I just purchased a gripper kit and a LSS HT1 motor to go with it.
My question is this:
How do you implement a control so that the gripper stops and holds once an item is grasped without being crushed by the high-torque motor? With other grippers, I am used to using a force sensing resistor (like flexiforce) … would that be unnecessary by using the internal functionality of the LSS HT1 servo or one still has to use a flexiforce with it? If flexiforce is used, what would the installation configuration be? Any suggestions?
Thanks!
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@cosmicone To stop the LSS gripper from crushing the object being held, it needs to be in serial mode. You would use the CH modifier described here. For example, if to grasp an object such that it is touching the metal of the gripper (foam fully compressed), the gripper would need to be set to angle 45 degrees. For argument’s sake, in this fully compressed position, the servo draws 500mA.
Assuming the ID of the servo is 5, and the angle is 45.0 (450), the normal position command would be
#5D450<cr>
If you want to to stop before this, you could have it hold the position should the current reach 300mA. The command would be
#5D450CH300<cr>
The servo will have pushed against the foam, but not to the point where it contacts the metal. You would need to experiment a bit in order to determine the right current at which the actuator will stop and hold.
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So I won’t need a force sensor at all? That particular gripper always moves 45° max?
So I won’t need a force sensor at all?
Correct, if you do some experimentation (since the current is an indirect measurement of force / torque) you should not need a force sensor.
You’ll need to experiment with the angle and the CH value.
That particular gripper always moves 45° max?
Negative. That was just an example.
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Thank you.
I am impressed by Robotshop’s informative, timely and cogent advice and explanations. It is a pleasure to deal with such a responsive company.
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I tried it the way you suggested, Even at 250mA it holds the object tightly – beautiful.
What I noticed was that the motor gets hot when it is held at that high current. Is that detrimental to the motor or is it safeguarded up to a certain ampearage for a defined period of time?
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The lower the current the better - always.
There are two built-in safety methods regarding current: “soft limit” and “hard limit”:
“Soft limit”: current increases up slowly over time, so the firmware takes a moving average, and once a threshold is crossed, it goes into safety mode.
“Hard limit”: specific value for current (fairly instantaneous) which, if surpassed, the servo goes into safe mode.
This having been said, the safety cutoffs are not foolproof and are there as a last resort to try to save the servo.
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What are those soft and hard limits? (or ranges)
Also, what are the limits for the temperature of the motor?
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They are per servo, with the HT1 being different since the motor is different.
https://wiki.lynxmotion.com/info/wiki/lynxmotion/view/lynxmotion-smart-servo/lss-specifications/
See the sections on temperature & current. Re: hard / soft current limits, they are normally above the peak current at 12V.
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