Would you consider changing the geometry of the castor to eliminate the problem?

Specifically you need to change the axis of rotation os that it is not vertical, and the intersection of the steering axis with the ground may need to be moved.

I think you will striggle to control it with a damper, although motorbikes sometimes do, using a little friction damper or a hydraulic unit. So check out motorcycle steering dampers.

Cheers

Greg Locock

This is the supermarket shopping trolley effect.

Am I right in thinking you control steering by changing the speeds of two separate drive motors (one for each driven wheel)?
I which case you need the other two wheels to caster fully through 360 degrees, which in turn requires the vertical steering axis.

Have you tried pneumatic tyres for the caster wheels? This might provide some damping on its own.

Most motorcycle friction dampers are linear units attached to brackets on the frame and front suspension legs, and are only suitable for a limited amount of steering.

It sounds like you want to try a fully rotary damper, with perhaps a rotating vane in a pot of grease or oil.
I have no idea if such a damper is commercially available, but it should be fairly easy to fabricate a prototype to see if it helps.

I used to have a rotary friction plate damper on my motorbike, so they definitely exist.

Cheers

Greg Locock

Caster wheels with a different amount of trail?  Or different wheel composition?

Norm

The two caster wheels have the same amount of trail and have the same composition.

Sjoerd

I didn't intend to imply that they currently differ from side to side (guess I posted a little too quickly and with too little proofing re how it might be interpreted).

How about changing those parameters?

Norm

Thanks all for your reactions!
We may have found a supplier who could provide us with a rotary damper. We will also do some tests with changing trail.

Regards,

Sjoerd

I did not know where castor shimmy came from (still don't actually) but below cliped from a web site.

2: Castor assembly (front wheel) (see figure 2)
a. To minimise rolling resistance the castor stem should be perpendicular to the
ground. Also if the castor stem is not in an angle of 90 degrees with the ground
you will notice that the front part of the wheelchair will go up or down when
turning the wheelchair.
b. The castor fork is bent backwards. This bending provides how big the castor trail
will be (see figure 3). When castor shimmy appears the castor trail is not big
enough. The manoeuvrability of the wheelchair depends on the castor trail but it
is not possible to define an optimal position.

Well, it is possible to define the optimum trail, so long as you know the tyre characteristics.

In this case you don't, admittedly, but that statement is not true in the general case.

Cheers

Greg Locock