Many times, in industrial automation we hear the term “safe speed” in motion conversations. It’s important to define safe speed and how it applies to control design, in relationship to functional safety. First, let’s define a safe speed.
Safety terms include safe reduced speed (SRS) and safe limited speed (SLS). SRS reduces a speed to a speed that minimizes risk of harm. This applies during situations when an operator might have to interact with moving parts. SLS is a speed that is set to a specific, predefined value. For instance, carts may be set to a specific speed for jogging in manual mode. In this instance, max speeds are reduced to match machine state.
What are the regulations for safe speeds?
ISO 12100: This standard addresses machine speeds and is a guide for setting up machine design requirements for reducing risks based on understanding machine speeds and functional safety.
ISO 13849: This standard provides guidelines for the design and implementation of safety-related parts of control systems. It includes requirements for safe speed functions to ensure machinery operates within safe limits.
IEC 62061: This standard focuses on the functional safety of electrical, electronic and programmable electronic control systems. It outlines the necessary measures to achieve safe speed and other safety functions.
OSHA regulations: The Occupational Safety and Health Administration (OSHA) provides guidelines for safe operation speeds. OSHA emphasizes that machinery should be operated at speeds that allow it to stop safely under all conditions.
How does this relate to machine controls? Controls engineers must use sensors to show states or state changes.
Let’s look at some examples. If a person must load trays in a robot area, as soon as the gate opens, or a person passes into the space where the robot is moving, then code can acknowledge that a human is in the zone. If a human is in the zone, and the robot is moving, then it will automatically be given a reduced speed for the purpose of acknowledging the interaction. Then, once it finishes its move, it can be set to pause or stop so that the next step is not completed until the human clears.
A robot may continue as the human backs away. ISO 10218-1 shows a speed of less than 250 mm/s is usable in robotics for tool center point speeds. There are numerous studies that show 130 to 150 mm/s is less likely to cause harm. Newton’s Law would tell us that higher speeds cause more harm, so apply some sense to physics. An apple falling on your head does not have the same harm factor as a bowling ball falling on your head.
Safety devices today allow robot zones, whereas the closer the human gets to the robot, the robot slows and eventually stops. Safe stops can also be categorized. Safe operating stops are different than safe torque offs, but there is still an expectation that the motor control does not deviate within a range. Safe stop is something to consider with safe speed regulation, as speeding and stopping are related, but in this case the focus is on speeds.
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Reduced speeds are not just applicable in robot scenarios. Reduced speeds could be used in staged startup sequences, as well. Why? What if an operator needs to back out of startup? At a reduced speed, it may be a quicker stop than at a max speed.
Limiting the machine speeds per states allows reduced risk. A startup condition can have a range of idle to normal speed. A normal automatic condition may have a range from low speed to high speed. Sensor inputs can frame this, or machine states initiated by conditions and agreed to by the operator.
In another instance, if an operator notices a jam, or something wrong with a conveyor, the operator might call maintenance and put the machine conveyor in manual or setup mode. In a manual mode, the speeds will automatically get reduced to a set jog speed. The cause here is that the operator said, “I want the line to be slower,” based on putting it in manual mode. It’s not any different than a car slowing down in the parking lot as opposed to using highway speeds in a parking lot.
If a machine has a cart that loads equipment and will have people walking in the area, the speed must be less than walking speed. This is a different regulation, which is ISO 12100. The related regulation is ISO 13849, which sets requirements for safety-related parameters.
The idea of ISO 12100 is that the control system defines changes in speed based on risk. Using parameters such as range, speed, acceleration, deceleration and load capacity in relationship to machine state can allow the speed references used to deter any uncontrolled speed change. Machine cleaning is another example of a mode change. Thus, manual mode, automatic mode, cleaning mode, initialization mode are all specific machine states that can have speed tables for each moving device. Putting a speed table in the functionality of the code for a control system can automatically reduce risks during machine operations.
