The following article by Kayla Walsh appeared in Rail Express on July 29, 2025
PRM’s Sentinel Height and Slew Limiters can be used on both new and existing vehicles.
When it comes to working near power lines, around bridges and inside tunnels, safety is paramount.
That’s where PRM Engineering Services’ Sentinel Height and Slew Limiting Systems come in.
The systems can be used on both new and existing vehicles, from excavators to loaders to skid steers and backhoes, protecting against operator error and maximising safe use of equipment in all conditions.
An updated version of the product has recently been released, with new and improved safety features. We spoke to the design engineer at PRM Engineering Services to find out more.
How it works
Sentinel Height and Slew Limiters can be used on any machine that has an articulating boom. They work using angle sensors that attach to the boom, which measure the height reached by the machine. The system integrates with on-screen and audible alerts to warn operators when they are approaching unsafe operating limits, and there are several different types of stops that can then be activated.
A new slow-stop option decelerates fast-moving machine booms before the height limit is reached, to prevent a sudden jolt.
“If the boom is moving at speed and it stops suddenly, the machine will rock back and forth,” PRM’s engineer explained.
“With a slow stop, you’re minimising bucket spillage and reducing that unpredictability of movement for the operator. If the boom is moving slowly or is far from the limit, the full machine strength is maintained.”
Predictive stops also prevent overshooting at any speed by taking into account the speed of the machine.
“If you try to stop a machine when the boom reaches the limit, and it is going fast, it will overshoot. PRM’s Sentinel systems will begin to stop the machine earlier, so by the time it stops fully it is right at the limit, where we want it to be,” he said.
Alarms warn operators when they are approaching unsafe operating limits, and there are several different types of stops that can then be activated.
Site supervisors have the ability to set height limits, via a key switch and password.
“As an example, Queensland Rail and Aurizon both use the Sentinel system, and they have specific height limits that they work to – often 3 to 3.4 metres, depending on the site.
“Our system allows their supervisors to input those limits without the operators being able to change them themselves, which is an added layer of safety protection.”
Each arm on a machine can be limited individually, to maximise the available operating room.
Different machine attachments can also be set and selected from the user interface, which can automatically adjust the height settings for the current attachment, saving a lot of time for operators with a wide range of attachments.
PRM’s Sentinel Height and Slew Limits also come with a safety lock level input, to reduce current draw when the machine is not in use and improve battery life.
Meanwhile a black box functionality records all events when the system is running.
“Internal logging records every time the system starts, every time it hits the limit, and every time the operator changes something.”
Made in Australia, for Australia
PRM’s Sentinel Height and Slew Limiters are designed and made in Australia, with local support, national installers and local engineering.
The product has already been tried and tested by operators across the country, with excellent results and feedback.
PRM’s engineer said the system is robust, can withstand harsh weather conditions and is suitable for use on construction sites and around railways.
“It has industry standard screens and protection for the sensors as well. In some cases, the sensors have five millimetres thick steel plates protecting them.”
Sentinel’s SIL 2/ PLd (safety integrity level 2/ performance level 2) system option is suitable for high-risk use cases such as working under rail power lines.
It is currently the only system designed in Australia that meets the requirements set out in SIL 2, with improved redundancies in the latest version of the system.
“When you’re working with rail power lines, there is inherently a greater risk, with high voltage which can jump, leading to injuries and even loss of life, so you need to achieve SIL 2,” the engineer continued.
“That means the system needs to have redundancies and components that we know won’t fail. Even if they do fail, the system detects failure and has failsafe.
“We use two angle sensors in redundancy to measure the angle of the boom. If one of them is giving us the wrong angle, we can stop the machine, get it out of harm’s way and repair it before we end up in a dangerous situation.
“The system eliminates the risk of human error, keeping rail industry and construction workers safe with consistent, real-time protection.”
