The mechanical claw has no strength and can’t hold it firmly? Try servo control

Maybe you encountered a similar situation at that time, in winter. My fingers were frozen and stiff. I wanted to pick up a cup, but I always felt unable to do so. It is obvious that I have caught it, but the feeling that it may slip off at any time makes me irritated for no reason. You want your hands to be more flexible and precise. This hope is actually the same as our experience when debugging a mechanical claw.

What you need is not brute force. What you need is for every angleCrawl accuracy。

Many people will encounter a common confusion when they first start building their own mechanical claws. The claws should be opened too wide and grab something empty, or they should be closed too tightly and crush the object. This is just like when you try to pick up a hard-boiled egg with your hands. If you use less force, it will slip through your fingers. If you use more force, the eggshell will crack. This lack of sense of propriety is the root of the problem.

Let’s look at an ordinary case. An enthusiast planned to use a mechanical claw to carry a paper cup. The motor he used rotated very fast, but it was difficult to stop at a precise position. The claw either squeezed the paper cup with a "click" and deformed, or just touched the edge of the paper cup slightly, causing the paper cup to not move at all. He repeatedly debugged for a long time, but was never able to find a perfect intermediate state. This is because ordinary motors cannot provide precise position feedback.

At this time, the steering gear shows its own value. There is a motor and a gear set inside the steering gear, and there is also a small component that plays a decisive role hidden in it - the potentiometer. This potentiometer is like a loyal sentinel. It consistently senses the position of the output shaft. When you give a command to the steering gear, asking you to turn to 90 degrees, the control circuit will compare the current position with the target position. If it has not reached the corresponding position, it will drive the motor to continue rotating. If it has already turned, it will adjust the direction of rotation and turn back. Until it stays at the 90-degree position accurately.

This closed-loop control process is the basis for achieving precise grasping. Your robotic claw is no longer a simple device that is either fully open or fully closed. It can stay at any angle you want. For example, if you want to open it 30 degrees to pick up a piece of paper, this is feasible. For another example, if you want to hold an apple at an angle of 60 degrees, this is also feasible. It is this continuous position control capability that gives the mechanical claw a preliminary form similar to "touch" for the first time.

In the spring, I once saw someone making an ingenious little device with the help of a steering gear. This person uses a mechanical claw to grab the seedlings, and then places the seedlings into the pit. The opening and closing angle of the claws is set just right, which will not cause damage to the fragile roots and can firmly grasp the stems. This case shows us that the right tools combined with the right controls can achieve extremely fine work.

However, the mere presence of position control is far from sufficient.If you want a mechanical claw to pick up a blueberry without cracking it in the process, then what you will be controlling is another dimension: force.。

This brings us to the second keyword:force control。

Precise positioning does not equate to appropriate power. The amount of torque that the servo can output is determined by its model and voltage. However, the power that the same servo can output at different angles is actually slightly different. More importantly, your control signal only tells it where to go, but not how much force it should use.

A common way to solve this problem is to use the operating current of the steering gear. When the mechanical claw clamps an object and starts to apply force, if the object is hard, the claws will not be able to close further. At this time, the motor of the steering gear is blocked and the current will rise rapidly. If your control system can detect this current change, it can immediately issue a stop or hold command.

For example, there is a mechanical claw that wants to grab a grape. If you rely solely on position control, you set a closed angle. However, the size of each grape is different. If the set angle is too small, the grapes will be pinched. If the angle is set too high, it will be impossible to catch it.At this time, if the current monitoring method is used, the process will look like this: the mechanical claw begins to close, and when the claw touches the grapes, the current begins to rise slightly.. The control system caught this change and immediately told the steering gear to say "enough, keep it here now". As a result, the mechanical claw used just the right amount of force to steadily pick up the grapes.

This technique has been repeatedly verified in many real-life application scenarios. An engineer engaged in sorting once said that he used the current feedback of the servo to successfully distinguish materials of different hardness. Soft sponges and hard plastic blocks can be grasped with the same set of mechanical claws and servo, without the need for additional force sensors, relying only on the keen interpretation of the current.

Maybe you think so, it sounds like it involves extremely complicated programming operations. However, this is not the case. Many relatively simple microcontroller development boards have the function of reading the servo current. You just need to set a threshold in the code that defines the current. Once the current exceeds this set value, it is deemed that the paw has touched the object. Then, based on the characteristics of the object, you decide whether to stop immediately or use a little more force.

Admittedly, this is just an entry-level application of "force control". A more advanced way is to use a strategy called "force-position hybrid control". However, understanding and grasping the relationship between force and position is the first critical step to move your mechanical claw from "clumsy" to "dexterous".

In summer, the sun is extremely strong and the air is very hot. People's patience has become limited. If you're debugging a scraping job, you're likely to get increasingly irritated with each failure. At this time, you will realize how valuable a steering gear with sharp response and precise control is. It reduces your frustration and allows you to focus on higher-level design.

We are going to talk about a deeper issue, how to make the mechanical claw understand whether the object it grasps has slipped, or whether it has firmly grasped it.？

This leads to the third keyword:closed loop feedback。

It's just that the servo control alone can only tell you the current position. It doesn't know whether the paw and the object are in a relatively stationary state, or whether they are slightly sliding. In order to obtain this level of information, we usually need to introduce other sensors.However, what is interesting is that experienced developers will use the characteristics of the servo itself to indirectly determine whether there is slippage.。

There is a clever way to observe the position changes of the steering gear in a very short period of time.When the mechanical claw is caused to grab an object and maintain it at a fixed angle, if the object starts to slide down, the claw will also be pulled slightly open.. This extremely small angle change can be detected by the potentiometer inside the steering gear. If your control system reads this change quickly enough, it can tell "Oh, something is falling."

The system can respond immediately. For example, it can send commands to turn the servo a few degrees again and tighten the claws a little more. This process can continue back and forth, creating a dynamic stability, just like your hand holding something. When you feel that it is about to slip, your hand will subconsciously tighten it slightly.

The method used to achieve closed-loop control by relying on the feedback of the steering gear itself is extremely low-cost, but the effect is unimaginably good. It does not require additional sensors or complicated installation structures. All you need is that your control software is sensitive enough and the sampling frequency is high enough.

I once saw in a teaching case that the teacher asked the students to use this method to grab a fully inflated balloon. The surface of the balloon was slippery and the shape was irregular. It could not be grasped by position control alone. However, after using this closed-loop feedback method, the mechanical claw could detect the slight shaking of the balloon at any time and adjust the opening and closing of the claw in real time. In the end, the balloon was picked up steadily. This experiment profoundly demonstrated the power of feedback.

So, you see, from precise position control, to intelligent force control, to smart closed-loop feedback. These three levels gradually make your mechanical claw more powerful and smarter.

Q/A: The servo vibrates badly when rotating. What should I do?

Check whether the power supply voltage is sufficient. A lack of voltage is a common cause of jitter. Try increasing the current of the power supply or shortening the length of the servo cable.

Q/A: The mechanical claw always grasps things in the wrong direction. How to solve this problem?

Calibrate the zero position of the servo. There may be an installation error in the mechanical structure. Set the software midpoint in the code and test the grabbing range again.

Q/A: Will the servo burn out if it grabs objects for a long time?

There are risks. When in the locked-rotor state, the current is large and the degree of heat is high. It is recommended to add force control or use a servo with overload protection to prevent continuous application of force.

Q/A: How to make two mechanical claws move synchronously?

The two servos are driven by the same control signal. However, before assembly, the initial angles of the two servos must be manually adjusted to the same state.。

Q/A: Can the signal cable for controlling the servo be connected to a very long length?

There is no opinion that more than one meter is appropriate. Long lines will introduce interference and cause signal distortion. It is preferable to use shielded wires or add signal repeaters.

Now, please once again recall in your mind the feeling of picking up a cup in winter, the feeling of holding it just right, the state of neither light nor heavy force, and the steady control ability. This is exactly the kind of ability that the servo can give you the mechanical claw. It is not a machine without temperature, it is a tool that can have a detailed and nuanced dialogue with you. You give it clear instructions, and it gives you precise movements.

Suggestions for action

Don't think about completing all the complicated crawling tasks at once. Start with the simplest goal, such as using a mechanical claw to grab a stationary building block of the same size. Achieve stable position control first. Next, try to grab objects of different sizes and introduce the idea of ​​force control. Finally, deal with sliding, irregularly shaped objects. Each step builds on the previous one, rising slowly like a spiral. Remember, true intelligence does not come from a single power, but from precise control, gentle power and sensitive feedback. Sowing in spring and harvesting in autumn, every bit of your patience in adjusting the details will eventually be transformed into the smooth and reliable opening and closing of the mechanical claw.