How to detect usage of height adjustable table?

Hi Mads,

Have you considered using a capacitive sensor mounted to the desk?

The TI sensor system can accurately detect objects up to about 50 cms.

Another way would be to use a light and a camera to measure spot size, but now you are adding some complexity.

Maybe an ultrasonic sensor?

DAB

On the note of what you're trying to do, you've giving detail of your method, but it still leaves your end goal rather vague.... especially with "acceleration of a table".

It leaves the questions: "Is this to be done on multiple/different floorings under the table", "is the table moving", and "how frequently does the table change heights".

If you want to stick with the accelerators: I haven't used them before to know their accuracy, but I remember an Arduino SHARP library that averaged 5 IR scans into one value.... you could use 2 accelerators, one on each end of the table, and run each end through that same thing to alleviate some of the fluctuation.

IR has the highest fluctuation of any simple sensor I've used. It varies based on ambient light, and even EM waves. Sonar varies by texture of what it's to hit, which is why a question is the flooring. I also have an idea for determining if you're on carpet or hard flooring if you wanted it.

Your limit switch idea is a good one. Optical limit switches can easily be shielded from stray light or you can use regular electro-mechanical limit switches.

Like a garage door opener function.

I'm not sure I do fully understand. Do you suggest to use a capacitive sensor to measure the electrical field of the motors? Or, how do you figure it's possible to detect movement with a capicitive sensor?

And a camera might be a bit too complex and we do experience users linking cameras to surveillance, which ain't good.

We have been using ultrasonic sensors and they do work, however they are very dependent on the environment.

I'll try to be a bit more specific So we are trying to register everytime the users changes working position from sitting to standing and from standing to sitting. So our ultimate goal is to register every time the height adjustable table changes from a height between 65 to 85 cm to a height between 90 and 125 cm. A height adjustable table moves up and down with an average speed of between 18 mm/sek to 40 mm/sek. The table is used between 0 and 8 times a day (8 hours). The table can be placed in whatever office environment you can think of, so the room height is different, the floor material is different etc. Hope that helps a bit

2 accelerometers might not be a bad idea. Think we will try and see if we can make it work.

Are there any restrictions on fitting sensors to the table ? Are you trying to have a device that just sits on the table and logs the information, or can the sensors be physically attached to the table ? Are the tables of the same design ?

If you can attach the sensors to the table then:

- mechanical limit switches would be a simple choice

- a linear encoder could give you more detail about the heights the table was adjusted to

- a rotary encoder with rack and pinion arrangement could also be used

If the floor covering is giving problems with the likes of ultrasonic sensors then perhaps locate the sensor on the floor so that the reflection is coming off the underside of the solid table rather than off a carpet.

To measure height use a Maxbotix ultrasonic sensor or a Sharp  GP2Y0A02YK0FGP2Y0A02YK0F or a string pot or a spring and force sensor

Should be easy.

Just lay down a conductive strip near the keyboard and connect it to the capacitive sensing circuit.

When the compute keyboard is on the desk, you get a good reading when the hands are on the keyboard.

If the keyboard is raised to a standing position, then the reading will be less and can probably provide you with a fairly precise distance from the desk top.

A simple program can check the height every time the computer is in use, so you can record trend data over time.

Clean, simple and automatic.

DAB

Hello,

is the table height adjusted by some electronic control or a manual handle?

I use two types of these tables (both electronically controlled) and the manufacturer offers them with two types of controllers. The first one has only up and down keys, the second one has up and down keys, 7 segment display to display the height in centimetres and four keys to store / restore value from a memory. These controllers are interchangeable which means there is probably a rotary encoder on the motor shaft and only one of these controllers can make use of the output The controller of course has then no idea about the ground clearance etc., it just knows the relative position to the lowest position. (Note: I haven't personally dismantled the mechanism.) I would try to do my own version of the controller in this case.

If you wanted to attach the measurement sensor to all kinds of mechanical tables (parallel with the floor), an ultrasonic sensor is one of the options. You don't even need to measure the absolute ground clearance, just the difference between the positions.

So we are trying to register everytime the users changes working position from sitting to standing and from standing to sitting

You could use an ultrasonic sensor and detect the height to the ceiling.

That would give you a relatively clear path and is uniform.

Some code to establish when it's stationery and then record it, or transmit it to a collection device.

Cheers

Mark