Machine safety: Two-hand Control Device

A Two Hand Control device or ‘THC’ is a protective measure for safety systems that requires simultaneous actuation of both hands in order to initiate and to maintain any hazardous operation of a machine. It protects only the operator, and not the person standing near you. Since both hands are required for enabling movement, both hands of the operator are protected. An example of this is controlling a press machine by a THC device.

Selection and Design

According to AS4024.1501/1503 standards, a risk assessment will need to be performed to ascertain the category or performance level required to control the hazardous movement.

Based on these results and to create the output signals required, a safety relay or safety controller will be recommended to evaluate the THC signals. Additionally, the hardware and software (if applicable) used in the design of the safety system will also need to be validated according to the AS4024.1502 standard.

Other characteristics

To provide measures against ‘defeat’ for your THC device, accidental actuation prevention

means such as shields or covers can be used in the design of the THC device. In addition, the design of the actuator must take into account how much force is required to initiate signals to prevent from accidental actuation.  Actuation of both hands is required to initiate a movement. For example using only one hand or using one hand and another part of the body still allows the other hand to potentially become a hazardous movement. To facilitate this, a minimum distance of 260mm between the two actuators is required. Covers can be installed on the actuators to prevent the use of one hand and another part of the body.

Another way of defeating is to block one signal so that one signal is permanently sent to the safety system. When a signal is blocked the operator can use only one hand to create a movement and the other hand can still get injured. To prevent from this defeat possibility, the safety system has to be designed to accept two signals only when they are created within 0.5 seconds of each other. Both actuators are then released in order to initiate another movement the next time.