For a better understating of the robot start-up and performance in cold environments and to guide the user on how to proceed on these scenarios some Q&A are presented in this article as well as a program to perform a start-up warm up routine.

the robot under cold Enviroments

Universal Robots joints use strain wave gear technology and are sealed, self-contained, and self-lubricated for the life of the product, with no need to exchange or add grease. During normal operation, the lubrication is naturally recirculated and distributed around the gears and bearings to keep the mechanical systems properly lubricated. The viscosity of the grease and the friction in the gears are affected by temperature, which is why it is recommended to power on the robot and perform slow movements until the robot warms up before increasing speed.

High-speed operation may be limited when the robot is started with low internal joint temperatures. As a practical guideline, performance typically improves as the joints warm up, and 21°C internal joint temperature can be used as a recommended reference point when ramping toward top speed, but it should not be treated as a universal mandatory threshold for all robots or all applications.

Depending on the robot, the application, and the settings, some robots may be able to run at higher speed before reaching this temperature, while others may need more warm-up time. The recommended approach is therefore to increase speed gradually and avoid entering protective stop.

Q&A Regarding Cold Environments​

Some common questions and their respective answers are presented to clarify the content and recommendations contained in this article:

Q/A: What can be considered cold environments and what operating temperature?

The robot can operate in ambient temperatures between 0 and 50°C. However, if the robot is started with low internal joint temperatures, warm-up may be needed before higher speeds can be reached reliably. In this context, a cold environment is any condition where the internal joint temperature is low enough to affect robot performance during start-up.

Q/A: What is considered internal operating temperature?

A useful reference for internal joint temperature is 21°C, and this value can be used as a guideline when ramping up toward higher speeds. However, this should not be interpreted as a strict pass/fail requirement for robot operation or for full speed in every situation. Every robot application setup behaves differently, and the actual temperature needed for stable high-speed operation can vary depending on the robot, the application, the load, and the settings. Internal joint temperature can be seen in the log tab:

Robot Safety limits

Robot limits configured in the Safety settings can influence whether protective stops occur during cold start-up. Universal Robots provides factory preset levels ranging from Most Restricted to Least Restricted, and the user can also define custom limits for parameters such as force, power, speed, momentum, stopping time, and stopping distance.

If a configured safety limit is reached or exceeded, the robot can trigger a protective stop. Safety settings should be verified against the application risk assessment, since unnecessary restrictions can contribute to nuisance protective stops in some applications.

Increasing the safety force limit can make the robot more robust to protective stops, provided that the application risk assessment allows such a change. For that reason, if protective stops occur in low-temperature operation, it can be relevant to review whether the selected robot safety limits are more restrictive than required for the validated application. Any adjustment to safety settings must always remain consistent with the application risk assessment.

Warm-Up Routines for Cold Environments​

Showcasing the above, the following program has been developed to gently warm the robot to enable it to move at higher speeds in cold environments. The resulting values are the outcome of internal tests that have been performed in a climate chamber, where robots were cooled to joint temperatures of -15 °C, in -20 °C ambient.

The program has a strict limitation of the size of the movements, which means that it only moves the robot within a very small footprint.

Note: The warm-up program and routine followed is just one example showing the principle. Requires to be adapted to specific use. Also, depending on the work cell, it may be necessary to re-teach the robot's initial position.

Program Description

• The temperature of the elbow joint is continuously monitored, as this joint generally seems to be cooler than the others.
• To start the routine, the robot is moved to the starting position at joint speed = 10°/s.
• The program moves the robot between two waypoints.
• Depending on the temperature, one of 4 different speed/acceleration parameters is chosen for this movement.
• If the joint temperature is above 20 °C, the program will remain idle until the joint temperature drops below 20 °C.

The table below shows the target speed and acceleration depending on the temperature of the elbow.