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 which are sealed, self-contained, and self-lubricated for the life of the product (no need to exchange or add grease). During normal operation, the lubrication gets naturally recirculated and distributed around the gears and bearings to keep the mechanical systems properly lubricated.

Temperature has an impact on the grease viscosity and gears friction values, Universal Robots recommend to power on the robot and do slow movements until it reaches internal operating temperature before reaching higher speeds. 

Robot cannot operate with high speed until the joints reach at least a recommended operating temperature above 21° C. 

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 temperatures between 0 and 50 °C. However, Universal Robots considers the internal operating temperature to be 21°C. Every temperature that brings the internal operating temperature below 21°C can be considered a cold environment and will need to follow the warm-up steps presented below.   

Q/A: What is considered internal operating temperature?

Universal Robots considers the internal operating temperature to be 21°C but since this can be hard to obtain during this process and that every robot behaves differently, it is recommended to create a program that gradually increases the speed without going into protective stop.

We recommend at least 21°C internal joint temperature should be reached before top speed can be achieved.

This might not work for all robots, and some might need to reach higher internal temperature. Internal joint temperature can be seen in the log tab:

General guidelines for starting up robots that are operating cold environments:

1. Bring robot to room temperature before powering on (if stored in a colder environment). 
2. Brake release- If brake release isn’t ready, set the robot to Back Drive and then manually exercise joints slowly.
3. Exercise all joints slowly in freedrive, one-by-one, without any payload and tool installed.
4. Create a program that exercises the robot joints slowly and continuously for 20 minutes at least.  
5. Limit the output speed to approx. 10 deg/s with standard acceleration. Lower speeds are more effective. If possible, slowly raise the speed without entering protective stop until maximum speed can be achieved.  
6. Continue operating the robot arm with lower speed until internal operating temperature has been reached.  

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.