Closed Loop Stepping Servo System for Co-bot Grippers

Collaborative robots (cobots) are increasingly taking on delicate assembly and handling tasks in modern automation. A critical component of any cobot is its gripper – the “hand” that enables the robot to interact with objects. Today’s cobot grippers need to be compact, precise, and intelligent to safely work alongside humans and handle a variety of objects. As automation demands grow, engineers are seeking gripper solutions that offer more feedback and programmability than traditional pneumatic or open-loop electric grippers.

MinebeaMitsumi’s new closed-loop stepping servo system for cobot grippers exemplifies this next generation of smart end-effectors. It combines a hybrid stepping motor with an integrated feedback controller (the ST-Box) to deliver servo-like performance in a lightweight, user-friendly package. Below, we explore how this innovative gripper design enhances cobot applications by providing compactness, precision with feedback, programmability, and flexible mounting – all essential for modern robotic automation.

Compact, Lightweight Design with Hybrid Motor Technology

At the heart of the gripper is a hybrid stepping motor driving a simple yet effective mechanism. A patent-pending spiral cam drive shaft converts the motor’s rotation into linear motion for the gripper fingers. This clever mechanical design, supported by high-precision NMB ball bearings for smooth, low-friction operation, enables an ultra-compact form factor without sacrificing grip force. The entire unit is lightweight and streamlined – a major advantage for cobots, which often have limited payload capacity. The small size and weight also allow the gripper to be mounted in various orientations on a robot arm (e.g., attaching from the side or top of the gripper body), offering flexible mounting options to suit different robot models and workspaces. In short, the hybrid motor and spiral cam mechanism provide a reliable, efficient gripper that fits where others can’t, making integration into tight or weight-sensitive setups much easier.

Function of Cobot Gripper with ST-Box

• Simple Control – software can be programmed to precisely control gripping force, speed, and position without external sensors.
• Detection Function – users can detect the size of an object and also whether an object was actually picked up during conveyance.
• Limit-setting Function – because it is possible to define the fully open position and fully closed position while operating the gripper, users can reduce the cycle time by aligning the finger(s) more closely with the size of each object.
• Low Gripping Force – electric grippers can precisely control gripping force when dealing with fragile objects. It is possible to lower the gripping force in order to accommodate different scenarios.
• Program-defined Gripping Conditions – users can define up to 16 types of programs along with gripping conditions.

• Interface – IO and RS485 communication enables the gripper to communicate with the PLC and cobots.
• Parameter Settings – Easily manage and set the parameters of each gripper with MinebeaMitsumi application software.

Precision and No Missed Steps via Closed-Loop Feedback

Unlike conventional stepper-based grippers that run open-loop, this system uses the ST-Box controller to close the loop with feedback. A high-resolution sensor (encoder) in the gripper continuously monitors the finger positions. The ST-Box compares this feedback to the commanded position in real time, ensuring the gripper fingers move exactly as instructed with no missed steps. If a finger’s motion is obstructed or a heavy object causes a slight slip, the controller detects it instantly and corrects the position. This closed-loop stepping servo approach gives the gripper servo-like precision and reliability – every motion is verified and accurate.

For engineers, this means the gripper can apply a precise gripping force and speed profile confidently, without needing external sensors to confirm movement. The integrated feedback also enables some smart capabilities during operation: the gripper can detect when it has picked up an object (or if an expected object slips out) by sensing the change in position or force. It can even infer the size of the object it grasped based on finger travel, allowing the system or operator to verify that the correct part was gripped. This level of built-in intelligence and accuracy increases trust in automated handling, which is especially important in collaborative environments where consistent performance equates to safety.

Intelligent, Programmable Control for Adaptive Gripping

Beyond its mechanical prowess, the closed-loop gripper offers a suite of programmable functions that give engineers fine-grained control over how objects are gripped. Users can define and store up to 16 distinct grip programs or profiles, each with specific parameters like force limits, speed, and finger positions. This means a single gripper can seamlessly handle a variety of part shapes and materials by switching modes on the fly. For example, one program might be tuned for a delicate component that needs a gentle touch, while another is set for a larger, heavier item that requires a wider grasp and higher force. The ability to adjust gripping force precisely (even down to very low levels) is ideal for fragile objects – the gripper can be gentle enough to handle glass, electronics, or other delicate items without damage, then immediately ramp up to handle sturdier parts as needed.

An especially useful feature is the limit-setting function for the gripper’s travel. Engineers can program custom, fully open and fully closed positions for the fingers to match each object’s size. By not always driving the fingers to the mechanical extremes, the gripper only moves as much as necessary for a given object. This reduces cycle time and unnecessary motion, speeding up operations and minimizing wear. It’s a simple software-defined tweak that optimizes efficiency for each pick-and-place task.

Easy Integration and ST-Box Connectivity

Implementing this gripper into a robotic cell or automated station is straightforward. The ST-Box controller provides both discrete I/O signals and an RS-485 communication interface, making the system compatible with a wide range of industrial controllers, PLCs, and robot platforms. For a simple setup, the gripper can be controlled via standard I/O – open, close, or mode select signals – directly from a cobot’s tool port or a PLC output. For more advanced control and feedback, the RS-485 serial link allows two-way communication, enabling status monitoring and on-the-fly parameter adjustments from a host controller or computer.

MinebeaMitsumi also offers easy-to-use PC application software for the gripper, which simplifies configuration and parameter settings. Through this application, engineers can quickly tune each program’s parameters (force, speed, positions, etc.), calibrate the gripper, and update firmware if needed. Managing multiple grippers is convenient as well – each unit’s settings can be saved and replicated, ensuring consistent performance across several robots on a production line. This emphasis on plug-and-play integration means that upgrading to a closed-loop stepping servo gripper doesn’t require reinventing the control system; it’s designed to slot into existing automation setups with minimal fuss.

Originally published at Closed Loop Stepping Servo System for Co-bot Grippers

Conclusion

The Closed Loop Stepping Servo System for cobot grippers demonstrates how advanced motion control can elevate collaborative robotics. By merging a compact hybrid motor mechanism with intelligent closed-loop control, MinebeaMitsumi has created a gripper that is precise, adaptable, and user-friendly. For engineers and developers, this translates into higher productivity and peace of mind – the gripper handles a range of objects with accuracy, adjusts on the fly, and integrates easily into current workflows.

At Pacific International Bearing Sales (PIB), we are proud to partner with MinebeaMitsumi to bring this state-of-the-art gripper technology to our customers. Our team is ready to support your automation projects with technical expertise and personalized service. Contact PIB Sales ([email protected]) today to discuss how a closed-loop stepping servo gripper can enhance your robotic system. Whether you need detailed specifications, a demo unit, or guidance on integration, our experts are here to help you leverage the latest in precision cobot gripper solutions.

FAQ

Q: What is a “closed-loop” stepping servo system?
A:

• It’s a stepper motor system enhanced with feedback control, so it behaves like a servo drive.
• In a normal stepper (open-loop), the motor moves in fixed steps without verifying position. A closed-loop stepper uses a sensor (encoder) to monitor the actual position.
• A controller (like the ST-Box) constantly compares the commanded position to the actual position and corrects any errors in real time.
• This means the motor never “misses a step” – if the load causes a deviation, the system adjusts to maintain precise synchronization. You get the precision of a servo with the simplicity of a stepper.
  

Q: What does the ST-Box controller do for the gripper?
A:

• The ST-Box is an integrated control unit that turns the gripper’s stepper motor into a closed-loop system with feedback. It reads the encoder on the motor and ensures the gripper fingers reach the target position or force.
• It enables precise control of gripping force, speed, and position without needing any external sensors – all the intelligence is built in.
• The ST-Box also provides the gripper with object detection capabilities. It can sense if an object is present or if it has been successfully picked up, using changes in motor load/position.
• With the ST-Box, you can define custom open and close limits for the gripper. This helps align the finger movement to each object’s size, reducing wasted motion and speeding up cycle times.
• The controller offers easy connectivity (digital I/O and RS-485) so you can integrate the gripper with PLCs or robot controllers and program up to 16 different grip modes for various tasks.
  

Q: Can this gripper handle delicate or irregularly shaped objects?
A:

• Yes. The gripper is designed to deal with a wide range of object types, including very fragile items. Its closed-loop control allows you to set a low gripping force with fine resolution, so you can pick up delicate objects (like glass, electronics, or soft materials) without cracking or squashing them.
• The system’s feedback will detect when the object is gripped – it stops applying more force once it senses contact, preventing damage. You can trust it to grip just enough to hold the item securely.
• For irregularly shaped or variable-sized objects, the gripper’s adaptive features shine. You can program the finger travel limits to match the item’s geometry, ensuring a snug grasp every time. The gripper will conform to the object’s size (within its stroke range) and adjust on the fly via the feedback loop.
• Because you can store multiple gripping programs, one gripper can quickly switch between handling a small, fragile component and a larger, awkwardly-shaped part. This versatility makes it ideal for cobot applications where the types of objects can change frequently.

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