Industrial Safety Device for Hexapod 6-DOF Motion Systems / Parallel Robots

Effective workplace safety measures are especially important in manufacturing environments when machines carry out fast movements with large forces. Physical barriers, such as fences that spatially separate people from machinery, are common and easy-to-integrate solutions. However, if mechanical systems cannot be installed or if the work process is influenced by them, contact-free safety concepts such as a light grid or a light curtain can be used. A light curtain forms a close-meshed protective field and, therefore, secures the access to the danger zone.

Hexapod robots are multi-axis positioning systems with a limited workspace that can often be safely integrated into industrial setups. However, if people can move into the workspace of the hexapod or of the setup on the hexapod platform then a safety measure must be set in place.

This blog explains:

• when it is useful and necessary to use a safety device;
• how a safety light barrier (or light curtain) works;
• how the risk assessment for hexapods is carried out;
• how the safety device can quickly be integrated in the controller environment.

1.1 Parallel-Kinematic Hexapods Compared to Serial-Kinematic Systems

Serial-kinematic systems consist of individual axes or actuators that build on one another, which means that they are mechanically connected in series. For example, a platform-bearing Z axis can be mounted on a Y axis, which in turn can be mounted on an X axis (Fig. 1) and so on.

In hexapods, all six actuators act directly on a single platform. This allows a considerably more compact setup than is the case with serially stacked multi-axis systems.

Since only a single platform is moved, which is also often equipped with a large aperture, the mass to be moved is considerably smaller. This results in considerably improved dynamics.

In addition, hexapods exhibit higher accuracy since they usually do not contain guides with corresponding guide errors and the errors from the individual drives do not compound. Since the cables are not moved with the platform, the precision is not influenced by corresponding friction or torques. In addition, the parallel structure increases the stiffness and thus the natural frequencies of the overall system as well.

In stacked systems, however, the lower drives not only have to move the mass of the payload, but also the mass of the drives that follow, along with their cables. This reduces the dynamic properties. Guiding errors that accumulate on the individual axes also impair accuracy and repeatability.

Manufacturing and quality assurance processes in electronics production, mechanical engineering, and the automotive industry increasingly require compact multi-axis positioning systems. At the same time, the requirements for accuracy are also increasing.

Depending on the model, parallel-kinematic hexapod robots can move and position tools, workpieces, and complex components with masses ranging from a few grams, several hundred kilograms, to even several tons in any spatial orientation with high precision.

The PI hexapod controller with integrated EtherCAT® interface makes it easy for users to integrate hexapods into automation systems without having to do challenging kinematic transformations.

An essential feature of hexapods is the freely definable reference coordinate systems for the tool and workpiece position (Work, Tool). This allows the movement of the hexapod platform to be specifically adapted to the respective application and integrated into the overall process.

1.3 Safety-Relevant Applications

Generally, the risk of injury from hexapods, designed for precision positioning, is low because of the relatively low speed and the limited range of movement.

The situation is different for dynamic motion hexapods (see Fig. 3) because of their high speed and acceleration which can become a hazard for people who work in their immediate vicinity and can’t react quickly enough to avoid an impact. When a collision occurs high impulse forces due to mass inertia can cause harm. A safety system can protect people and minimize this risk of injury.

For these situations, PI provides hexapod controllers that feature a motion stop input (e.g. model C-887.522). The input is used for connecting external hardware (e.g. push buttons or switches) and it deactivates or activates the power supply of the hexapod drives. However, the motion stop socket does not offer any direct safety function in accordance with applicable standards (e.g. IEC 60204-1, IEC 61508, or IEC 62061).[2]

Hereafter, the integration of a light barrier (or safety curtain) in the control of the hexapod is described. The light barrier itself is compliant and approved in accordance with EN/IEC 61508 and EN/IEC 61496-1/-2.

Safety light curtains are made of an emitting (transmitting) and a receiving unit. In the emitting unit, light sources that radiate toward the receiving unit are arranged at an even distance from each other (e.g. LEDs). The receiving unit is equipped with detectors in the same arrangement (e.g. photodiodes), each being responsible for the detection of its opposite light source. The resolution, i.e. the distance between the light sources, determines the level of protection provided (finger, hand, or body protection).

If one of the light beams is interrupted, the light barrier outputs the information via two so called OSSD (Output Switching Signal Device) outputs, which nowadays are standard for barrier-free safety devices. In the standard state, the outputs are at 24V and fall to 0V briefly and asynchronously from each other for the purpose of self- monitoring. To be able to interpret the output signals, a suitable and certified safety relay is required that switches off or stops the machine movement in the event of a failure.

Different things need to be considered when selecting and dimensioning the safety device.

1.       The relevant EN ISO 13849-1 standard specifies the safety levels for the components used.

2.       With the EN ISO 13855 standard, the minimum distance between the safety device and the danger zone is calculated.

3.       Depending on the degree of protection, the minimum distances need to be increased accordingly.

The safety level is defined as described in the EN ISO 13849-1 standard. With this standard, a tree diagram can be used to determine the safety level that a system must meet. In Fig. 4 the risk diagram according to the EN ISO 13849-1 standard is depicted. To reach a defined safety level, all safety components must meet the requirements of that level.[1]

In this chapter, an example is shown of how the safety function can be implemented when written in the Python programming language for the PI Python library. It is always possible to choose a different software solution or to use a PLC to connect the hexapod, for example, via EtherCAT. The PI Python library facilitates the use of the General Command Set (GCS) from PI. GCS offers a standardized set of commands that is independent of the connected controller or the drive principle used.

When using Python, each controller error throws an exception in the running program as soon as the next GCS command is executed. A controller error occurs, for example, when the motor voltage is disconnected by the safety circuit. This is why, in Python, try-except blocks can be used to ensure a controlled continuation of work after the activation of the safety function.

In the following example, a Python program is shown which moves the hexapod in an infinite loop in the Z axis. After the safety relay is triggered, the program tries to switch the hexapod's servo back on. The safety relay can be reset to its normal operation by pushing the reset switch. The hexapod controller keeps retrying to switch on servo. This only succeeds when the safety relay returns to normal operation while no new controller error has occurred.