I purchased the 2515 board to control linear actuators to set the parking brakes on my vehicle. The actuators will pull on brake cables to apply the drum brakes (one actuator/cable per side). I would like to retract the LAs until the cable force reaches about 150#, using current sense to remove power from the LAs and hold them in that position.
My concern is that the load will increase quickly after the slack is pulled out of the cables and the shoes contact the brake drum and the controller may not catch the preset max allowable current. . I understand that I can limit the LA travel speed with PWM, and my plan is to slow the LA speed once it senses some tension on the cable.
So, I’m looking for advice on my approach for this application.
As an aside, others have used LAs for this service by simply adjusting the position of the LA so that the brakes clamp at the end of the stroke and the built-in limit switches remove power, but as brakes wear or cables stretch the distance between the LA arm and the end of the cables need to be adjusted to compensate. I’m hoping I can make those adjustments unnecessary.
Please keep in mind that our products are not specifically designed for use in automobiles or other applications where failure could result in injury or significant property damage, so I do not recommend proceeding unless you really know what you are doing.
What you are proposing might be possible with the current sense capabilities on our G2 High-Power Motor Drivers, but it is hard to know for sure in advance if your linear actuators’ current draw will be consistent enough or if the current sense measurements will be precise and accurate enough for that to work well. Using separate in-line current sensors with greater precision might increase your chance of success, but in either case I would anticipate needing to do a significant amount of testing and tuning to get your control scheme working well.
One thing I’m concerned about is whether the current sense can respond quickly enough once the LA clamps the brakes and tensions the cable - so that the LA is applying force very quickly and may go into stall before the arduino has time to react. I thought about using PWM to slow things down. Hoping someone has experience in this kind of application and can share it.
I want to make sure I have the correct board for my project. The linear actuators have a full load current of 3A at the rated physical load of 220 lbs. I would like to sense current and shut off power to the actuator when the load reaches 150 lbs. I understand that when the motor starts it will draw ~10 times full load current. Do I understand that the 50A default current limiting threshold for this board will allow the motor to start?
You mentioned that it is hard to know for sure in advance if your linear actuators’ current draw will be consistent enough or if the current sense measurements will be precise and accurate enough for that to work well.
What is the accuracy of the current sense measurements of the 2515 board? I intend to use a spring scale to document the current sense readings at my desired load. Assuming I get reasonably consistent current readings, is there any reason why this board will not work for my application?
Based on your description I think the 18v18 should be okay, but if you post a datasheet for your linear actuator or a link to where you got it I might be able to offer a more confident assessment.
We have not characterized the current sense measurement’s accuracy for these boards in detail, but we generally expect it to be tougher to measure low currents well. As described in our documentation on the product page and in the user’s guide the sensitivity for the 18v18 is approximately 20mV/A, so your 3A full-load current would only correspond to a 110mV output (20mV/A +50mV offset). However, if you decide to try it, using a spring scale like you suggested sounds like a good procedure to characterize the accuracy of the current sensing with your specific board.
As I mentioned before our products are not specifically designed for use in automotive environments which can be very harsh, so there are lots of ways your board could be damaged and not work beyond the specific detail we are discussing, so I do not recommend proceeding unless you really know what you are doing.
