3x18 socket head ISO 4762
  • x M2x15 socket head ISO 4762
  • 2x M2x20 socket head ISO 4762
  • 4x 2x6mm cylinder pin DIN 6325
• Gear
  • 8x M3x6 button head ISO7380
  • 1x M3x14 socket head ISO 4762
• Adapter
  • 6x M3x10 button head ISO7380

Gripper Pads

• Foam rubber sheet 2-3mm thickness
• Double-sided tape
• 3M gripping material gray 25mm ~45cm

Feetech SM85CL Smart Servo

• Includes servo cable connectors

Servo Cable

• PUR 4x0.25mm2 shielded 4m or similar

RS-485 to USB Adapter

• e.g., In-Circuit USB RS-485 Bridge

12V 50W Power Supply

• e.g., Mean Well LRS-50-12

3D Models

• Interactive Fusion 360 Model: https://a360.co/35agyxS

Drivers

• ROS Package on GitHub: https://github.com/machinekoder/ar_gripper

Building the Gripper

Required Tools

• 3D printer with heatbed, all-metal hotend, and hardened steel nozzle - or capable of printing carbon fiber filaments. Alternatively, most of the parts can also be printed out of PETG or Nylon, but at least the gripper fingers and slider need to be printed with a stronger material.
• PH1 3.5mm Phillips screwdriver for the smart servo
• 1.5mm Allen (hex) key for preparing the gripper fingers
• 2mm Allen (hex) key for sunken and button head screws
• 2.5mm Allen (hex) key with ball end for cylinder head screws
• M2x0.4 hand tap
• M3x0.5 hand tap
• Tap wrench
• 1.6mm metal/plastic drill to prepare M2 holes for tapping
• 2mm metal/plastic drill to clean up holes
• 2.5mm metal/plastic drill to prepare M3 holes for tapping
• 3mm metal/plastic drill to clean up holes
• Electric drill
• A metal/plastic file to improve sliding quality
• Blade cutter or scissors for cutting the foam rubber and tapes
• A pen to mark the foam rubber before cutting
• Optionally, basic electric equipment to extend the cable if needed
• Optionally, a vise to hold the parts during drilling, tapping, filing
• Optionally, screw lock glue medium

3D Printing

The gripper is designed to be 3D printed. I tested the printed design with PETG and Colorfabb XT CF-20. At least the gripper fingers and the sliders need to be printed with carbon fiber filament; the rest of the gripper can be printed in PETG or a similar material. If your printer is capable of printing Nylon (SLS or with enclosure), this might work as well.

Furthermore, parts of the gripper fingers and sliders, and the gear need to be printed with 100% infill to become as sturdy as possible. The rest of the fingers and slider can be printed with 70% infill. The side parts can be printed with 15% infill.

Preparing the Prints

First, use the 2.5mm drill to prepare the holes for tapping. The holes that need to be tapped are located on the 2 base parts and the sliders.

On the base part, all holes except the ones with chamfer need to be tapped.

On the sliders, only tap the holes that go through the hole parts; the other 2 holes are for alignment bolts.

Next, use the 3mm drill to clean up the rest of the holes so M3 screws can slide through easily.

When done, use the 3mm M3 tap to tap the holes.

The next step is to clean up the slider and top part so the sliders can move freely. Usually, it is required to file away the rest of the support material on the underside of the sliders and some plastic bits on the inside of the top part.

Side note on metal inserts

The holes are tapped directly into the 3D printed parts. This works nicely with Colorfabb XT CF-20 if you prepare the holes accordingly; else, there is a chance of the part splitting or delayering. I have also tested M3 brass inserts and come to the conclusion that they work but are more complicated to insert (using heat and/or pressure). If you really want to use metal inserts, use a suitable drill to prepare the holes. The slider part needs very long inserts (≥ 10mm); else, the slider will break apart when the finger is under pressure.

Preparing the Fingers

First, we need to prepare the gripper fingers by drilling and tapping the three M2 threads into the side of the gripper. The bolts that will go into these sockets are not used to attach the fingers to something but instead prevent breaking of the 3D printed part in the direction of the layers.

Use the 1.6mm drill to drill out the holes. Then use the M2x0.4 tap to create a thread as long as possible. Don't skip those 2 steps; driving the M2 bolts in without drilling and threading them will eventually break the gripper finger.

When done, use the 1.5mm Allen key to insert the M2x15 and M2x20 cylinder head screws. To prevent the part from splitting while inserting the screws, you can put it in the vise, pressing together the layers of the print.

Then clean up the mounting holes for the M3 screws using the 3mm drill so the screws can move freely. Additionally, you can use a 2mm drill to clean up the holes for the 2x6mm pins, which are located between the M3 mounting holes on the underside of the gripper finger.

Put the 2x6mm steel pins into their sockets on the slider; when mounting the fingers, those pins make sure the gripper fingers are aligned correctly.

Now mount the gripper fingers using two M3x20 sunken head screws and one M3x18 cylinder head screw per finger.

Next, prepare 2 rubber foam pads 82x42mm using a blade cutter or some scissors and cut four pieces of the gripping material with a length of 110mm.

Use the double-sided tape to mount the rubber foam pads on the inside of the gripper fingers. When done, glue the gripping material stripes on the rubber foam pads, starting from the inside, align them to the center. The stripes are slightly wider than the foam pad; just put the tape around the grippers' fingers' edges.

Finish the preparation by cutting away parts of the gripping material tape that extend from the side of the fingers.

Assembly

Remove the free-spinning wheel from the servo using the PH1 screwdriver.

Mount the servo on the side part using the M3x12 sunken head screws and the 2mm Allen key.

Do not tighten the screws too much at the moment; it is best to tighten them when all screws are in place to align the 2 parts of the base.

Use the M3x8 sunken head screws to mount the bottom part of the servo.

Then continue by mounting the second part of the base.

Once all screws have been inserted, tighten them to align the 2 parts of the base.

Now mount the gear using the 8 M3x6 button head screws and one M3x14 cylinder head screw using the 2 and 2.5mm Allen keys.

Mount the top part of the gripper with 8 M3x20 sunken head screws using the 2mm Allen key. Do not apply too much torque to prevent the part from breaking.

Next, insert the sliders; yours might not have metal inserts but tapped holes directly in the slider.

Now you can mount the gripper fingers. Use one M3x18 cylinder head screw for the slotted hole on the back of the finger and the 2 M3x20 cylinder head screws for the middle and front holes of the finger. You will need a 2.5mm Allen key for the screws.

If you don't use metal inserts, make sure to use the long screws as described. Don't be tempted to use shorter screws, as the slider might break apart more easily when high force is applied to the fingers.

Last but not least, mount the gripper on the robot using the adapter.

Troubleshooting

• If the slider seems loose, you can apply copper tape (or any other tape with a smooth surface) to the bottom of the sliding assembly.