As a complex integrated system, industrial robots have undergone a transformation from difficult to flexible applications due to constraints imposed by other related industries. This paper discusses the problems and solutions for robots operating in low-temperature environments, addressing these application scenarios.
Every winter, engineers working on-site face the challenges of severe weather. During this season, many have encountered the problem of robots failing to start due to low temperatures. Besides conventional heating methods to keep robots "alive," are there any solutions that align with the characteristics of our high-end intelligent equipment?
Existing problems
The ambient temperature ranges from -10 to 40℃. Currently, the industrial robots in the welding workshop are in the production phase. In the early winter, when the ambient temperature is low (below 10℃ without the workshop air conditioning on), some industrial robots experience problems such as collisions with external equipment, deviation from the robot's running trajectory, excessive joint load, or overcurrent on a certain axis during welding, handling, and rolling processes. After the robot manually returns to its original position, production resumes after reducing the robot's automatic running speed, and the robot malfunctions disappear.
Next, we will use theory and practical experience to tell you how to deal with the above problems!
//1. ABB Robots
During the early winter season in the welding workshop, especially during the morning production line startup, ABB robots frequently triggered "joint overload" and "motion monitoring" alarms. The actual torque of robot joint ROB_X (as shown in Figure 1, where X represents the robot joint axis and ROB1_2 is the second joint axis of robot 1) was excessively high, potentially due to incorrect load data, excessive acceleration, excessive external process force, low temperature, or hardware malfunction. After on-site confirmation by technicians, it was determined that the robot had not collided with any equipment or workpieces, ruling out jamming or collisions as causes for excessive torque or load. The robot motion monitoring was triggered, and the movement of the robot's mechanical unit was immediately interrupted.
//2. KUKA Robot
Similarly, at the beginning of winter in the workshop, the air conditioning was not turned on and the ambient temperature in the workshop was below 10°C. Especially during the morning when the production line was running, the KUKA robot frequently reported "exceeding the maximum hysteresis error (X)" (as shown in Figure 2, where X is the robot or auxiliary mechanical joint axis, KSP is KUKA SERVO PACK, and 1 is the servo control of the first axis).
KUKA robot alarm interface //3. FANUC robot
Japanese robots often use grease for lubrication. However, when the temperature is low or the robot is idle for an extended period, the grease can solidify, increasing resistance. This increased resistance triggers collision detection.
FANUC robot alarm
Problem Analysis and Solutions:
//1. Uneven robot movement trajectory
The robot's trajectory on site is mainly generated through manual teaching. However, the trajectories vary during the teaching process, and the resulting trajectories may not be smooth or even. If the robot encounters a position where the motion of a certain axis is large, and the speed and acceleration are also large, an instantaneous overcurrent will occur on that axis.
This is caused by improper settings of the position and motion parameters of a certain trajectory point in the program. You can observe the position where the alarm is generated each time to see if it is in the same position. If so, optimize the position of the point and appropriately reduce the speed and acceleration values of the point.
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If the problem persists on-site, meaning it occurs frequently, then the hardware needs to be checked, including related components such as motors, cables, and servo drives. The replacement method can be used to replace the corresponding modules. It's important to note that only the relevant parts of the alarm axis need to be replaced.
This article cites ABB robots and KUKA robots, and specifically points out that the KPP/KSP of KUKA robots cannot be replaced at the same time, otherwise an error will occur, and they must be replaced one by one.
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Working in low-temperature environments causes the grease inside the robot to become more viscous. When the robot starts running, this high viscosity creates greater resistance, leading to increased friction within the gearbox and requiring higher motor torque to overcome this resistance. Consequently, the motor current for one (or more) axes will reach its maximum. Simultaneously, low temperatures also affect the robot's control system circuit board. Since circuit boards are primarily made of semiconductors, semiconductors are unstable at low temperatures, potentially causing problems such as robot control system crashes or prolonged robot startup times.
ABB industrial robots have the following temperature characteristics: The robot body's storage temperature range is -25°C to 55°C, and the maximum temperature it can withstand when stored for less than 24 hours is 70°C. The maximum humidity the robot body can withstand at room temperature is 95%. The robot body's operating temperature range is 5°C to 50°C, and the maximum humidity the IRC5 robot can withstand at room temperature is 95%.
The IRC5 robot has a storage temperature range of -25 to 55°C, and can withstand a maximum temperature of 70°C under storage conditions. The robot body can withstand a maximum humidity of 95% at room temperature. The IRC5 operates at a temperature range of 5 to 45°C, and can withstand a maximum humidity of 95% at room temperature.
KUKA industrial robots have the following temperature characteristics: body storage temperature -40～60℃, robot body operating temperature 10～55℃. The KRC4 robot has a storage temperature of 5～45℃.
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Our proposed solution for robot gear grease is to add antifreeze to the grease in low-temperature environments. Robot grease consists of additives and grease (the grease is generally synthetic oil), but its composition is usually monopolized by manufacturers and kept secret. ABB robot greases are from two companies: Kyodo Yushi and Mobil Yushi, while KUKA robot greases are from a certain type of Castrol oil.
Because the composition of the grease is confidential, it is not recommended to add any additives without consulting the manufacturer or professionals, as adding any ingredients will affect the overall properties of the gear grease and may damage the gears of the robot shaft and the seals of the gearbox.
For more questions about greases, please consult the more professional Zhinanche! Zhinanche's special lubricants are more resistant to high and low temperatures.
In addition to the methods mentioned above, ABB robots, KUKA robots, and other brand robots can run at low speed for a period of time at the start of production, gradually increasing the speed to return to full external speed or the customer-required cycle time. The motor current of KUKA robots can be monitored through a preheating function; once a set value is reached, the control system automatically reduces the movement speed, thereby reducing the motor current.
FANUC robots have a speed selection option.
Menu - Settings - DI Speed Selection - Enable, configure IO, and set the required speed.
The application of industrial robots is to complement production processes and improve automation levels. We need to pay sufficient attention to the management and troubleshooting of robots in the early stages of equipment installation and commissioning. This article aims to provide some reference for addressing the issue of shaft overcurrent in industrial robots.
