Thin is the New Strong: Flat Profile Brakes for Humanoid, Collaborative, and Exoskeleton Robots

Next-generation robots are designed with strict space constraints. In applications ranging from humanoid robots and collaborative arms to exoskeletons, maintaining joint position safely, quietly, and efficiently depends on the brake integrated within each joint.

Conventional cylindrical brakes do not meet the geometric requirements of these applications. Designers require brakes that are thin, deliver high torque for their size, consume minimal holding current, and provide sufficient bore space for power and signal cables. This flat, often called a pancake brake profile is now a critical specification in modern robotics.

Why Robot Joints Place Unusual Demands on Brakes

While industrial and humanoid robot joints share some requirements, humanoid robots impose stricter constraints. Each component, including actuators and brakes, must fit within human-scale dimensions. For example, a hip joint module must integrate a motor, gearbox, encoder, drive electronics, and holding brake within the width of a human hip. Similar constraints apply to joints throughout the kinematic chain.

As a result, brakes for these joints must be thin, with a flat profile relative to their diameter. The axial dimension is more limited than the radial, so a brake measuring 100 mm in diameter and only 12 to 15 mm thick can fit where a standard 50 mm cylinder cannot.

Beyond geometry, three other requirements consistently surface in robotics brake specifications:

• Low power consumption during holding. Humanoid robots and exoskeletons are primarily battery-powered. A brake that draws 40 to 50 watts across 12 joints is a meaningful load on a system running from a 24V or 48V battery pack. As such brake power consumption must be considered.
• Large internal bore diameter for through-bore cable routing. Modern robot joint modules route power cables, encoder cables, and communication buses through the center of the joint axis rather than along the exterior of the limb. This eliminates cable flexing problems at the joint, prevents cable interference with the range of motion, and produces a cleaner form factor. A brake with a large through-bore, relative to its outer diameter, enables this routing strategy without compromise.
• No rare earth materials. Supply chain resilience is a priority for robot manufacturers scaling to volume production. Brakes that rely on rare earth permanent magnets introduce sourcing risk and cost variability. Technologies that achieve high torque density without rare earths offer a more predictable bill of materials.

KEB Flat Profile Brakes for Robotics

KEB manufactures two types of power-off brakes for robotic applications: spring-applied and permanent magnet brakes. KEB America produces over 90 percent of brake components in-house, including wound copper coils, machined steel and aluminum housings, and friction systems.

Watch: Brake & Clutch Manufacturing at KEB America, Inc.

This manufacturing depth makes it possible for KEB engineers to produce custom flat profile brakes that are matched to the specific torque, bore, thickness, and voltage requirements of a given joint, rather than asking the customer to adapt their design to a catalog part.

COMBISTOP S1 Flat

KEB’s current standard flat profile offering is the COMBISTOP S1 Flat, a spring-applied holding brake developed specifically for AGV and robotics applications. It uses a patented friction system that generates braking torque from the friction interface between the friction disc, armature, and flange, converting kinetic energy to heat and stopping motion reliably. This design achieves torque density comparable to permanent magnet brakes without any rare earth materials, which simplifies sourcing for volume production.

COMBISTOP S1 Flat Standard Product Data:

 
The ambient temperature range of -40 to +80°C is consistent with outdoor and industrial deployment scenarios that humanoid robots and exoskeletons increasingly must handle. Integrated cable routing is designed into the S1 Flat, supporting the through-bore cable management strategy that collaborative and humanoid robot designers require. The plug-and-play installation and pre-set air gap reduce commissioning time during robot assembly.

Custom Flat Profile Brakes

The standard S1 Flat covers a torque range of 3.4 to 10.8 Nm in two sizes. However, many robotics joints require torque values outside this range, different bore diameters, modified voltage ratings, or specific axial thickness targets that do not match a catalog part. Because KEB winds its own coils and machines its own housings, these parameters are tunable.

KEB engineers have produced customized flat profile brakes for applications including:

• Spring-applied flat brakes with modified coil winding to achieve custom release voltages (12 V, 36 V, 48 V) suited to specific robot power bus architectures
• Custom torque settings achieved through friction disc and spring preload adjustment within the same flat housing envelope
• Connector and cable assembly options that allow direct harness routing out of the joint without additional junction hardware
• Material certifications (DIN EN 10204 2.1 and 3.1) for programs requiring documented component traceability

The ability to modify any of these parameters without changing the fundamental flat profile geometry means that a robot designer can specify the brake by the joint’s geometric and torque constraints and work with KEB to produce a part that meets both without compromise.

Power Consumption and Battery Life

Spring-applied brakes engage by spring force and release electromagnetically. During normal operation, when the joint is powered, the brake coil draws current to release. When power is removed, whether at end of task, during an emergency stop, or during a power failure, the brake engages automatically without any additional power. This behavior is called fail-safe, and it is the correct operating mode for any joint that must hold a load when power is interrupted.

For battery-powered robots, the implication is significant. If the robot is standing or holding a posture, and the joint actuators are managing that posture with motor torque, the brake coils are drawing their release current.

However, reducing voltage to maintain release is a technique that can be used to conserve power. After a spring-set brake has been released, the voltage can be lowered to a predefined and tested level. The lower voltage still allows the brake to remain disengaged, and the lower voltage results in less energy consumption. KEB offers rectifiers that provide this functionality, and many adjustable power supplies can be programmed to do this as well.

KEB engineers can work with robot designers to match coil power ratings to the operating duty cycle, selecting the minimum coil wattage that meets the torque and release time requirements without over-specifying power consumption.

Working with KEB Application Engineers

Selecting a flat profile brake for a robot joint is not a catalog exercise. The torque, bore diameter, axial thickness, voltage, and mounting geometry are all interdependent, and the right solution for a specific joint module is rarely identical to the right solution for an adjacent joint. KEB application engineers work directly with robot design teams to define the brake specification from the joint geometry outward.

This process typically starts with the joint envelope, the torque load analysis, the power bus voltage, and the cable routing strategy. From there, KEB engineers can recommend a standard product, propose modifications to a standard product, or scope a custom design if the application requires a geometry or performance point outside the current product range.

Because KEB manufactures both spring-applied and permanent magnet brake technologies, the conversation is not limited to a single operating principle. Some joints are better served by spring-applied brakes, particularly where fail-safe behavior on power loss is the primary requirement. Others benefit from permanent magnet designs, where response time and size constraints favor a different approach. KEB engineers can evaluate both and recommend based on the actual application requirements, not on which product family is easier to supply.

Talk to a KEB Application Engineer

Bring your joint geometry and torque requirements. KEB engineers can specify a flat profile brake, whether standard or custom, that fits your design. Contact KEB America, Inc. today.