How Safety Relay Guaranteed Security of Two-Hand Controls
Button size and shape: The size of the buttons should be moderate in order to adapt to the different hand shapes of the operators. The shape is usually round or oval, which can be easily pressed by the fingers. At the same time, the buttons should have obvious tactile feedback, such as some travel and force feedback, to enhance the real sense of operation.
Button layout:
Spacing: The distance between two buttons should be large enough to prevent a hand from pressing two buttons at the same time. Usually, this distance should be at least 250mm to 260mm, and the maximum inner distance should not exceed 600mm to ensure that both hands must be operated at the same time to start the machine. This is because the worker may be under time pressure and may try to hold the piece. Because workers may be pressed for time, they may try to hold the part in place with one hand and then push the two-hand controls with their arms - elbow on one button and index finger on the other.
Height and Angle: Buttons should be at a height and angle that allows the operator to reach them easily while standing or sitting, without over-extending or twisting the wrist and arm.
Visibility: Add visual and tactile cues to the HMI, such as icons, colors, shapes, and tactile feedback on buttons. These cues help the operator clearly understand the method and process of two-handed operation and improve the safety and efficiency of the operation.
Space: The work area should be spacious and unobstructed to ensure that the operator has enough room for two-handed operation while avoiding collisions with other machine parts or tools.
Lighting: The work area should be adequately lit to minimize visual fatigue and the risk of misuse.
The safety relay monitors whether both control buttons are pressed at the same time. Only if both buttons are pressed simultaneously within a predetermined time (e.g., within 0.5 seconds) does the relay trip and send a start signal to the machine. This ensures synchronized operation of the operator's hands and eliminates potential hazards caused by single-handed or unsynchronized operation.
Protective design: A guard or baffle is placed around the pushbutton to prevent workpieces or other objects from accidentally touching the pushbutton. At the same time, it ensures that the guard does not interfere with the normal operation of the operator.
Fault warning and protection: When designing a two-hand control, consider incorporating fault indication and protection functions. The safety relay can monitor abnormalities in the control circuit in real time, such as short circuits, broken circuits, or damaged buttons. When a fault is detected, the relay immediately shuts down the power supply to prevent the fault from spreading or causing an accident. For example, if the system detects abnormal operation or malfunction, it can automatically cut power or activate the emergency stop device.
Standards compliance: Ensure that the design of the two-hand control complies with national and industry safety standards, such as ISO 13850. These standards specify the design principles, test methods, and safety performance requirements for two-handed controls.
Power supply: 24 V DC
Relay safety output: 3NO + 1NC (PNP or NPN)
Maximum switching capacity: 12 A (Distributed on all safety output contacts)
Transistor signal output: <500 mA 24 V DC
Contact resistance: <100 mΩ
Size: 117 × 100 × 22.5 mm
It is not acceptable to install two pushbuttons in series to create the appearance of a two-handed safety situation. This means that by connecting the output of one pushbutton to the input of the second pushbutton, both pushbuttons must be pressed simultaneously to obtain an output from the second pushbutton. The following is a simple test to verify the presence and use of a proper two-hand safety relay.
1. Press and hold down one of the two-hand controls/buttons;
2. Wait 5 seconds;
3. Press the second two-hand switch or button.
If the machine starts, it is not properly designed and may not have a two-hand safety relay because the other side button must be pressed within 0.5 seconds. Two-hand controls without special safety relays allow the machine to start in this way because there is no synchronized drive forcing. Without this forcing, one of the buttons could be taped or pushed in with a heavy object, destroying the two-hand safety feature and allowing the machine to start with only one button. Using a two-hand safety relay forces the user to press both buttons simultaneously within half a second, preventing the user from bypassing one of the buttons.
The correct installation of two-hand control devices is one of the most important steps in improving the safety of industrial machinery. Through preliminary preparation, fine installation, professional training and continuous supervision, it can ensure that the two-hand control device can play its proper safety protection role in industrial production. Enterprises should attach great importance to the installation and management of two-hand control devices to create a safer and more efficient working environment for employees.
